Input and Output

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

cout

cout displays text or values. Add << endl at the end of the output expression when a new line is needed.

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

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

cout << "\"Hello" << endl;
cout << "World\"" << endl;

Output:

"Hello
World"

Formatted output

Use fixed together with setprecision() to control the number of decimal places, then chain text and values with <<.

• fixed: use fixed-point notation
• setprecision(2): keep two digits after the decimal point
• <<: send each following value to the output stream

cout << fixed << setprecision(2)
     << "Area = " << length << " * " << width << " = " << area << endl;

Output:

Area = 10 * 5 = 50.00

fixed << setprecision(2) selects fixed-point formatting with two decimal places.

#include <iomanip>
#include <iostream>

using namespace std;

int main() {
    int length = 10;
    int width = 5;
    double area = length * width;
    cout << fixed << setprecision(2) << "Area = " << length << " * " << width << " = " << area << endl;
    return 0;
}

Controlling the line ending

cout does not add a new line automatically, so outputs can be chained; add << endl when the current line should end.

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

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

Output:

1, 2, 4, 8...

cin input

cin reads standard input with the extraction operator >>; the destination variable's type controls parsing.

cin >> word can store one token in a string, while int and double variables receive numeric input.

#include <cmath>
#include <iomanip>
#include <iostream>

using namespace std;

int main() {
    cout << "Radius: " << endl;
    double r;
    cin >> r;
    double area = 3.141592653589793 * pow(r, 2);
    cout << fixed << setprecision(2) << "Area = " << area << endl;
    return 0;
}

One possible run:

Radius: 5
Area = 78.54

Here, cin reads a double directly, while 3.141592653589793 is the numeric value of pi used in this example.