Convert case based on first character

Logic. convert() first checks the first character: islower(s[0]). If it is lower case, every character is changed with toupper(), otherwise with tolower(). The word is read in main(), converted in the function (the array is passed by address) and displayed in main().

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

void convert(char s[])
{
    int toUpper = islower((unsigned char)s[0]);   /* decided by first character */
    for (int i = 0; s[i] != '\0'; i++)
        s[i] = toUpper ? toupper((unsigned char)s[i]) : tolower((unsigned char)s[i]);
}

int main(void)
{
    char w[50];
    printf("Enter a word: ");
    scanf("%49s", w);
    convert(w);
    printf("Converted: %s\n", w);
    return 0;
}

Sample run

Input: hELlo

Enter a word: Converted: HELLO

Because the first letter h is lower case, the whole word becomes upper case. For an input such as Hello the result would be hello.