The crux of the problem is here:
string dayOfWeek(int day);
using namespace std;
The using is after code that depends on it, so the compiler goes looking for string and can't find it. It doesn't go looking for std::string because it hasn't been told to yet.
You could move using up a few lines, but for the many reasons covered in the answers to Why is "using namespace std" considered bad practice? and a few not covered, you're probably better off not putting
using namespace std;
in your code at all. Prefix the std:: explicitly and you can avoid a litany of nasty surprises.
I also recommend following @πάνταῥεῖ example with where the function was placed. Forward declaring and then declaring gives you two places to have to change code and one more place to have a bug. Declaring the function ahead where it will be used mean you only ever have to change one declaration.
Putting it all together with a few other small tweaks:
#include <string>
#include <iostream>
std::string dayOfWeek(int day)
{
switch (day) // for stuff like this, I like a switch. I find it looks nicer.
{
case 0:
return "SUNDAY";
case 1:
return "MONDAY";
case 2:
return "TUESDAY";
case 3:
return "WEDNESDAY";
case 4:
return "THURSDAY";
case 5:
return "FRIDAY";
case 6:
return "SATURDAY";
default:
return "No such day"; // functions must always return something.
// in this case what will cout print if you
// don't return a string? Don't know?
// Don't feel bad. Neither do I.
// Welcome to Undefined Behaviour
}
}
int main()
{
int day; //this was missing
std::cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";
std::cin >> day;
std::cout << "The name of the day of the week is: " << dayOfWeek(day) << std::endl;
}
There's one other trick you can use to get rid of the if or switch entirely
#include <string>
#include <iostream>
// define an array of days of the week to print. const means dayOfWeek cannot be changed
const std::string dayOfWeek[] =
{
"SUNDAY",
"MONDAY",
"TUESDAY",
"WEDNESDAY",
"THURSDAY",
"FRIDAY",
"SATURDAY"
};
int main()
{
unsigned int day;
//Note unsigned int. Negative numbers are now disallowed
std::cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";
// keep looping until user provides a good number
while (!(std::cin >> day) // user MUST input a number
|| day > 6) // number is a usable number
{
std::cin.clear(); // clean up bad input
std::cout << "Please enter a day of week (0 for Sunday, 1 for Monday, etc): ";
}
// since we know day is a usable (0..6) number we can read the day out of the array
std::cout << "The name of the day of the week is: " << dayOfWeek[day] << std::endl;
}
