change lowercase string to uppercase

[confusedduck123]

I am writing a function that changes the case of a lowercase string into uppercase. However, it's not doing so. Any help on what I am doing wrong will be appreciated!

#include <iostream>
#include <string>
#include <stdio.h>
#include <ctype.h>
using namespace std;

string changeCase(const string & s);

 int main() {

	cout << changeCase("blah") << endl;

}

string changeCase(const string & s) { 
	string ret(s);
	for (unsigned i = 0; i < ret.size(); i++)
	{
		if (islower(ret[i])) { toupper(ret[i]); }
	}
	return ret;
}

[JayhawkZombie]

if (islower(ret[i])) {
ret[i] = toupper(ret[i]);
}

[naraku9333]

#include <algorithm>
string changeCase(const string & s) { 
	string ret(s);
	std::transform(ret.begin(), ret.end(), ret.begin(), ::toupper);
	return ret;
}