How to get a function to redelcare a variable in Main()? - c++

Question

Im trying to do a paycheck calculator thing that takes out taxes, etc. after a gross income is inputted. I am using functions to calculate the each tax seperately, and then in my Main() I want to then subtract it all from the gross total to get the net pay. My problem is that I cannot figure out how to get the totals from the functions to subtract it in my main().
#include "stdafx.h"
#include <iostream>
#include <iomanip>
using namespace std;
double federalTax(double userInput, double total1) {
double total = total1;
total = userInput * 0.2;
cout << "Federal Tax: " << setw(12) << "-$" << total << endl;
total = total1;
return 0;
}
double stateTax(double userInput, double total2) {
double total = total2;
total = userInput * 0.04;
cout << "State Tax: " << setw(15) << "-$" << total << endl;
total = total2;
return 0;
}
double Medicare(double userInput, double total3) {
double total = total3;
total = userInput * 0.0275;
cout << "Medicare: " << setw(16) << "-$" << total << endl;
total = total3;
return 0;
}
double Pension(double userInput, double total4) {
double total;
total = userInput * 0.06;
cout << "Pension: " << setw(17) << "-$" << total << endl;
total4 = total;
return 0;
}
int main()
{
double userInput;
double total1 = 0, total2 = 0, total3 = 0, total4 = 0;
double sum;
cout << "What is your gross income?" << endl;
cin >> userInput;
cout << "Gross Income: " << setw(10) << "$" << userInput << endl;
federalTax(userInput, total1);
stateTax(userInput, total2);
Medicare(userInput, total3);
Pension(userInput, total4);
cout << "Health Insurance: " << setw(10) << "-$80" << endl;
sum = userInput - total1 - total2 - total3 - total4 - 80;
cout << "Net Pay: " << setw(15) << "$" << sum << endl;
system("pause");
return 0;
}
When I try to subtract it (which you can see with my declaration of double = sum) it is just taking the 0's for totals 1 through 4 that I initialized.

Answer 1

In such case you have to pass values by reference.By default, C++ does not accept values by reference.
#include "stdafx.h"
#include <iostream>
#include <iomanip>
using namespace std;
double federalTax(double userInput, double &total1) {
double total = total1;
total = userInput * 0.2;
cout << "Federal Tax: " << setw(12) << "-$" << total << endl;
total = total1;
return 0;
}
double stateTax(double userInput, double &total2) {
double total = total2;
total = userInput * 0.04;
cout << "State Tax: " << setw(15) << "-$" << total << endl;
total = total2;
return 0;
}
double Medicare(double userInput, double &total3) {
double total = total3;
total = userInput * 0.0275;
cout << "Medicare: " << setw(16) << "-$" << total << endl;
total = total3;
return 0;
}
double Pension(double userInput, double &total4) {
double total;
total = userInput * 0.06;
cout << "Pension: " << setw(17) << "-$" << total << endl;
total4 = total;
return 0;
}
int main()
{
double userInput;
double total1 = 0, total2 = 0, total3 = 0, total4 = 0;
double sum;
cout << "What is your gross income?" << endl;
cin >> userInput;
cout << "Gross Income: " << setw(10) << "$" << userInput << endl;
federalTax(userInput, total1);
stateTax(userInput, total2);
Medicare(userInput, total3);
Pension(userInput, total4);
cout << "Health Insurance: " << setw(10) << "-$80" << endl;
sum = userInput - total1 - total2 - total3 - total4 - 80;
cout << "Net Pay: " << setw(15) << "$" << sum << endl;
system("pause");
return 0;
}

Answer 2

Your issue is that you pass by valuenot by reference. So variables which are passed by value will be copied so that any change to those copied values within the function won't affect the original variable from main, or wherever. So changing total4 = total doesn't touch the total4 in main. Instead, try returning from the functions.
#include "stdafx.h"
#include <iostream>
#include <iomanip>
using namespace std;
double federalTax(double userInput) {
double total = 0;
total = userInput * 0.2;
cout << "Federal Tax: " << setw(12) << "-$" << total << endl;
return total;
}
double stateTax(double userInput) {
double total = 0;
total = userInput * 0.04;
cout << "State Tax: " << setw(15) << "-$" << total << endl;
return total;
}
double Medicare(double userInput) {
double total = 0;
total = userInput * 0.0275;
cout << "Medicare: " << setw(16) << "-$" << total << endl;
return total;
}
double Pension(double userInput) {
double total;
total = userInput * 0.06;
cout << "Pension: " << setw(17) << "-$" << total << endl;
return total;
}
int main()
{
double userInput;
double total1 = 0, total2 = 0, total3 = 0, total4 = 0;
double sum;
cout << "What is your gross income?" << endl;
cin >> userInput;
cout << "Gross Income: " << setw(10) << "$" << userInput << endl;
total1 = federalTax(userInput);
total2 = stateTax(userInput);
total3 = Medicare(userInput);
total4 = Pension(userInput);
cout << "Health Insurance: " << setw(10) << "-$80" << endl;
sum = userInput - total1 - total2 - total3 - total4 - 80;
cout << "Net Pay: " << setw(15) << "$" << sum << endl;
system("pause");
return 0;
}

Answer 3

Passing by value issues in creating new copies of the original parameters. On contrary to passing by reference or by pointer which means passing the very address of the original variables; which means any change affects the original one. Because no copy is created. The law of thumb Pass by reference as much as possible.
You can in your program to change it to look like reasonable:
1- Change the function to return void as long as you are not interested n the returned values.
2- Make the pass by reference.
3- Remove the local variables total_x in all the functions to affect the original ones.
One of the functions will look like this:
void federalTax(double &userInput, double& total1) {
total1 = userInput * 0.2;
cout << "Federal Tax: " << setw(12) << "-$" << total1 << endl;
}
Do the same for the remaining functions.