Make own strchr() function but case-insensitive

Question

My exercise requires that I can use case-insensitive input. My approch is that I use the tolower and toupper function.

How can I convert the array to lowercase letters?

void KULstrcichr(char *arr, char search)
{
    printf("Return value when uppercase character %c is passed to isupper(): %d\n", search, isupper(search));
    // The  strchr() function returns a pointer to the first occurrence of the character c in the string s.
    if (isupper(search))
    {
        printf("Groß\n");
        char lowercasesearch = tolower(search);
        printf("Das ist der Output: %s", arr);
        char *ptr = strchr(arr, lowercasesearch);
        printf("Das ist der Buchstabe: %s", ptr);
    }
    else
    {
        printf("Klein\n");
        char upercasesearch = toupper(search);
        printf("Das ist der Output: %s", arr);
        char *ptr = strchr(arr, upercasesearch);
        printf("Das ist der Buchstabe: %s", ptr);
    }
}

Answer 1

According to the title of the question

Make own strchr() function but case-insensitive

your code does not make any sense. You should write your own function similar to strchr that is declared in the C Standard like

char * strchr(const char *s, int c);

The function should be declared and defined the following way

char * my_strchr( const char *s, int c )
{
    c = tolower( ( unsigned char )c );

    while ( *s && tolower( ( unsigned char ) *s ) != c ) ++s;

    return c == '\0' || *s != '\0' ? ( char * )s : NULL;
}

Here is a demonstration program.

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

char *my_strchr( const char *s, int c )
{
    c = tolower( ( unsigned char )c );

    while (*s && tolower( ( unsigned char )*s ) != c) ++s;

    return c == '\0' || *s != '\0' ? ( char * )s : NULL;
}

int main( void )
{
    char s[] = "Hello";
    char *p = my_strchr( s, 'h' );

    if (p != NULL)
    {
        printf( "position = %td, substring = %s\n", p - s, s );
    }
}

The program output is

position = 0, substring = Hello