Formatted Printing

In an old assignment description, I suggested students use the command

System.out.printf("%-13s%8d%8d%n", "Assignments:", asgnPct, asgn);

To print out a line in a table. If asgnPct has the value 100, and asgn has the value 25, then this will produce the output

Assignments: 100 25

The String "Assignments:" has been printed at the left end of a field 13 characters wide. The numbers 100 and 25 have been printed at the right ends of fields 8 characters wide.

As the code shows, System.out has a printf method -- "print formatted". This method takes a String (the format String) that says how the rest of the arguments should be printed. In the sample code, the format String is "%-13s%8d%8d%n". The same format String was used for each of the lines in the table. It says (roughly):

(%-13s) Print a String left-justified in a field 13 characters wide.
(%8d) Print an integer right-justified in a field 8 characters wide.
(%8d) Print another integer right-justified in a field 8 characters wide.
(%n) End the line.

As you probably guessed, it's the % sign that tells printf to do something special. The number after the % sign says how wide to make the field (13 characters, 8 characters), and whether to left-justify (negative number) or right-justify (positive number) in the field. The letter s tells printf that the thing to print is going to be a String. The letter d tells it to print an integer. The letter n tells it to print a newline character.

Any other characters in the format String itself are printed "as is". For example,

System.out.printf("Here's my message: >%5s<", "Hi!");

Would print the String

Here's my message: > Hi!<

The format String from the old assignment mentions three values to be printed -- a String and two integers. That means we need to give three more arguments to printf -- the String "Assignments:" and the integers asgnPct and asgn. If only one argument were mentioned (as in the format String "Here's my message: >%-5s<" above), then only one more String argument would be provided. The arguments can be literals (as the Strings above were) or variables (as the integers above were).

printf can also deal with numbers with decimal points. The letter f is used for those, and you can say how many spaces come after the decimal point --

System.out.printf(">%10.2f<", 57.1468902);

prints

> 57.15<

Notice that there are five spaces in front of the number, and that the number itself takes up the other five (of ten) spaces available. Also note that the number is rounded in the printed text. It rounded off to two decimal places because of the .2 in the format String.

Another thing you can do with numbers (%d and %f) is to say to add "leading zeroes". For that you just put a zero in front of the field width. Thus

System.out.printf(">%010.2f<", 57.1468902);

prints

>0000057.15<

Instead of putting five spaces at the front, it put five zeroes.

Yet another thing you can do with numbers is to add commas in the output. For that you put a comma in front of the field width. Thus

System.out.printf(">%,10d<", 5714602);

prints

> 5,714,602<

Note that the total length of the output is still 10.

There's plenty more to learn about formatted printing, but that's enuf to get you started. Google java printf for more information.