1.3 Input and Output

Programs need to communicate with users. The most basic interactions are printing output and reading keyboard input.

std::cout

std::cout displays text or variable values on the screen. The << operator sends values into the output stream.

Some characters have special meaning in code, so they need escape sequences.

• \\: backslash
• \': single quote
• \": double quote
• \n: newline
• \t: tab

std::cout << "\"Hello\nWorld\"" << std::endl;

Output:

"Hello
World"

Formatting output

C++ can print several values by chaining <<.

int length = 10;
int width = 5;
double area = length * width;
std::cout << "Area = " << length << " * " << width << " = " << area << std::endl;

Output:

Area = 10 * 5 = 50

To control decimal places, include <iomanip> and use std::fixed with std::setprecision.

#include <iomanip>

std::cout << std::fixed << std::setprecision(2);
std::cout << "Area = " << area << std::endl;

Output:

Area = 50.00

Controlling the line ending

std::cout does not add a newline automatically. You can use std::endl or print \n yourself.

int num1 = 1, num2 = 2, num3 = 4, num4 = 8;

std::cout << num1 << ", ";
std::cout << num2 << ", ";
std::cout << num3 << ", ";
std::cout << num4 << "..." << std::endl;

Output:

1, 2, 4, 8...

std::cin

std::cin reads text from the keyboard and stores it in variables. The >> operator extracts values from the input stream.

Important: the variable type decides how the input is parsed. If r is a double, then std::cin >> r; reads a floating-point value.

#include <iostream>
#include <iomanip>

int main() {
    double r;

    std::cout << "Radius: ";
    std::cin >> r;

    double area = 3.14159 * r * r;
    std::cout << std::fixed << std::setprecision(2);
    std::cout << "Area = " << area << std::endl;
    return 0;
}

One possible run:

Radius: 5
Area = 78.54

Here, std::cin >> r; reads one value and stores it directly into r.