Multiplying a variable by itself(squaring) without using pow function

[megsweatshirt]

So my instructor for my c++ class wants us to make multiple functions that do various operations on an initial value given by the user. He told us to essentially create user-defined functions, even for operations that have predefined functions such as pow, abs, etc.
Unfortunately, I have never run into a problem where I have to multiply a variable by itself without using the pow function, so I am not sure how exactly to go about it.

Edit: the error message comes about when I attempt to multiple initial by initial and assign it to squared.

 #include "stdafx.h"
#include<iostream>
#include<iomanip>
#include<cmath>
#include<fstream>
using namespace std;
double amountOfValues, initial, interval, squared = 0.0;

double valueSquared(double initial, double squared) {
	double squared = double initial * double initial;
	//initial = squared;
	return squared;
}

[integralfx]

double squared = double initial * double initial;
You're declaring doubles which are uninitialised, so when you do arithmetic on them, you will most likely end up with junk values, usually large negative/positive values.

double valueSquared(double initial) {
	return initial * initial;
}

I'm not sure what you intended the parameter squared to do. Maybe it's the exponent? Maybe you meant to assign initial² to it? (you'll need to pass it by reference if you intend to do that)