making a function of turning lowercase string into uppercase string in c - c

Question

I'm still a beginner at making function and C programming.
I'm trying to make a function for turning it into uppercase, but it seems I messed it up at the pointer(?)
#include <stdio.h>
void mytoupper(char *s[]) {
int i = 0;
while (s[i] != '\0') {
if (s[i] >= 'a' && s[i] <= 'z') {
s[i] = s[i] - 32;
}
i++;
}
return s;
}
int main(void) {
char s[32];
printf("Insert string:");
printf("%s", s);
printf("%s", mytoupper(s[32]));
return 0;
}

Answer 1

There are multiple problems in your code:
the definition for mytoupper is incorrect: it should take a char *s argument instead of char *s[], and return char *.
Changing the character from lower to upper case should not use a hard coded value of 32 that only works for ASCII, use a more generic approach with s[i] = s[i] - 'a' + 'A';
To read the string, use scanf("%31s", s); instead of printf("%s", s); and it is highly recommended to test the return value of scanf()
The argument in printf("%s", mytoupper(s[32])); is incorrect: you should just write printf("%s", mytoupper(s));
Here is a corrected version:
#include <stdio.h>
char *mytoupper(char *s) {
int i = 0;
while (s[i] != '\0') {
if (s[i] >= 'a' && s[i] <= 'z') {
s[i] = s[i] - 'a' + 'A';
}
i++;
}
return s;
}
int main(void) {
char s[32];
printf("Insert string:");
if (scanf("%31s", s) == 1) {
printf("%s\n", mytoupper(s));
}
return 0;
}

Answer 2

There are a few spots that will not work as expected in your code.
You are not properly getting user input, instead you are trying to print an 'empty' string.
printf("%s",s);
You could change this to:
scanf("%31s",s);
You are accepting an array of char pointers in your function mytoupper, but this is not needed. You can instead simply pass it an array of chars.
void mytoupper(char s[])
You are attempting to use a return value of a void function. You can either call the function then print the string, or have the function return the string.
If you want to change the string then print it seperately you will need to change mytoupper to no longer return anything and keep it as a void return type.
mytoupper(s);
printf("%s", s);
Or change the function to:
char* mytoupper(char s[]) {
Then print the string:
printf("%s", mytoupper(s));