C++ :: Simple Calculation Going To 0 Every Time

I tried to write a simple program to calculate monthly yield, APR, and principle in various directions. Anyway, here's some code to get the APR from the principle and monthly yield. When I run it though, it spits 0 at me every time! What the problem is; the other functions work just fine and the code line for the APR calculation is just what it ought to be - I see neither a math nor tech problem here.

Here is the offending function:

Code:
void calculateAPR() {
int principle, monthlyYield, apr;
cout<<"
Please input the principle:";
cin>>principle;
cin.ignore();

[code]....