[c++] Find largest and smallest number in an array

big=small=values[0]; //assigns element to be highest or lowest value

Should be AFTER fill loop

//counts to 20 and prompts user for value and stores it
for ( int i = 0; i < 20; i++ )
{
    cout << "Enter value " << i << ": ";
    cin >> values[i];
}
big=small=values[0]; //assigns element to be highest or lowest value

since when you declare array - it's unintialized (store some undefined values) and so, your big and small after assigning would store undefined values too.

And of course, you can use std::min_element, std::max_element, or std::minmax_element from C++11, instead of writing your loops.