Detailed explanation of printf() and scanf() functions of C language

Preface

printf() 、scanf() I believe you are not unfamiliar with the two functions , however , Do you really understand these two functions , Through the study of this article , I believe you can realize that these two functions are different . This article will make a specific explanation for you through the combination of theory and code .

One 、printf() Function details

1. The function prototype

Standard format output function

int printf(const char *format, ...)

Parameters 1 : format Is a string , His content is the content displayed to the screen terminal . Its contents can be replaced by the following parameter values , The final formatted output to the screen terminal .
Parameters 2 : … It represents a variable length parameter , You can set several parameters of this function .
Return value : It returns an integer . The successful return is the number of characters printed on the screen , But does not include ‘\0’ , An error returns a negative number .

2. Detailed explanation of parameter specifier

format The specifier form is ：%[ sign ][ Width ][. precision ][ Type length ] type
Except for the type , The others are optional .

sign ：

sign	meaning
-	Align the standardized output left
+	Make the output result with a plus or minus sign (+ or -). Under normal circumstances, integers do not take +, Negative numbers will bring -
Space	Add a space before the output result
#	And 0 or x When used together with specifiers, it will add 0 or 0x, It is mainly convenient for viewing when printing .
0	Use... Before output 0 Fill instead of space .

Width and accuracy ：
Width : If the length of the number to be output is greater than the specified width , Then output as is ; If the length of the number to be output is less than the specified width , Then fill the space to the left .
precision ： That is, the decimal places to be reserved , If the length of decimal places to be output is greater than the specified precision , The specified precision is retained by rounding ; If the decimal length to be output is less than the specified precision , It will make up for 0.

Examples are as follows

#include <stdio.h>

int main()
{
    
    printf("%f\n", 13.5);       // 13.500000
    printf("%3.2f\n", 13.5);    // 13.50

    return 0;
}

In the above example , Output 13.5 If it is output in floating-point mode, it will retain the decimal by default 6 position , But after adding the width and precision control character output , It is output. 13.50.

Type length ：

length	meaning
h	Parameters are interpreted as short integers
l	Parameters are interpreted as growth shaping
L	Parameters are interpreted as long double

type ：

type	meaning
d	Output as decimal signed integer
o	Output in octal form
x,X	Output in hexadecimal form
u	Output as decimal unsigned integer
f	Output in the form of single precision floating-point numbers
lf	Output in the form of double precision floating-point numbers
e,E	Output single in exponential form 、 Double precision real number
g,G	With %f or %e Short output width in output single 、 Double precision real number
c	Output in character form
s	Output as a string
p	Output variable address

3. Code examples and error prone points

Source code :

#include <stdio.h>

int main()
{
    
    char a = 'X';
    char b[15] = "hello world";
    int c = 100;
    float d = 13.14;
    double e = 161.123321;
    int ret = 0;

    printf("%c \n", a);   // Output characters should be  %c
    printf("%s \n", b);   // Output string to use  %s
    printf("%d \n", c);   // Output shaping should use  %d
    printf("%f \n", d);   // To output single precision floating-point numbers, use  %f
    printf("%lf \n", e);  // To output double precision floating-point numbers, use  %lf

    ret = printf("hello world\n");   // Twelve characters will be output to the screen terminal , The return value is 12
    printf("%d %d\n", ret, ret++);   // First run  ret++  expression , Running  ret  expression 

    return 0;
}

Running results :

X 
hello world 
100
13.140000
161.123321
hello world
13 12

Two 、scanf() Function details

1. The function prototype

Standard format input function

int scanf(const char *format, ...)

Parameters 1 : format Is a string , Accept the string from the screen terminal . The accepted content can be formatted into the following parameters .
Parameters 2 : … It represents a variable length parameter , You can set several parameters of this function .
Return value : It returns an integer . The successful return is the number of data items read , Error return EOF.

2. Detailed explanation of parameter specifier

format The specifier form is ：%[*][ Width ][ Type length ] type
Except for the type , The others are optional .

Parameters	meaning
*	Lag character , Ignore the parameters read from the standard input stream , It is not assigned to the following parameters .
Width	Specify the maximum number of characters that can be assigned .
Type length	It's common for h or L, Such as hd,hu It will be stored in a signed or unsigned short integer

type ：

Convert specifiers	meaning
%c	Receive the input single character and pass it into the following parameters
%d	Receive the input integer and pass in the following parameters
%e、%E、%f、%F、%g、%G	Receive the input floating-point number and pass it to the following parameters
%i	Receive the signed decimal input and pass it into the following parameters
%o	Receive the input octal input to the following parameters
%x,%X	Receive the hexadecimal input and pass in the following parameters
%s	Receive the input string and pass in the following parameters . encounter Space 、 Line breaks and tabs end
%u	Receive the input unsigned decimal and pass in the following parameters
%p	Receive the input address and pass in the following parameters

3. Code examples and error prone points

Source code :

#include <stdio.h>

int main()
{
    
    char a = 'a';
    int b = 13;
    float c = 24;
    int ret = 0;

    printf("a = %c, b = %d, c = %f\n", a, b, c);
	/*1.  The variables received later must be received with address symbols */
	/*2.  The input content should be consistent with the content in double quotation marks except for the format controller */
    ret = scanf("%c,%d,%f", &a, &b, &c);   

    printf("a = %c, b = %d, c = %f, ret = %d\n", a, b, c, ret);

    return 0;
}

Running results :

a = a, b = 13, c = 24.000000
x,34,13.14
a = x, b = 34, c = 13.140000, ret = 3