如果输入字符代替 integer 如何终止程序

Question

I would like to validate the user input. For example, if I enter a character in the place of a integer, then the program should terminate.

Answer 1

The simplest way to read an integer from the user with basic input validation would be the following:

#include <stdio.h>
#include <stdlib.h>

int main( void )
{
    int num;

    printf( "Enter a number: " );

    switch ( scanf( "%d", &num ) )
    {
    case 1:
        //input is valid
        break;
    case 0:
        printf( "That was not a valid number!\n" );
        exit( EXIT_FAILURE );
    default:
        printf( "Unexpected input error occurred!\n" );        
        exit( EXIT_FAILURE );
    }

    printf( "You entered the following valid input: %d\n", num );
}

However, this solution is not perfect, as it will accept for example 6sdfh4q as valid input for the number 6 , although that input should probably be rejected. This is because scanf is not designed for line-based user input.

For line-based user input, it is probably better to use fgets instead of scanf . In contrast to scanf , fgets will generally read one line at a time, so it is the perfect function for reading user input. After reading one line of user input, you can attempt to convert the input to a number using the function strtol or sscanf .

Performing full input validation is rather complex in C. This is how I would write it:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>

int main( void )
{
    char buffer[1024], *p;
    long num;

    //prompt user for input
    printf( "Enter a number: " );

    //get one line of input from input stream
    if ( fgets( buffer, sizeof buffer, stdin ) == NULL )
    {
        fprintf( stderr, "Unrecoverable error reading from input!\n" );
        exit( EXIT_FAILURE );
    }

    //make sure that entire line was read in (i.e. that
    //the buffer was not too small)
    if ( strchr( buffer, '\n' ) == NULL )
    {
        printf( "Line input was too long!\n" );
        exit( EXIT_FAILURE );
    }

    //attempt to convert string to number
    errno = 0;
    num = strtol( buffer, &p, 10 );
    if ( p == buffer )
    {
        printf( "Error converting string to number\n" );
        exit( EXIT_FAILURE );
    }

    //make sure that no range error occurred
    if ( errno == ERANGE )
    {
        printf( "Range error!\n" );
        exit( EXIT_FAILURE );
    }

    //make sure that remainder of line contains only whitespace,
    //so that input such as "6sdfh4q" gets rejected
    for ( ; *p != '\0'; p++ )
    {
        if ( !isspace( (unsigned char)*p ) )
        {
            printf( "Unexpected input encountered!\n" );
            exit( EXIT_FAILURE );
        }
    }

    //number was successfully converted, so we print it
    printf( "You entered the following valid number: %ld\n", num );
}

Example output of this program when entering a valid number:

Enter a number: 65
You entered the following valid number: 65

Example output of this program when entering invalid input:

Enter a number: 6sdfh4q
Unexpected input encountered!

Answer 2

When any character was being input in my program in the place of integers, it was the same as if I were inputting the integer 4256577. So, I simply wrote an if statement if(myint==4256577){exit(3);}. Though I am uncertain about whether this number is defined somewhere or it just happened to show in my program that if I ran on another computer, it would be different, thus my program wouldn't eventually work

Answer 3

终止程序的正常方法是调用“退出”函数。

exit(ERROR_CODE);