我如何在 c 中编写一个程序，显示不允许名字不超过 10 个字符？

Question

I am trying to create a program to output firstname not more than 5 characters.

#include <stdio.h>
#include <stdlib.h>
int main()
{
     //declaring fisrtname
    char firstname[5];
    printf("Enter your name: ");
    scanf("%s", &firstname);
    printf("\nYour name: %s", firstname);
    return 0;
}

While running a program, i am getting this in my command prompt:

Enter your name: newton

Your name: ←
Process returned 0 (0x0)   execution time : 4.719 s
Press any key to continue.

Answer 1

There are a couple of errors holding you up:

• The parameter passed into scanf is the wrong type (you are passing a pointer to an array, but it is just expecting a pointer). I would change that line to:

scanf("%s", firstname);

• The scanf function will happily overflow your filename[5] buffer. The quick fix would be to just up the size of the buffer to something like 256. But to do it the right way you'll need to switch to using something like fgets (passing in stdin), or the likes.
• To truncate the name to five characters, you will need to write '\\0' (the null terminator) to the sixth memory location in the buffer (again, make sure your buffer is large enough).

filename[5] = '\0';

Here is a working version of your code:

#include <stdio.h>
#include <stdlib.h>
int main()
{
     //declaring fisrtname
    char firstname[256];
    printf("Enter your name: ");
    scanf("%s", firstname);
    firstname[5] ='\0';
    printf("\nYour name: %s", firstname);
    return 0;
}

Answer 2

In your program, your array firstname only has space for 5 characters including the terminating null character. That means you can only store 4 other characters in it.

When writing newton into the array, you will be writing 7 characters (including the terminating null character) into the array, although it only has space for 5 characters. This means that you are writing to the array out of bounds, causing undefined behavior .

If you want to limit the number of characters that are written into the array to 5 characters (not including the terminating null character), then you should change the scanf format specifier from "%s" to "%5s" . However, in that case, you must also increase the size of the array from 5 to 6 characters, so that you also have space for the terminating null character:

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

int main( void )
{
    char firstname[6];

    printf( "Enter your name: " );
    scanf( "%5s", firstname );

    printf( "Your first name is: %s", firstname );

    return 0;
}

However, using scanf for line-based user input is generally not recommended. It is usually better to use fgets instead. See this guide for more information:

A beginners' guide away from scanf()

The %s format specifier of scanf will attempt to read exactly one word of input. It won't read a whole line of input. And it won't read a whole first name, if the first name consists of several words. For example, with the first name John Paul , the "Paul" is also part of the first name, not the middle or last name. Therefore, it is generally also important to read the second word of the first name.

This can be best accomplished by reading a whole line of input using fgets :

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

int main( void )
{
    char line[100];

    //prompt user for input
    printf("Enter your first name: ");

    //attempt to read one line of input
    fgets( line, sizeof line, stdin );

    //remove newline character, if it exists
    line[strcspn(line,"\n")] = '\0';

    //print input back to user
    printf( "Your first name is: %s\n", line );

    return 0;
}

If you want to limit the output to 5 characters, then you can change the line

printf("Your first name is: %s\n", line );

to:

printf("The first 5 characters are: %.5s\n", line );

If you instead only want to allow the user to enter up to 5 characters, and want to reject the input and prompt the user to enter valid input again, then you can do that too:

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

int main( void )
{
    char line[100];

    //this loop will run until the user enters valid input
    for (;;) //infinite loop, equivalent to while(1)
    {
        //prompt user for input
        printf("Enter your first name: ");

        //attempt to read one line of input
        fgets( line, sizeof line, stdin );

        //remove newline character, if it exists
        line[strcspn(line,"\n")] = '\0';

        //reject input if it is too long
        if ( strlen(line) > 5 )
        {
            printf( "Error: Input is too long!\n" );
            continue;
        }

        //input is ok, so break out of infinite loop
        break;
    }

    //print input back to user
    printf( "Your first name is: %s\n", line );

    return 0;
}

One problem with the code above is that it does not have any error handling. It always assumes that the call to fgets will succeed and that the input will never be too large for the input buffer. If any of these assumptions are false, then the program will misbehave.

A more robust version of the code above would look like this:

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

int main( void )
{
    char line[100];
    char *p;

    for (;;) //infinite loop, equivalent to while(1)
    {
        printf("Enter your first name: ");

        //attempt to read one line of input
        if ( fgets( line, sizeof line, stdin ) == NULL )
        {
            fprintf( stderr, "input error!\n" );
            exit( EXIT_FAILURE );
        }

        //find newline character
        p = strchr( line, '\n' );

        //verify that newline character was found
        if ( p == NULL )
        {
            int c;

            //check if unable to read whole line due to input failure
            if ( ferror( stdin ) )
            {
                fprintf( stderr, "input error!\n" );
                exit( EXIT_FAILURE );
            }

            //if end-of-file is encountered, then it is ok if
            //a newline character was not encountered
            if ( !feof(stdin) )
            {
                printf( "Error: Line was too long for input buffer.\n" );

                //discard remainder of line
                while ( ( c = getchar() != '\n' ) )
                {
                    if ( c == EOF )
                    {
                        fprintf(
                            stderr,
                            "input error discarding remainder of line!\n"
                        );
                        exit( EXIT_FAILURE );
                    }
                }

                //prompt user for new input
                continue;
            }
        }
        else // p != NULL
        {
            //discard newline character
            *p = '\0';
        }

        //verify that input satisfies our special requirements
        if ( strlen(line) > 5 )
        {
            printf( "Error: Input is too long!\n" );
            continue;
        }

        //input was ok, so we can break out of infinite loop
        break;
    }

    printf("Your first name is: %s\n", line );

    return 0;
}

This is what the output of the program looks like:

Enter your first name: Michael
Error: Input is too long!
Enter your first name: Jimmy
Your first name is: Jimmy