Thread: checking input from user

I am doing this exercise where I have to draw a pyramid like below:
**
***
****
*****
******
*******
********
*********

Here's the code I have, it does the job but it doesn't work well if user enters strings. It strangely goes on infinite loop. Why is that? I thought that scanf ignored strings so it would return 0 and it would fail condition but it's not working like what, what am I missing here?
Also I'd like to know how you guys would code this. I have variable i map to number of rows and jto the the number of blank space to print.

Code:

#include <stdio.h>


int main(void)
{
    int height, i, j, k, readOkay;
    do
    {
        printf("Enter height(0 to 8): ");
        readOkay = scanf("%d", &height);
    }
    while ((height < 0) || height >8 || readOkay < 1);
    for (i = 0; i < height; i++)
    {
        j = (height + 1) - (i + 2);
        k=0;
        while (k < j)
        {
            printf(" ");
            k++;
        }
        while (k < (height + 1))
        {
            printf("*");
            k++;
        }
        printf("\n");
    } 
}

If "scanf()" fails, whatever made it fail remains on the input buffer. Your code is going into an infinite loop because it keeps trying to read the same thing, failing, and looping again.

If "scanf()" fails, you need to clear the bad data from the input buffer. An quick way is to use the "while getchar does not equal newline" loop if the input is invalid.

A better idea is to use "fgets()" to read the input, and parse as needed (either with "sscanf()" or "strtol()").

If the user enters only letters then readOkay will be < 1 and you loop back.
But the letters are still sitting there in the input buffer!
So scanf returns 0 yet again.
Etc.
You need to empty the input buffer before trying again.
You could loop with getchar() until you get a newline.

It's probably better to read the input a line at a time:

Code:

char line[512];
int height = 0;
do {
    fgets(line, sizeof line, stdin); // ignoring NULL return possibility
    if (sscanf(line, "%d", &height) != 1) {
        printf("Numbers, man, NUMBERS!!\n");
        continue;
    }
} while (height < 0 || height > 8);

I tried to use strtol but it's not converting right, *ptr is poiting to 10 and not a '\0' or '\n' when i insert a valid number such as 8.

Code:

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


int main(void)
{
    int height, i, j, k, readOkay;
    long int a;
    char aux[20], *ptr;
    do
    {
        printf("Enter height(0 to 8): ");
        fgets(aux, 20, stdin);
        a = strtol(aux, &ptr, 20);
        printf("%ld", a);
        printf("*ptr = %d", *ptr);
    }
    while ((a < 0) || a >8 || aux[0]=='0' || *ptr!='\n' || *ptr != '\0');
    for (i = 0; i < height; i++)
    {
        j = (a + 1) - (i + 2);
        k=0;
        while (k < j)
        {
            printf(" ");
            k++;
        }
        while (k < (a + 1))
        {
            printf("#");
            k++;
        }
        printf("\n");
    } 
}

You're making it more complicated than it needs to be. You don't need need the second parameter, so it can just be NULL (no *ptr necessary, and no need for the last 3 checks in the expression to "while").
Note that the third parameter is the base (you probably want 10).

If this is tricky, try the "sscanf()" method suggested by algorism and myself.

Originally Posted by telmo_d

I tried to use strtol but it's not converting right, *ptr is poiting to 10 and not a '\0' or '\n' when i insert a valid number such as 8.

And what ASCII character has the value 10?