Why is message[i] returning an uninitialized value in this string?

Question

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

void vowel_check(char message[30], int i);

int main() {
    char message[30];
    
    while (fgets(message, 15, stdin) != NULL) {
        int i = 0;
        while (i < 30) {
            vowel_check(message, i);
            i++;
        }
        printf("%s\n", message);
    }
    
}

void vowel_check(char message[30], int i) {
    
    if (message[i] == 'a' 
    || message[i] == 'e' 
    || message[i] == 'i' 
    || message[i] == 'o' 
    || message[i] == 'u') {
        message[i] = '\0';
        i++;
    }
}

the compile error I'm getting

So it was my first time learning strings, and I want to make a program that gets rid of all the vowels in the program.

ex) white milk -> wht mlk

I think I'm not stuck in something massive and extremely challenging, I think I didn't get the hang of strings just yet. Can anyone point out what part of my code I did wrong? Thanks!

Answer 1

You declared a character array with 30 elements

char message[30];

but in the call of fgets you are using the magic number 15 by an unknown reason

while (fgets(message, 15, stdin) != NULL) {

The user can enter a string that contains even much less than 15 symbols. In this case this while loop

while (i < 30) {
    vowel_check(message, i);
    i++;
}

invokes undefined behavior.

Also changing a vowel to the zero character

message[i] = '\0'

does not remove the element that contained a vowel from the string.

And this statement within the function

i++;

has no effect.

It seems you need to write a function that will remove vowels from a string.

Here is a demonstration program that shows how such a function can be written.

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

char * remove_vowels( char *s )
{
    const char *vowels = "aeiou";

    char *src = s, *dsn = s;

    do
    {
        if ( *src == '\0'  || strchr( vowels, *src ) == NULL )
        {
            if ( dsn != src ) *dsn = *src;
            ++dsn;
        }
    } while ( *src++ );

    return s;
}

int main( void )
{
    char s[] = "white milk";

    printf( "%s -> ", s );
    puts( remove_vowels( s ) );
}

The program output is

white milk -> wht mlk

Your main function can look like

int main( void ) 
{
    char message[30];
    
    while ( fgets( message, sizeof( message ), stdin ) != NULL &&
            message[0] != '\n' ) 
    {
        printf( "%s", remove_vowels( message ) );
    }
}