It's saying on line 13: name lookup of 'i' changed for ISO 'for' scoping
#include <iostream> using namespace std; int main() { int Fib[40]; Fib[0] = 1; Fib[1] = 1; cout << Fib[0] << Fib[1] << endl; for (int i=2; i<40; i++) Fib[i]=Fib[i-1]+Fib[i-2]; cout << Fib[i] << endl; } You forgot to scope the instructions you want to be executed in the loop:
for (int i=2; i<40; i++) { Fib[i]=Fib[i-1]+Fib[i-2]; cout << Fib[i] << endl; } Loops will only execute a single instruction or a single scope of instructions. In your code, only the first line was part of the loop. The second was not, and therefore i was out of scope. Whether you indent the lines or not doesn't matter at all in C++. The compiler doesn't care about indentation; it is used only to aid the programmer, not the compiler.
The last line has [i] it is not inside the loop cout << Fib[i] << endl; the scope of i is only within the for loop as i is declared within the loop according to new ISO standard so the correct line would be
for (int i=2; i<40; i++) { Fib[i]=Fib[i-1]+Fib[i-2]; cout << Fib[i] << endl; } 
cout << Fib[i] << endl;line is outside the for loop. Also fix your indentation.