Modern C++: The Good Parts/Number crunching

The machine in front of you is really just a very fancy calculator, so let's make it calculate.

Code

#include <iostream>
#include <string>

int main()
{
	std::string input;
	int a, b;
	
	std::cout << "Enter two integers to multiply.\n";
	
	// Take input.
	std::cin >> input;
	// Parse it as an int.
	a = std::stoi(input);
	
	// Take input.
	std::cin >> input;
	// Parse it as an int.
	b = std::stoi(input);
	
	std::cout << a << " * " << b << " = " << a * b << "\n";
}

Explanation

So first we have a std::string variable named "input" and two int variables named "a" and "b". An int is an integer or whole number, so it can't have a decimal point.

Important: The = (equals sign) does not mean equality. In C++ it's the assignment operator; it puts the value on the right into the variable on the left, overwriting any previous value of that variable.

std::stoi is a function, and the name stands for string to int. It takes a string (in this case, the value of input) and converts it to an integer. If this isn't possible, or you enter a very large number, the program will crash and you'll probably see something about an "unhandled exception". How to prevent this issue will be addressed a few chapters later.

You can read about std::stoi at cplusplus.com or cppreference.com. It has some friends, too.

The multiplication operator in C++ is * (asterisk). Division is / (forward slash).

Try entering 3.14 as one of the integers. Oops! std::stoi finds the valid integer 3 and ignores the invalid input after it. You'll get the same result for 3sdjgh. The next few chapters should allow you to write a better integer parser.

Exercises

• Modify the above program so it accepts decimal points (floating-point numbers or float values). Hint: Check the links on this page.

Vocabulary

int

an integer or "whole number". Cannot represent fractions.

assignment operator

puts the value on the right into the variable on the left, overwriting any previous value of that variable. Syntax: =

function

a named piece of code. More on these later.

float

a floating-point number or "decimal number". (That last one is a little confusing.) Can represent some fractions.