Input and Output

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

printf()

printf() displays text or values. It only starts a new line when the format string contains \n.

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

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

printf("\"Hello\nWorld\"\n");

Output:

"Hello
World"

Formatted output

You can also insert values into a string with placeholders.

• %d: integer
• %f: floating-point number
• %s: string

int length = 10;
int width = 5;
double area = length * width;
printf("Area = %d * %d = %.2f\n", length, width, area);

Output:

Area = 10 * 5 = 50.00

The placeholders correspond to the following arguments in order; %.2f formats a floating-point value with two decimal places.

printf("Area = %d * %d = %.2f\n", length, width, area);

Controlling the line ending

printf() does not add a newline automatically, so calls can continue on the same line. Put \n in the format string when the current line should end.

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

printf("%d, ", num1);
printf("%d, ", num2);
printf("%d, ", num3);
printf("%d...\n", num4);

Output:

1, 2, 4, 8...

scanf input

scanf reads standard input according to a format string: %d reads an int and %lf reads a double.

scanf("%127s", text) stores one whitespace-delimited token in a character array while limiting its width.

printf("Radius: ");
double r;
scanf("%lf", &r);
double area = 3.141592653589793 * r * r;
printf("Area = %.2f\n", area);

One possible run:

Radius: 5
Area = 78.54

Here, scanf("%lf", &r) reads a double directly, while 3.141592653589793 is the approximation of pi written explicitly in this example.