Lesson 2 - Input, Variables, Computation, And Output

Prerequisites

• Lesson 1. You can build and run a complete program that prints a fixed line.
• A working C++ setup, as described in Lesson 1.

What You Will Learn

• How to declare an int variable with a clear name.
• How to read values from standard input with std::cin.
• How to compute a value and print it with std::cout.

Output That Depends On Input

In Lesson 1 the output was fixed. Now the program reads numbers first, so the output depends on what it is given.

Three new pieces appear:

• A variable is a named place that holds one value. int firstNumber = 0; declares an integer variable named firstNumber and starts it at 0.
• std::cin is the standard input stream. std::cin >> firstNumber; reads one integer from the input and stores it in firstNumber.
• >> moves data from the stream on its left into the variable on its right. It points the opposite way from the << used for output.

Whitespace between input numbers does not change how they are read. Spaces, tabs, and line breaks all act as separators, so 7 5 on one line and 7 then 5 on two lines are read the same way.

Worked Example

Read two integers, store them in named variables, and print their sum.

#include <iostream>

int main() {
    int firstNumber = 0;
    int secondNumber = 0;

    std::cin >> firstNumber >> secondNumber;

    int sum = firstNumber + secondNumber;

    std::cout << sum << "\n";
    return 0;
}

Input:
7 5
Output:
12

Notes on the code:

• std::cin >> firstNumber >> secondNumber; reads two integers in order. The first value read goes into firstNumber and the second into secondNumber.
• int sum = firstNumber + secondNumber; computes the result once and gives it a name. Naming the result keeps the printing step simple.
• std::cout << sum << "\n"; prints the value and then a newline. Printing a variable needs no quotation marks, because sum holds a value rather than text.

Exercise

Write a complete program that reads one integer and prints twice its value.

Start from the worked example. These are all of the source changes required:

1. Delete the line int secondNumber = 0;, because only one input value is needed now.
2. Change std::cin >> firstNumber >> secondNumber; to std::cin >> firstNumber;, so the program reads one integer instead of two.
3. Replace int sum = firstNumber + secondNumber; with int doubled = firstNumber * 2;, which multiplies the input value by two.
4. Change std::cout << sum << "\n"; to std::cout << doubled << "\n";, so the printed name matches the new variable.

Leave #include <iostream>, int main(), and return 0; unchanged.

Exercise Check

Input:
7
Expected output:
14

Check these before moving on:

• The output is 14 and nothing else
• The output still ends with a newline, as in the worked example
• Input 0 prints 0, and input 100 prints 200

Next Steps

You can now read input, name values, compute, and print. The next lesson uses only these tools on a task whose input and output format is fixed in advance.