Thread: declared int but entered a char

#include <stdio.h>

int main()
{
int n;
printf("Please enter one integer: ");
scanf("%d", &n);

printf("Entered integer = \n");

return 0;
}

In this program I prompted the user to enter their desire integer value but when the user accidentally or purposely enter an alphabet, the program generated an error. How do I prevent this kind of problem. Is it we have to declare a buffer first, then copy what ever inside the buffer which is a valid integer into 'int n'? If yes, how? If no, any other way?

You can check the return value of the scanf function

Code:

#include <stdio.h> 

int main() 
{ 
    int n; 
    printf("Please enter one integer: "); 
    if(scanf("%d", &n))
        printf("Entered integer = %d\n", n); 
    else
        printf("This is not a number\n");

    return 0; 
}

You can use the fgets function but you need to check if the string begins with an integer (atoi returns a zero on error).

Code:

#include <ctype.h>

if(isdigit(buffer[0]))
{
    /* It's (begins with) an integer */
}

First of all, thank you for replying but there is a problem in this source code (posted by 'Prelude'). When I accidentally or purposely entered a float number something like this 1.23, it doesn't print the expected answer or the 'else' statement. When I entered an alphabet, it printed zero (the else statement did not execute) but when I entered key something like ctrl 'z', 'x' and etc the else statement was executed.



Problem in the source code posted by 'Monster'.
When I accidentally or purposely entered a float number like 1.33, it printed the integer number only (this is true because I declared an 'int n'). How do I actually warn the user that he or she has entered an invalid integer.


Sorry for being so fussy.

Originally posted by lockpatrick
Sorry for being so fussy.

No problemo, if we stop answering your questions you know it's too fussy. Here's something you can try. Check all characters in your buffer with the isdigit function (but not the last character because this is the newline character).

Code:

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

int main ( void )
{
    unsigned int i;
    char buffer[BUFSIZ];
    printf ( "Enter a number: " );
    if ( fgets ( buffer, sizeof buffer, stdin ) != NULL )
    {
        /* watch out for the ';' at the end of the next line (for) */
        for(i = 0; (i < strlen(buffer)-1) && isdigit(buffer[i]); i++) ;

        if(i == strlen(buffer)-1)
            printf ( "%d\n", atoi ( buffer ) );
        else
            printf("Invalid number\n");
    }
    else
        printf ( "Error: Input validation\n" );

  return 0;
}