C++ Calculator Skipping Else Statement - c++

Question

I was making a simple calculator in C++. However the program does not completely function the way it should. When run, the trig if statement executes fine, however, the basic arithmetic else statement doesn't work. I have determined that the code is not executing the else statement and was wondering how to fix it. The code inside the else statement works fine, as I have commented out the if statement. Help?
Here is my code:
#include "stdafx.h"
#include <iostream>
#include <string>
#include <cmath>
int main()
{
double input = 0;
double firstnumber = 0;
double secondnumber = 0;
std::string function;
std::string operation;
std::cout << "Enter your calculation: ";
std::cin >> function;
if(function == "sin" || "cos" || "tan")
{
if(function == "sin")
{
std::cin >> input;
std::cout << "The sine is " << sin(input) << std::endl;
system("PAUSE");
}
else if(function == "cos")
{
std::cin >> input;
std::cout << "The cosine is " << cos(input) << std::endl;
system("PAUSE");
}
else if(function == "tan")
{
std::cin >> input;
std::cout << "The tangent is " << tan(input) << std::endl;
system("PAUSE");
}
}
else
{
firstnumber = ::atof(function.c_str());
std::cin >> operation;
std::cin >> secondnumber;
double valueadd = firstnumber + secondnumber;
double valuesubtract = firstnumber - secondnumber;
double valuemultiply = firstnumber * secondnumber;
double valuedivide = firstnumber / secondnumber;
if(operation == "+")
{
std::cout << " = " << valueadd << std::endl;
system("PAUSE");
}
else if(operation == "-")
{
std::cout << " = " << valuesubtract << std::endl;
system("PAUSE");
}
else if(function == "*")
{
std::cout << " = " << valuemultiply << std::endl;
system("PAUSE");
}
else if(function == "/")
{
std::cout << " = " << valuedivide << std::endl;
system("PAUSE");
}
else
{
std::cout << "Error" << std::endl;
return 0;
}
}
return 0;
}

Answer 1

This line is wrong.
if(function == "sin" || "cos" || "tan")
It should be
if((function == "sin") || (function == "cos") || (function == "tan"))
Note that the check is actually meaningless because you already check for them each individually. You could tidy this up by doing this in a if, else if, else chain.

Answer 2

You must write out each condition separately. The following line of code compiles but it doesn't do what you think:
if (function == "sin" || "cos" || "tan")
Change it to the following:
if (function == "sin" || function == "cos" || function == "tan")

Answer 3

Since you want to do something different for each trig function, you should just have a single if...else if...else if...else if...else chain. There is no need to nest the if statements as you have. In fact, it is probably less efficient because you check each condition twice.

Answer 4

Change:
if(function == "sin" || "cos" || "tan")
into:
if ((function == "sin") || (function == "cos") || (function == "tan"))
What you have first calculates the expression "sin" || "cos" || "tan" and then tries to compare the string with that.
But, in fact, it's not really necessary to have this two-step process. You can simply do something like this:
if (function == "sin") {
std::cin >> input;
std::cout << "The sine is " << sin (input) << std::endl;
system ("PAUSE");
} else if (function == "cos") {
std::cin >> input;
std::cout << "The cosine is " << cos (input) << std::endl;
system ("PAUSE");
} else if (function == "tan") {
std::cin >> input;
std::cout << "The tangent is " << tan (input) << std::endl;
system ("PAUSE");
} else {
// It's neither sin, cos nor tan if you get here.
firstnumber = ::atof (function.c_str ());
// and the rest of your stuff in here.
}