Getting string input in C unless empty string is found

Question

I am trying to get the user to enter string in a loop unless the string is found to be empty.

This is my attempt and I tried couple of different ways but I have the same problem. the loop keeps going even when I don't enter any thing.

Code

char string[100];

do{
    fgets(string,100,stdin);
} while (string[0] != '\0');

Unfortunately, when I run this the output is something like this:

Output

> hello
> world
> test
>
>

I also tried using scanf() instead of fgets() but the same issue is still there.

but what if I used scanf? Don't use scanf() if you want to be able to handle bad input with any reliability. If scanf() fails, you really don't know what state your input stream is in. When you do something like fgets() and then maybe sscanf() to parse the string, at least then you know exactly where your input stream is even if the data is bad. If you're reading from something like a pipe (because the user redirected input with something like yourProgram < inputDataFile you can't seek back to a previous location to recover. — Andrew Henle
– Andrew Henle, Commented Sep 29, 2019 at 22:13

user3121023 · Accepted Answer · 2019-09-29 23:18:43Z

Using fgets is pretty simple. As a feature, you can test to see it the input contains a newline. If not, there are pending characters.
scanf is possible using a scanset. %99[^\n] will scan up to 99 characters that are not a newline. %*c will consume the newline. If the only character is a newline then the scanset will fail and scanf will return 0. If more than 99 characters are entered, %*c will consume a character that is not a newline. fgets does not have that problem.

#include <stdio.h>

int main ( void) {
    char string[100] = "";
    int result = 0;

    printf ( "using fgets\nenter a blank line to stop\n");
    do {
        fgets ( string, sizeof string, stdin);
    } while ( string[0] != '\n');

    printf ( "using scanf\nenter a blank line to stop\n");
    do {
        result = scanf ( "%99[^\n]%*c", string);
    } while ( result == 1);

    return 0;
}

With ungetc, if scanf reads too many characters, the last character can be put back in the stream if it is not a newline.

    char last = 0;

    do {
        result = scanf ( "%99[^\n]%c", string, &last);
        if ( EOF == result) {
            fprintf ( stderr, "scanf EOF\n");
            return 0;
        }
        if ( '\n' != last) {
            ungetc ( last, stdin);
        }
    } while ( result == 2);

Lion King · Accepted Answer · 2019-09-30 01:32:34Z

0

You can use strcmp function to check whether the input is empty or not:

char str[50];
while(strcmp(gets(str), "") != 0){
    printf("Output: %s\n", str);
}

strcmp function returns 0 when both strings are found to be identical.

answered Sep 30, 2019 at 1:32

Lion King

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