CODEWITHSUNDEEP
cvalidationinput
user3731938
user3731938

Reputation: 21

C Determine if user input is an integer

Hi I need to prompt a user for some input and then validate it. The input must only be validated if it is a positive integer and not greater then 23. The only problem I am having with this is when the user enters a non-numerical input like "hello." The code below does not successfully detect that any input is non-numerical and though I have tried many methods to do this, none of them seem to work. Below is the closest I seem to have gotten by taking the input as a string then converting it to an integer, however it still does not work. Any help would be appreciated.

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


int main(void) {
    int height;
    char input[50];
    int cont = 0;
    while (cont == 0) {
        printf("Please provide a non-negative integer no greater than 23.\n");
        scanf("%s", &input);
        height = atoi(input);
        if (height <= 23 && height >= 0) {
            cont = 1;
        } else {
            //do nothing
        }
    }
    printf("Valid Input.\n");
    return 0;
    }

Upvotes: 1

Views: 134

Answers (3)

Arpit
Arpit

Reputation: 775

I am assuming that you have to consider the whole input into the consideration rather than only certain parts like "12jjj" and "23h" as invalid.

In my opinion, since 23 is only 2 char, so there is no harm in checking the length of the string and the individual characters.

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

bool ValidateInput (char * input, int &output)
{
    if (strlen(input) > 2)
        return false;

    for (int index = 0; index < strlen (input); index++)
    {
        if ((input[index] < '0') || input[index] > '9')
            return false;
    }

    output = atoi(input);

    return true;

}


int main(void) {
    int height;
    char input[50];
    int cont = 0;
    while (cont == 0) {
        printf("Please provide a non-negative integer no greater than 23.\n");
        scanf("%s", input);

        if (ValidateInput (input, height))
            break;
    }
    printf("Valid Input.\n");
    return 0;
    }

I hope this helps.

Upvotes: 0

BLUEPIXY
BLUEPIXY

Reputation: 40145

#include <stdio.h>

int main(void) {
    int height;

    while(1){
        printf("Please provide a non-negative integer no greater than 23.\n");
        //if(2==scanf("%d%c", &height, &nl) && nl == '\n' && 0<= height && height <= 23)//more limited for "number\n"
        if(1==scanf("%d", &height) && 0<= height && height <= 23)
            break;
        //Clear of invalid input
        while(getchar()!='\n')
            ;
    }
    printf("Valid Input(%d).\n", height);
    return 0;
}

Upvotes: 0

Greg Hewgill
Greg Hewgill

Reputation: 992747

The atoi() function has no provision for returning an error indicator. Instead, you can use the strtol() function:

char *end;
height = strtol(input, &end, 10);
if (end == input) {
    // no digits were entered
    puts("Invalid input.");
    continue;
}

Upvotes: 4

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