How to use toupper on char * in C?

Question

So I have a problem with using toupper on char *. Let me show you what I tried.

#include <stdio.h>
#include <ctype.h>
#include <string.h>
int main() {
    char* shi = command->args[0]; //which is "binance"

    while(*shi) {
        toupper((unsigned char) *shi);
        shi++;
    
    }

    printf("Capitalized version is: %s",shi); // which should be "BINANCE".

return 0;
}

Sorry ! I am working on that project for 6 days straight, codes went into each other. Take a look again! — round_protector, Nov 16 '22 at 09:30
@Jabberwocky command line argument, which in this case is "binance". — round_protector, Nov 16 '22 at 09:31
`toupper((unsigned char) *shi);` is wrong anyway, it should be `*shi = toupper((unsigned char) *shi))`; — Jabberwocky, Nov 16 '22 at 09:31
`for (int i = 0; shi[i]; ++i) { shi[i] = toupper(shi[i]); }`? — Retired Ninja, Nov 16 '22 at 09:31
@round_protector please [edit] and provide a [mcve]. Your code does not compile. — Jabberwocky, Nov 16 '22 at 09:31
"does not work"is not sufficient to help you. Please describe in detail what happens. — Marcus Müller, Nov 16 '22 at 09:31
@MarcusMüller it prints "Capitalized version is: ". So it does not see the string. — round_protector, Nov 16 '22 at 09:33
@round_protector it is quite normal that your last version prints `"Capitalized version is: "`. Hint: where does `shi` point after the while loop? — Jabberwocky, Nov 16 '22 at 09:35
At the zero at the end of the string, just like you wrote your loop condition — Marcus Müller, Nov 16 '22 at 09:36
@round_protector yes, more or less. Now you should understand why nothing is printed. — Jabberwocky, Nov 16 '22 at 09:37
@round_protector Either you did something wrong or `command->args[0]` is not terminated properly or isn't writeable. That's the problem with code we can't see. Works fine here: https://godbolt.org/z/vj637djqr — Retired Ninja, Nov 16 '22 at 09:39

Vlad from Moscow · Accepted Answer · 2022-11-16T09:50:35.053

This statement

toupper((unsigned char) *shi);

has no effect. The result of the call of toupper is not used.

Also after this while loop

while(*shi) {
    toupper((unsigned char) *shi);
    shi++;

}

the pointer shi points to the terminating character '\0' of the string.. So the following call of printf

printf("Capitalized version is: %s",shi);

will deal with an empty string.

You should write for example

for ( char *p = shi; *p; ++p ) {
    *p = toupper((unsigned char) *p);
}

How to use toupper on char * in C?

1 Answers1