printf in Java - What is %3d for?

Question

I’m seeing a java arrays exercises, but I can’t understand what the %3d, if it’s a int.

for (i=0; i<n; i++) {
    System.out.printf("a[%d] = %3d   b[%d] = %3d\n", i, a[i], i, b[i]);
}

Answer 1

The %d is used to format whole variables, basically can be used as follows:

• %d - formats an entire variable with how many digits there are;

• %4d - formats an entire variable with how many digits there are, but if the variable has a quantity less than 4 digits, fills the missing ones with white space on the left;

• %04d - formats an entire variable with how many digits there are, but if the variable has a quantity less than 4 digits, fills the missing ones with zeros on the left.

Examples:

• %d applied to 1234 would stay 1234;

• %4d applied to 1234 would be 1234, but applied to 123 would be _123 (where _ is a blank);

• %04d applied to 123 stay 0123.

Take an example: http://ideone.com/0JxzzH

Answer 2

The explanation of %3d can be divided as below:

• % prints a variable here,
• 3 use at least 3 characters to display, filling in spaces if necessary,
• d the variable is of the int.

See a demo on Ideone.

Source: What does "%3d" Mean in a printf statement?