Do while loop won't run in a program to only take upper case letter from user

Question

> #include <stdio.h>

int main(void)
{
    char a;
    printf("Enter a capital alphabet: ");
    do{
    scanf("%c", &a); }
    while(a>=65 && a<=90);
}

I am building a program which only takes one uppercase letter as input. But it doesn't prompt to input again, even if I input a lowercase letter, a number, or a symbol.

Where am I going wrong in this?

`while(a>=65 && a<=90);` is "read again if input is ASCII uppercase letters". — MikeCAT, May 15 '20 at 13:13
Your prompt is outside of the loop so it will only ask the once. — FreudianSlip, May 15 '20 at 13:13
@FreudianSlip even without the printf, it doesn't show the prompt for input. It executes and closes. — brownputin, May 15 '20 at 13:16

MikeCAT · Answer 1 · 2020-05-15T13:29:12.753

2

Add a whitespace character before %c to have it skip newline character (from hitting Enter)
Negate the loop condition to read again if the input is out of desired range.
Use character literals for better understanding.
Use isupper() for even better understanding.

Manual comparision version:

#include <stdio.h>

int main(void)
{
    char a;
    printf("Enter a capital alphabet: ");
    do{
    scanf(" %c", &a); }
    while(!(a>='A' && a<='Z'));
}

isupper() version:

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

int main(void)
{
    char a;
    printf("Enter a capital alphabet: ");
    do{
    scanf(" %c", &a); }
    while(!isupper((unsigned char)a)); /* case to unsigned char to avoid trouble when a is negative */
}

edited May 15 '20 at 13:29

answered May 15 '20 at 13:22

MikeCAT

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...and add `\n` to `printf` so it will flush the output. – Paul Ogilvie May 15 '20 at 13:30
@PaulOgilvie But it will add extra newline to the output. Using `fflush(stdout);` should be better for flushing the output without affecting the contents. – MikeCAT May 16 '20 at 12:54

score 2 · Accepted Answer · answered May 15 '20 at 13:23

#include <stdio.h>

int main(void)
{
    char a;  
    do{
        printf("Enter a capital alphabet: "); // printf should be in while loop to print the request when you did not enter a capital alphabet.
        scanf(" %c", &a);  // add space character before %c

    } while(a <='A' || a >= 'Z'); // it's more clear when you use 'A' and 'Z' in stead of 65 and 90.
}

See scanf() leaves the new line char in the buffer

and While-loop ignores scanf the second time

Do while loop won't run in a program to only take upper case letter from user

2 Answers2