C programming. Trying to loop trap a user into entering a positive number

Question

Beginner here. I'm trying to trap a user into entering a positive number. However the while loop doesn't seem to be working for when the user enters and incorrect number.

Output:

Please enter a positive integer: -3
I'm sorry, you must enter a positive integer greater than zero: why?
I'm sorry, you must enter a positive integer greater than zero: -42
I'm sorry, you must enter a positive integer greater than zero: 42
The positive integer was : 42
Press any key to continue....

Code:

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
void clear_keyboard_buffer(void);
int main()
{
    int k;
    printf("Please enter a positive integer: ");
    scanf("%d", &k);
    if (k > 0)
    {
        clear_keyboard_buffer();
        printf("The positive integer was: %d ", k);

    }
    while (k<=0)
    {
        printf("I'm sorry, you must enter a positive integer greater than zero: ");
        scanf("%d", &k);
        return 0;
    }
}

void clear_keyboard_buffer(void)
{
    char ch;
    scanf("%c", &ch);
    while (ch != '\n')
    {
        scanf("%c", &ch);
    }
}

Can you try making an else statement for your if (k > 0) then put your while block inside that else. — CurseStacker
– CurseStacker, Commented Oct 16, 2015 at 1:29
@CurseStacker like this?else { while (k <= 0) { printf("I'm sorry, you must enter a positive integer greater than zero: "); scanf("%d", &k); return 0; } } — hfu0823
– hfu0823, Commented Oct 16, 2015 at 1:33
Compile with all warnings and debug info (gcc -Wall -Wextra -g). Then use the debugger (gdb). Read the documentation (e.g. of scanf etc...) And test the result of scanf. Consider also using fgetc to read a single character. Be aware of buffering. — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 16, 2015 at 8:11

score 1 · Accepted Answer · 2015-10-16 07:59:09Z

Ok, I think it's easiest to show you some code that does what you want, along with some comments about why you should do it that way:

#include <stdio.h>

int main(void) /* note the void here, it says "no parameters"! */
{
    int k;

    /* here we don't use printf() because there is no formatting to do */
    fputs("Please enter a positive integer: ", stdout);

    scanf(" %d", &k); /* note the space, it consumes any whitespace */

    while (k < 1)
    {
        fputs("I'm sorry, you must enter a positive integer greater "
                "than zero: ", stdout);
        scanf(" %d", &k);
    }

    printf("You entered `%d'\n", k);

    return 0;
}

Still you should check the return value of scanf() for production quality code because there could be errors (e.g. the user entering something that isn't a number ...)

That being said, for really reliable user input, I'd suggest to abandon scanf() altogether and just use e.g. fgets() to read a line of input (whatsoever) and then parse yourself. strtol() could come handy ...

Just to give you an idea what I'm talking about, here's a very simple but reliable solution:

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

static int readInt(void)
{
    char buf[1024];
    int input;
    char *endptr;

    fgets(buf, 1024, stdin);

    /* strip off "end of line" characters */
    buf[strcspn(buf, "\r\n")] = 0;

    input = strtol(buf, &endptr, 10);

    /* sanity check, only accept inputs that can be wholly parsed as
     * an integer
     */
    if (buf[0] == 0 || *endptr != 0) input = -1;

    return input;
}

int main(void)
{
    int k;

    fputs("Please enter a positive integer: ", stdout);

    k = readInt();

    while (k < 1)
    {
        fputs("I'm sorry, you must enter a positive integer greater "
                "than zero: ", stdout);
        k = readInt();
    }

    printf("You entered `%d'\n", k);

    return 0;
}

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