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C language

This is a mind map about C language, and its main contents include: pointers, functions, arrays, loops, switch statements, if statements, logical operators and logical expressions, relational operators and relational expressions, simple C programs, data types, operators and expressions, overviews, and exam questions.

Edited at 2025-03-01 10:32:53

Mason·Carter

Recent works View more works>>

C language

Mason·Carter

Recent works View more works>>

Recommended to you
Outline

No relevant template

C language

Exam question type

Single choice question

Judgment question

Fill in the blanks of the program

Program Analysis

Programming questions

Overview

Computer Development

Development of programming languages

The development of C language

Characteristics of C language

Intermediate Language

Access to hardware

Programmer language

Rich transport characters, high code quality, good portability

Modular design language

C language program structure

illustrate

The composition of program C

The function consists of two parts

Program execution

Statement

Programming style

Overall structure Alignment

Definition of identifier, please understand the meaning of

Comments

Steps to go to the machine

edit

Source program input

Modify the source program

save

Compilation

Check for syntax errors

Generate target program .obj

Link

Library functions

implement

.exe

Data types, operators and expressions

Program = Data Structure Algorithm Programming Method Programming Language and Environment

Data Type

Basic Type

Integer

Character type

Real type

Construct Type

Array

Structure

Shared body

Enumeration type

Pointer type

void type

Function Type

Constants and variables

constant

The amount whose value will not change during the execution of the program

Direct constants (literal constants)

Symbol Constants

variable

The amount whose value can be changed

The variable has a name

Allocation of storage units at compile time

Identifier

An identifier is a name

Classification

Keywords

Predefined identifiers

printf

scanf

User Identifier

Made of alphanumeric underline

The beginning can only be made with letters or underscores

Cannot be repeated with reserved words

case sensitive

Pay attention to length

Definition of variables

Define first and then use

The format defined

Type name Variable name 1, Variable name 2;

Location

Global variables

Outside the function

Local variables

Generally at the beginning of the function

Starting with compound statement

Data Type

Integer data

Integer constants

Decimal

123, -456

Octal

Start with 0 (zero)

011 =》9

-017=》-15

018, 019=》Wrong

hexadecimal

Start with 0X

Add L or l (lower case) to be considered as long integer constant

Add U or u afterwards, it is unsigned unsigned

123U

Integer variables

int a;

VC 6.0 takes up 4 bytes in memory

Classification

Modifier

signed

unsigned

Short

2 bytes

long

4 bytes

Real data

Real constant

Real numbers are also called floating point numbers

Expression form

Decimal

Consisting of numbers and decimal points

0.123 .123 123.

Index form

123e3 or 123E3 =》123*10^3

Note: There must be a number before and after e, and the exponent after e is an integer

For example: e3, 2.13e3.5, .e, e

Real variables

Single Accuracy

float

4 bytes

Valid numbers 6-7

Double precision

double

8 bytes

15-16

Long double precision

long double

Character data

Character constants

A character enclosed in single quotes

For example: 'A', 'a' are different, '' two consecutive single quotes are wrong

Escape characters

Characters of special forms begin with "\"

' ', '\t',

'\ddd' ASCII characters corresponding to 1-3 digit octal

char ch = '\101';

‘\xhh' 1-2-bit hexadecimal

char a = '\x61';

Character variables

Used to store character constants, only one can be stored, accounting for 1 byte

Definition form

char character variable 1, character variable 2;

char c1, c2; c1 = 'a'; c2 = 97;

The storage form of character data in memory

Store the ASCII code corresponding to this character

Similar to integers, they can be used as integers

0-127 The highest bit is 0, and the output is positive with %d. 128-255 The highest bit is 1, and the output is negative with %d.

String constants

Sequence of characters enclosed in double quotes

Note: The difference between “a” and “a”

Two consecutive "" are correct

the difference

Different representation

Double quotes

Single quotes’

Sub-theme 3

Different definitions

char ch; character variables store character constants

Character array storing strings

Quote

The two cannot be cited by each other

Store in memory

1 byte in memory

String: Ended with '\0' Number of bytes occupied in memory = actual character length 1 For example: "CHINA" has 5 characters, accounting for 6 bytes in memory

Features

Only string constants and no string variables

A string ends with '\0', without '\0', it cannot be called a string.

4 zeros

Number 0, accumulator, counter, subscript of array

'\0'

The ending character of the string, the ASCII code value is 0, and the characters cannot be displayed.

NULL

Generally used for pointer assignment, pointer is empty

‘0’

Character 0, ASCII value 48

char-char =》int

'7' - '0' => 7

char int => char

//Case conversion char ch = 'B'; char st = ch - 'A' 'a'; st = 'a' 'A' - ch

char char meaningless

Variable assignment

Assigning an initial value to a variable when defined is called initialization

int a = 10;

int a; a = 10;

int a, b, c=10;

int a, b, c; c = 10;

int a = b = c = 10; wrong

int a, b, c; a = b = c =10;

Know the radius to find the perimeter, area, and volume of the ball

Symbol Constants

definition

#define PI 3.14

Replace 3.14 with PI

Features

Simplified writing, easy to read

Easy to modify

illustrate

Generally, uppercase and lowercase variables

Scope

Start with #define

End at #undef or end of file

The value of symbol constants cannot be modified within scope

const double PI = 3.14;

Mixed operations of various numerical data

Mixed operations

High double

←float

↑long

↑unsigned

Low int

←char, short

Horizontal arrows indicate necessary conversions

For example, two variable operations of float type must be converted to double first and then operation, and the result is double type

Vertical arrows indicate the direction of conversion

Int and double operations, convert int directly into double and double operations, and the result is double

Calculation results

The highest type of operation involved

practise

ch/i f*d - ( f i )

Arithmetic operators and arithmetic expressions

Operator characteristics

Arithmetic operators

composition

- * / % --

%module operator

The operation object can only be integer

The result is the same as the symbol of the dividend

Application: Disassembly

n = 123 Take out single, ten and hundreds respectively

Single digit n Ten digit n/10 n 0/10 Hundred digit n/100

Divided by 10, downgrade

Find more than 10, get a single position

Operator priority and nodule

Evaluation first depends on priority

() Single item

The priority is the same, depending on the combination

Combination of left: conforms to arithmetic operation rules, from left to right

Right combination:

Monograph

Assignment operator

Conditional operator

practise

The following set of operators is arranged from high to low _____

Cast type conversion (display conversion)

5.7%3 =》?

Format (type name) (expression)

For example: (int) 5.7%3

(double)x y

Convert the result of x y into double?

(double)(x y)

Convert x to double and add it to y

use

Function parameters

Notice

Unsafe conversion

High precision -》Low precision, direct intercept (not rounding)

Low precision-》High precision, supplement 0

Temporary one-time

(int)x y

After converting the x value to int, add it with y, the value of x itself remains unchanged

double x = 5.7 (int)x 3.2 Result: 5 3.2 = 8.2 x is 5.7 unchanged

Self-increase and self-decrease operator

Monologic operator, --

Operation object: can only be used for variables, and it is also an assignment operation itself

3, (a b) Wrong

i = 3;

j = i ;

Operator, on the right, first use the value of i, and then i completes the operation of adding 1

j = i;

i = i 1;

j = i;

Operator On the left, first complete the operation of adding 1 to i, and then assign the value of i (after 1) to j

i = i 1;

j = i;

practise

int a = 3; while( a-- ) printf("%d ", a ); printf(" out while %d", a ); 2 1 0 out while -1

int a = 3; while( --a ) printf("%d ", a ); printf(" out while %d", a ); 2 1 out while 0

Notice

Binding right binding, from right to left

int a = 3; printf(" %d ", -a ); printf(" %d ", a );

i ; and i ;

No difference, complete i=i 1;

Assignment operators and assignment expressions

Assign symbol =

Format: Variable name = expression

Function: Assign the value of the right expression to the variable on the left

If a = 10 assign 10 to a, perform an assignment operation (put 10 into a memory unit of a)

Notice

Binding: Right binding

The left side must be an invariant, not a constant or an expression

d = a b = c 10; wrong

Mutual assignments between different data types

Real type (float, double) is given to integer type (int), discarding the decimal part, not rounding, need to round 0.5

int=》float, double, the value remains unchanged, stored in floating point form, and the tail is supplemented by 0

char=》int , char1 bytes, int 4 bytes, put into the lower 8 bits

Unsigned and unsigned of the same type as each other

double=》float intercepts the first 7 valid numbers of double to float

practise

Keep the floating point number n and the 3rd digit rounded. If n=3.4567, the result is: 3.46

Compound operator (arithm character assignment operator)

Priority

Bondability (right)

constitute

=, -=, *=, &=, ^=, != (relationship)

Notice

x = 3;

x = x 3;

x *= y 3;

x = x *(y 3)

There are several items on the right side of the assignment number, which is equivalent to brackets.

practise

a=3; a = a -= a * a

Assignment expression

Assignment is an operator

Right combination

The expression has a value

Comma expressions and comma operators

Format: Expression 1, Expression 2

Solving process

Comma, left combination

The lowest priority

First solve expression 1,..., use the value of the rightmost expression as the value of the entire comma expression

practise

x = ( y=3, 4*5 )

x value 20

x = y=3, 4*5

x value 3, the value of the entire comma expression is 20

use

Expression concatenation

for( i=1,j=100; i<j; i ,j-- );

Delimiter

int x, y;

Find the length operator

sizeof (variable|constant|type)

Find the number of bytes occupied by this type in memory

malloc( sizeof( struct link ) )

Simple C program

Overview of C statements

Control statements

Function call statement

Function (real parameter table list);

scanf("%d", a);

Expression statement

Abortion statement

Compound statements

Enclosed in {}

Notice

In grammar is equivalent to a statement, as a whole

The start part of the compound statement can define variables

After the compound statement, no need to add it;

Three basic structures of the program

Sequential structure

Select (branch) structure

Loop (repeat) structure

Assignment statement

statement;

Difference from expressions

Expressions have values

Data input/out

There are no input and output statements, only input and output functions

stdio.h header file

Standard input and output header file

Output

putchar(ch) character output function

Output a character to the terminal

ch can be a char, int constant, variable, expression

printf (format control, output table column)

Format control is a string enclosed in double quotes

Format description

% and format characters %d

Normal characters

Output as is, space comma

Format characters

Different data types, different format characters

D format

Output decimal integer

%d outputs according to actual length

%md, m output field width, if the actual number of digits is less than m, fill the space on the left (right aligned) If the actual number of digits is greater than m, break through

%-md - (negative sign) alignment, left alignment (right space filling)

%ld long integer, %hd short integer

O format

Output in octal form

Sign bits are output together

X format

Output in hexadecimal form

%lx

U format

Unsigned output, decimal

C format

Used to output a character

The character memory occupies one byte, and the ASCII code is stored. 0-255 can be output according to the numerical value.

s format

Used to output a string

%s until we meet '\0'

%ms

%-ms

%m.ns

The output occupies m columns and takes the first n characters of the string

%-m.ns

F format

Used to output real numbers, floats and doubles, output in decimal form

All integer parts are output and 6 decimal places are output.

Notice

Not all values are valid

%m.nf

The output occupies m column, where n decimal places

Notice

One decimal point is also

When taking n bits, the first bit cut is rounded

%.0f does not output a fractional part

%-m.nf

e-format

Output real numbers exponentially

The system specifies 6 decimal places, the exponent part occupies 3 places, e accounts for one place, and ± accounts for one place

%m.ne

g format

Used to output real numbers, automatically select the shortest width in f or e format

Make the output number predecessor positive and negative sign

printf("% 5d% d ", a, b );

Notice

Lowercase characters except X, E, G

Data type matches format characters

type

Number of

Format control strings that can contain escaped characters

d,o,u,x,s,c,f,e,g, after % is the format character, and the others are output as is

Output % and use %%

Different systems vary

enter

getchar()

Character input, enter a character from the terminal

getchar() has no argument

scanf (format control, address table column)

The format control is the same as printf

Format characters

Normal characters are entered as-is

Address table column, address of variable, first address of string

Note: %d%d, spaces, tab keys, and carriage return cannot be used, commas

Format description

Short uses %hd, double uses %lf or %le to enter

For unsigned, you can use %u, %d, %o, and %x to input it.

You can specify the domain width of the input data and the system automatically intercepts it.

% followed by *, indicating that the specified number of columns is skipped

The number of columns can be specified, but the accuracy cannot be specified.

scanf("%7.2f", &f); wrong

Notice

scanf function, the address of the variable should be given

Normal characters are entered as-is

When %c enters characters, spaces and escaped characters are all valid inputs.

When entering data, the following situation is considered to be ended

Encounter space, Tab key, Enter

Encountered with specified domain width=

Illegal input

Relational operators and relational expressions

Relationships are actually more operational

Relational operators

>, >=, <, <=

!=, ==

Priority

Below arithmetic

Bonding

Left combination

Relational expressions

Link any legal expression

Value: is a logical value

True is represented by 1, fake is represented by 0

practise

a = 20; b = 10; the value of a>b is 1

The value of a=3; b=2; c=1; a>b>c is 0

Notice

The two sides are inconsistent, such as int and double operations, the system automatically converts it.

Qualified equality

int a==b?

fabs(a-b)<1e-6

Logical operators and logical expressions

Logical operators

&&, ||,!

Priority

! Monologic operator

Below relationship &&higher than||

Logical expressions

Value: Logical value, true is represented by 1, false is represented by 0

Objects participating in the operation, non-zero represents true, and zero represents false

Notice

Not all logical operations are performed during the solution process

Short circuit phenomenon

practise

if statement

Three forms of if statement

Single branch if statement

Format: if (expression) statement;

Expression: any legal expression

if (3);

Statement: can be a single statement, an empty statement or a compound statement

practise

Output two integers from small to large

Double branch if statement

Format: if (expression) Statement 1; else Statement 2;

Note: else is aligned with if

practise

Even n is true expression

n%2==0

n/2*2 == n

!(n%2)

ch is an English letter

ch>='a'&&ch<='z' || ch>='A'&&ch<='Z'

Judge leap year

Can be divided by 4, cannot be divided by 100, can be divided by 4, can be divided by 400, same as above

year%4==0&&year%100!=0 || year%400==0

year%4==0&&year%100 || !(year%400)

The integers a and b are the same number

a*b>0

Ladder-type if statement

Format: if (expression 1) Statement 1; else if (expression 2) Statement 2;

if( x<0 ) y = -1; else if( x==0 ) y = 0; else y = 1;

Nested if statements

if( x>=0 ) if( x>0 ) y = 1; else y = 0; else y = -1;

Note: if and else pair, starting from the innermost layer, else is always used with the closest, unpaired if on it

Solution:

The embedded if statement also contains else, and the number of if is as many as else

Use curly braces to limit the scope of the embedded if statement

practise

Conditional operator

constitute

? and:

Priority: arithmetic, relation, logic, condition, assignment, comma

Binding: Right binding

Conditional expression

Expression 1? Expression 2: Expression 3

Evaluation order: first solve expression 1, true solution 2, false solution 3, and use this value as the value of the entire expression

Expressions 2 and 3 will only solve one

illustrate

Binding: Right binding

int a = 1, b = 2, c = 3, d = 4; printf("%d ", a>b ? a:c>d ?c:d );

type

The types of expressions 1, 2, and 3 can be different

a>b ? 1.5:2

Value of high type

Relationship with if statement

When the true and false of the if statement are assigned a value to the same variable

if( a>b ) max = a; else max = b;

max = a>b ? a:b;

Switch statement

Handle multi-branches

Format: switch (expression) { case case case 1: statement 1; ... default: statement n 1; }

illustrate

Expressions in brackets after switch are generally, character type, integer type, enum type

The value of the case is the same as the expression type

The function of the case is equivalent to a label, representing the entry, executing the following statement until the end, and there is no matching execution of the following statement after default

The execution of the terminating statement requires break, and the switch statement can be directly jumped out.

Each situation must be different, otherwise it will be contradictory

The order in which each case and default statement appears does not affect the execution result (used in conjunction with break)

Multiple case statements can share a group of statements

practise

cycle

if and goto loop

goto statement

Format: goto statement label;

Statement labels comply with the naming rules of identifiers

goto label;

Notice

After the label, it must be:, placed before a statement and used as the code for the statement.

Jump location

In this function

Generally, it forms a loop with if

Use as little as possible and jump out of multiple loops

for ( ) for() for() if() goto end;

While

Format while (expression) statement;

Features

First judge the value of the expression, execute the statement in true form, and exit falsely

Notice

if, while, for effective range ends at the first semicolon

The statement can be a single statement, a compound statement, or an empty statement.

There should be a statement in the loop body (statement) that makes the loop end

while( 1 ) x ;

A dead cycle

i = 100; while( i>10 ) a b;

The difference between while( x = 'a' ); and while( x=='a' )

k = 2; while( k=0 ) k--;

Loop body execution? Second-rate

practise

1 2 … 100

do-while

Format do statement; while (expression);

Features

First execute the loop body, judge the value of the expression, true execution, false exit

Notice

Difference between while loop

The initial value of a loop variable, while may not execute the loop body at one time. do-while executes at least once.

Similarities

True execution, fake exit

practise

Disassemble and splice

Disassembled number

Get the final number

/10 Reduced level

Scratch number

Low position becomes high, high position becomes low

(0*10) a)*10 b)*10 c

High and low remain unchanged

t = 1; s = n * t; t *=10;

do statement; while( 1 );

A dead cycle

for

Format: for(Expression 1;Expression 2;Expression 3) statement;

illustrate

Expression 1, only 1 time is executed

Each expression can be omitted, and semicolons cannot be omitted

Expressions 1 and 3 can be simple expressions or comma expressions.

practise

Nesting

Multiplication formula table

Print graphics

Exhausted

Comparison of several cycles

Solve the problem

The four loops can be replaced by each other, and the loop composed of if and goto is not recommended.

initialization

while, do-while outside the loop, for expression 1

Termination conditions

while, do-while after while, for expression 2

i = 3; while(i--); while(--i); difference

Execution order

while and for first judge and then execute, do-while execute first and then judge

Break out of the loop

While, do-while, for can be used to break and continue to end this loop

break

Break out of the loop body and end the loop early

practise

Determine prime numbers

Enter a positive integer from the keyboard and output 5 prime numbers greater than or equal to the number. If you enter 17, then output 17, 19, 23, 29, 31

Continue continue

End this loop, skip the statements that have not been executed in the loop body, and judge whether to loop next time (while, for execution expression 3)

Array

One-dimensional array definitions and references

Array is a collection of data elements of the same type. Array name

definition

Data type Array name [Integer constant expression];

Array name: address constant, representing the starting address of the array

int a[10];

#define N 10 int a[N];

Quote

Array name [subscript]

Subscript starts from 0 to N-1

Subscript integer constants, variables, expressions

initialization

All elements assign initial values

int a[3]={1,2,3};

Assign initial values to all elements, you can not specify the length int a[] = { 1,2,3}; int a[] = { 0 };

Some elements assign initial values

int a[10] = { 1,2,3}; a[1]=2, a[3]--a[9] = 0

Some elements omitted are default values

int a[3] = { 0,0,0}; is equivalent to int a[3]={0};

The difference between int a[10]; and static int a[10];

When the array length N and element format M are inconsistent

N>M, only the first M elements of the array are assigned initial values, the rest are default values

N<M, syntax is wrong

application

Inverse order of elements

Fold half search

Sort

Bubble sort

Select Sort

Operation

insert

Move the latter first

delete

Move the front one first

Move (loop left, right)

Two-dimensional array

definition

Data type Array name [Integer constant expression 1] [Integer constant expression 2];

Store in memory by row, line order is preferred

int a[N][M]; N rows, M columns, how many elements are there before a[i][j] i*M j

Quote

Array name [row subscript] [column subscript]

initialization

Same as one-dimensional array, assign initial values one by one element, and are given in sequence

int a[2][3] = { 1,2,3,4,5,6};

All elements can omit the first dimension size

int a[][3] = { 1,2,3,4,5,6,7};

Assign initial values in branch, using curly braces

int a[2][3] = { {1,2,3},{4,5,6}};

int a[2][3] = { {1,2},{3,4},{5,6}};

wrong

Assign initial values to some elements

int a[3][4] = { {2}, {4}, {6} }; int a[3][4] = { {2}, {0,4}, {0,0,6} };

application

Row subscript

Yang Hui triangle

Special square matrix

Spiral square matrix

Character array

definition

char array name [integer constant expression];

initialization

Assign initial values one by one

char ch[] = { 'a', 'b', 'c' };

Specify a string as the initial value

char ch[] = { "hello"};

char ch[] = "hello";

char ch[] = { 'h','e','l','l','o','\0'};

The difference between a character array and a string

Each element in the character array is a character, and the last element is not required to be '\0'

The ending character of the string is '\0'

Input/out of character array

Single character input and output of string

for( i=0; i<5; i ) printf("%c %d | ", ch[i], ch[i] );

Access each element

Input and output of strings

%s format

char ch[80]; scanf("%s", ch ); printf("%s", &ch[3] );

scanf("%s", ch);

Indicates which address the input characters starts to be stored in memory

Enter, TAB key, space end

At the end, the system automatically fills '\0'

printf("%s", ch );

ch is the first address of the array, followed by %s, which means that the output starts from this address and ends with '\0'

Notice

The array name represents the starting address of the array. It should not be &, &ch is wrong, it is OK &ch[0]

You can input and output from the middle of the array

scanf("%s", &ch[3] ); printf("%s", ch );

String processing functions

String output function

puts( ch);

ch can be a character array name

Character pointer

String constants

The difference from %s

Automatically wrap the line after puts output

String input function

gets(ch);

ch can be a character array name

Character pointer (exact pointer)

gets(s1,s2); wrong

Difference from %s input

gets can only end with return

String processing function string.h

String link strcat(character array 1, string 2)

The former must be a variable, and the space size can put the sum of the two characters.

The latter, constant variables are available

Only '\0' in the latter result is retained

practise

String copy strcpy (character array 1, string 2)

The former must be a variable, and the space size can put the sum of the two characters.

The latter, constant variables are available

The result is that the content before character 2, '\0' is copied into the character array, and '\0' is also copied together.

practise

strncpy( str1, str2, n )

Copy the first n characters in str2 to str1

Don't copy '\0'

string comparison strcmp( string 1, string 2)

All constant variables are possible

Order: Compare ASCII values of characters one by one from left to right

End: Encounter '\0', or different characters

Result: 0 means equality, 1>2 positive value 1, 1<2 negative value -1

Determine whether a string is equal

if( strcmp(s1,s2)==0 ) ;

string length strlen (string)

Find the length of the actual string, excluding '\0'

Number of characters before the first '\0'

practise

Convert lowercase strlwr(string)

Convert capital strupr(string)

practise

Delete special characters in strings

String copy

String comparison

String connection

Statistics the number of words

Count the number of times a substring appears in the main string

Use the filtering method to find prime numbers within 100

Insert, delete

function

Overview

Definition: A program that can complete a specific function

Notice

Compilation unit: file

Compile in source programs

The execution of C starts with the main function

All functions are parallel, that is, the function definitions are independent of each other.

Definitions cannot be nested

Functions can be called mutually, and main functions cannot be called.

Function calls can be nested

Classification

User perspective

User functions

User-defined to solve specialized problems

System functions

Standard function, scanf

Functions' own form

There are parameter functions

sqrt(n)

No tragic function

getchar()

The angle of return value

There is a return value

strcmp(s1,s2); pow(x,y);

No return value

Angle of range of action

External functions

Internal functions

Function definition

Definition format with parameter functions

Definition format of non-parameter function

Empty function

dummy(){ }

Function parameters and function values

parameter

Formal parameters

Define functions, in brackets

Redirected

Actual argument

Calling functions to use parameters

Main tune

illustrate

When defining a function, you must specify the type of the formal parameter

Format: Data type Formal parameter name

The actual argument can be a constant, a variable, or an expression, but it must have an exact value.

Allocation and release of formal parameter storage units

Allocate during call, copy the value of the actual parameter to the formal parameter, and release it after the call

The real parameter is passed to the one-way value of the formal parameter. The real parameter is assigned different storage units. Modifying the formal parameter does not affect the actual parameter.

The return value of the function

The return value of the function is obtained through the return statement

The number of return statements, multiple, to which one is executed, which one works

The brackets after the return statement can be omitted, and can be constants, variables, and expressions require exact values.

Type of function value

The type of function return value

Determined at the time of definition

The function type is inconsistent with the type returned by the return statement

The function type determines the type of return value

There is no return statement in the function

Returns an uncertain value unless defined as void

No return value is required, defined as void

Return can still be used, but there cannot be a value afterwards

The default return value type of function is int

Calling of functions

General format

Function name (real parameter table column);

Comma interval between multiple parameters

The number, order, and type of parameters (assignment compatible) are consistent

Call method

Function statements

printf("OK");

Function expressions

x = 10 * sqrt(y);

Function parameters

max( max( x, y ), z );

Calling process

Registration of formal parameters is allocated to storage unit, real parameter value Assign to formal parameters

Practical parameters -》Formal parameters One-way value transfer

When multiple functions are called in nested, follow the first call and then return.

End of return statement, or close braces }

Declaration of the called function and function prototype

Call another function, meet the conditions

The called function must exist (i.e., the defined function)

Library functions

scanf

Custom functions

max(x, y )

Using library functions

Use #include at the beginning of the file to include the corresponding header file

stdio.h

math.h

abs(), fabs(), pow(), sin()

string.h

strcpy(), strcat(), strcmp()

strlen()

ctype.h

malloc.h(stdlib.h)

malloc(),free(), calloc()

Custom functions

Declare the called function

The difference between definition and declaration

significance

Function type Function name, formal parameter table column and a complete function body Definition is the establishment of function functions

Perform a defined (existent) function

Number of

The same name, in a file, there is only one

Multiple

Location

Function definitions cannot be nested

Which function is called, where is declared?

Scope

Internal function (this file), external function (other file)

Declare its function Declare its file

Function prototype

Function declarations in C language are called function prototypes

Function type Function name (parameter type 1, formal parameter 1, parameter type 2, formal parameter 2...)

Function type Function name (parameter type 1, parameter type 2...)

The following situations are not required to be declared

Defined first, call back

Before all function definitions, the function at the beginning of the file has been declared outside the function. No need to declare each main call function.

The return value of the function is int without declaration

Nested calls to functions

Call another function

e = 1 1/1! 1/2! 1/3! … 1/n!

double fun( int n ) calculates the factorial of n

double e( int n ) calculates n terms sum

void main()

Recursive call to functions

Direct recursion, indirect recursion

Similar to nested calls of functions, except that each time the same function is called

Conditions that can be implemented recursively

Initial situation (termination condition), such as: n<=1

Situation n can be described by situation n-1, for example: n! = n * (n-1)!

Note: Test first, then recursion. Recursion is conditional. Only when the conditions are met can you be recursive.

practise

Normal problem solving

Reverse the result from the termination condition

Copy 3 times

evaluate

Recursion can be converted to non-recursion

The recursive implementation uses the stack

Array as function parameters

Array elements make actual parameters

Same level as variable

Value pass

Array name as function parameters

Array name, address constant

Pass the address

Note

Definition: Arrays should be declared separately in the main and called functions

Type: Consistent

Size: The size of the shape parameter group and the real parameter group can be different. The shape parameter group can not specify the size. Suppose another parameter passes the number of elements void fun( int array[], int n )

The array name is used as a real parameter, instead of passing the value of the array to the shape parameter group, the starting address of the real parameter group to the shape parameter group. The shape parameter group and the real parameter group occupy the same storage unit. The elements in the shape parameter group change, and the image real parameter group

Multidimensional arrays make function parameters

Actual argument, use array name

Formal parameters, the size of each dimension can be specified, and the size of the first dimension can be omitted.

practise

Sort the first n elements in a one-dimensional array

Save the row number of the smallest element of each column in a two-dimensional array in a one-dimensional array

Local and global variables

Local variables

Location

Inside the function

In compound statement

Scope

Functions that define it

Compound statement that defines it

Survival period

Allocation of storage units when called, release at the end (except static)

illustrate

Variables defined in main function are only valid in mian function

Different functions can define variables with the same name, represent different objects, occupy different storage units in memory, and do not interfere with each other.

Formal parameters are also local variables

Global variables

Location

Outside the function

Scope

From definition to end of file declaration, declare its function

Survival period

Static storage area, until the program runs, it always takes up memory and does not release it

illustrate

Function: Increase communication channels between functions

It is recommended to use less

Survival period

General

readability

Any function can be modified, it is difficult to determine the value of a variable at a certain moment.

In a source file, the global variable is the same as the local variable

Global variables do not work within the scope of local variables

Take the nearest principle

Storage category of variables

Scope, life span

Variable and function properties

Data Type

Storage category

How to store in memory

Static storage

Dynamic storage

auto variables

When local variables in a function are not specifically declared, the default auto is

Allocate in dynamic storage area, and allocation and release are determined by the system

Scope

Functions that define it

Survival period

Allocate on call, end release

Features

The system cannot be initialized, and each call requires manual initial value

Local invariant declared by static

The call ends, the storage unit is not released, and the value of the last call ends

Scope

Same as local variables

Survival period

Always take up memory and not release

Features

The system automatically assigns initial values, or can manually assign initial values, and the initialization is only done once.

Keep the value at the end of the last call

Features

Local automatic variables and formal parameters, global variables are not

The number of registers is limited, so you cannot define too many

Declare external variables with extern

Declare external variables in file

Extend the scope of global variables

Location

In the function using it

Start of the file

extern int a, b;

extern a, b;

Declare external variables in programs with multiple files

One file refers to global variables in another file

Location

Start of the file

Static storage area

Global variables, local variables declared by static

The system can automatically assign initial values, or manually assign initial values.

Dynamic storage area

Local variables (no static)

Formal parameters

Manually assign initial value every time

Internal and external functions

Internal function (static function)

Functions that can only be called by other functions in this file

Add static before function name and function type when defining

static int fun( int a, int b ){}

External functions

When defining, the extern is first crowned, indicating the external function

Functions are implicitly external functions

extern int fun( int x, inty ){}

pointer

Overview

Direct access

By variable name

Indirect access

Relationships pointed by pointers

A variable that stores the address points to a normal variable

The variable that stores the address is called a pointer variable

Notice

pointer

address

1000

Pointer variable

Variables that store address

Pointer variable

definition

Base type * pointer variable name;

int *p;

When defining, * does not do any operation, which means that p is followed by it is a pointer variable, and p points to int, that is, p can store the address of the int variable

p is a pointer variable, and the address is stored in the variable p, not *p

Operation

Assignment

Take the address of the variable

int x, *p; p = &x;

Get the first address of the array

int a[10], *p; p = a; or p = &a[0];

Pointer variables assign each other

int x, *p1, *p2; p1 = &x; p2 = p1;

The types pointed to by p1 and p2 must be the same. When assigning values, the pointer variable on the right has an exact pointing point.

Application space return address

int *p; p = (int *)malloc( sizeof( int ) );

Contains malloc.h

Assign NULL value

int *p; p = NULL; p = 0; p = '\0'

Make p pointing empty to prevent misuse

initialization

Storage category Base type *Pointer variable name = Address;

int x, *p = &x;

Arithmetic operations

Pointer variable points to a continuous memory unit. It can complete addition and subtraction, self-increase and self-decrease operations with int.

int a[10]; p1=p2-1; Think of the addition and subtraction operations of characters int *p1, *p2; p2=p1 3; p1 = a; p2-p1; p2 = a 1;

p1 p2 meaningless

Relational operations

A continuous space

Notice

int *p; p is a pointer variable, not *p

Pointer variables can only point to variables of the same type

Address operator and pointer operator

The operation object must be a defined variable or array element.

The calculation result is the first address of the variable in memory

&3, &(x y), register variable Error

The operation object, the pointer that has been assigned, the address

Function: Take the content of the unit pointed to by the given address

* and & operations are inverse operations

The priority is the same, right-hand combination

*&a => *(&a) => *(1000) => *pa => a

&*pa => &(*pa) => &(a) => 1000 => pa

&*a => a

Note: & variables, * address

Pointer variables can only store addresses

Pointer variables as function parameters

Pointer variables are used as function parameters and pass address

During the execution of the function, the value of the variable pointed to by the pointer variable changes.

Pointer and one-dimensional array

The address of the variable

The first address of the array: the starting address, the array name

Pointer variable to array elements

int a[10]; int *p; p = a; or p = &a[0];

Reference array elements through pointer variables

p=a; a[i]<=>*(a i), bracket index operator p = a 3; p[5] = ? a[8] a: array name, address constant, a cannot be modified. Error p: pointer variable, p can be modified

The address of the i-th element&a[i]

p i

Sub-theme 4

The value of the i-th element a[i]

*(p i)

*(a i)

Pointer method

Pointers to one-dimensional arrays can also be subscripted

p[i]

a[i]

Subscript method