simple calculator

#include<iostream>
#include<cmath>
using namespace std;

void summation(double result, double x, double y)
{
    result = x+y;
    cout << "result of summation is: " << result << endl;
}

void subtraction(double result, double x, double y)
{
    result = x-y;
    cout << "result subtraction is: " << result << endl;
}

void multiplication(double result, double x, double y)
{
    result = x*y;
    cout << "result multiplication je: " << result << endl;
}

void division(double result, double x, double y)
{
    if (y==0)
    {
        cout << "you cannot divide by zero" << endl;
    }
    else
    {
        result = x/y;
        cout << "result division je: " << result << endl;
    }
}

void power(double result, double x, double y)
{
double p = 1;
if (y==0)
    {
        result = 1;
        cout << "result of powering: " << result << endl;
    }
else if (y<0)
    {
        y=abs(y);
    for (unsigned int i=1;i<=y;i++)
        {
        x=1/x;
        result = x;
        }
        cout << "result of powering: " << result << endl;
    }

else
    {
    for (unsigned int i=1;i<=y;i++)
        {
        p=p*x;
        result = p;
        }
    cout << "result of powering: " << result << endl;
    }
}

void faktorial(double result, double x)
{
    if (x==0)
    {
        x=1;
    }
    else
    {
        for (unsigned int i = x-1; i>0; i--)
        {
            x = x * i;
        }
    }
    cout << "faktorial is: " <<x << endl;
}

int main ()
{
double x,y,result = 0;
    char a;
    cout << "insert number, operator and number:" << endl;
    cin >> x >> a;
    if (a !='f')
        {
            cin>> y;
        }
        switch (a)
        {
        case '+':
            summation(result, x,y);
            break;
        case '-':
            subtraction(result, x, y);
            break;
        case '*':
            multiplication(result, x, y);
            break;
        case '/':
            division(result, x, y);
            break;
        case 'm':
            power(result, x, y);
            break;
        case 'f':
            faktorial(result,x);
            break;
        default:
            cout << "incorrect input" << endl;

        }
    return 0;
}