======================== PrintStream and printf ======================== You have already seen multiple time a basic way to print text and messages in programs:: System.out.println("hello!"); If we deconstruct this, we see three parts: - ``System.out`` is the object that provides a service; - ``println`` is the service provided, to print a message; - ``"hello!"`` is the argument to the service, the message to be printed. Next to ``println``, ``System.out`` provides other services to *print out* values towards the user. ``System.out`` is an object of type ``PrintStream``. The services offered by ``System.out`` are provided by any object of the type ``PrintStream``. Services provided by ``PrintStream`` ==================================== The full documentation of the services offered by ``PrintStream`` is available `in the JDK manual`__; for the time being, you need only remember the following: .. __: https://docs.oracle.com/javase/6/docs/api/java/io/PrintStream.html ``println(...)`` Print a message, then begin a new line of text afterwards. You knew this already. ``print(...)`` Print a message, but do not begin a new line of text afterwards. The next use of ``print`` or ``println`` will append its message to the same line of output. For example:: System.out.print("hello"); System.out.print("world"); will print “helloworld” on a single line. ``printf(...)`` Format zero or more values according to a format specification. See below for details. Formatted output ================ A common task to perform in programs is “alignment”: print data in columns, ensuring that values under each other are properly aligned. Another common task is to format numbers according to some application-defined standard, for example “all grades should be printed with one decimal of accuracy” to say the program should print “7.0” and not simply “7”. Unfortunately, ``print`` and ``println`` do not know anything about alignment, decimal places and the like. To perform these additional tasks, the service ``printf`` is provided. It is defined as follows:: printf(String format, ...) This syntax indicates ``printf`` takes at least one argument, something that evaluates to a string, and then zero or more additional arguments, which can be of any type. The semantics of ``printf`` are as follows: 1. ``printf`` begins processing at the start of the format string; 2. if the current character in the format string is not a “``%``” character, it is printed to the output as-is (unchanged); 3. otherwise, ``printf`` looks after the ``%`` sign until it encounters one of the following: - a “``d``”: the next argument in the remaining argument list is printed out in decimal; - a “``s``”: the next argument in the remaining argument list is printed as a string; - a “``c``”: the next argument in the remaining argument list is printed as a character; - a “``f``”: the next argument is printed as a number with a decimal point; - a “``e``”: the next argument is printed as a decimal number in scientific notation; - a “``%``”: the character “``%``” is printed; 4. if there are more characters in the format string, then ``printf`` moves to the next character and proceeds at step #2 above. Some examples: ===================================== ================================ Program text What is printed ===================================== ================================ ``printf("")`` (nothing) ``printf("hello")`` ``hello`` ``printf("hi %d", 123)`` ``hi 123`` ``printf("hi %d, again", 123)`` ``hi 123, again`` ``printf("%d - %d: %d", 4, 8, 12)`` ``4 - 8: 12`` ``printf("hi %s!", "bob")`` ``hi bob!`` ``printf("%s%s", "your", "world")`` ``yourworld`` ``printf("score = %d%%.", 13)`` ``score = 13%.`` ``printf("pi = %f", 3.14)`` ``pi = 3.140000`` ``printf("10^3 = %e", 1000)`` ``10^3 = 1.000000e+03`` ===================================== ================================ As you can see, each combination of ``%`` followed by a character in the format string becomes a “hole” to be filled in by one of the remaining arguments. This group formed by a ``%`` followed by a special character is called a *format specifier*. The last character in a format specifier (``s``, ``c``, ``d``, ``f``, etc) is called the *format conversion*. There exist other conversions, but in this course you will mostly use ``%d``, ``%s``, ``%c``, ``%f`` and ``%%``. Format specifier ================== The definition above says about specifiers, “``printf`` looks after the ``%`` sign *until* it encounters one of the following”. So there it is possible to express additional information between the “%” sign and the conversion, the special character that decides the type of value to convert. This extra information is used to control *how the formatting is done*. In this course, you will use use the following format specifiers: ========== ================================================== =========================== ================ Conversion Meaning Example Prints ========== ================================================== =========================== ================ ``%f`` Print the data as-is ``printf(":%d:", 12)`` ``:12:`` ``%Nf`` Print at least N columns, align right ``printf(":%5d:", 12)`` ``: 12:`` ``%.Mf`` Print M decimals (for floats only) ``printf(":%.3f:", 3.14)`` ``:3.140:`` ``%N.Mf`` Align right to N columns, print M decimals ``printf(":%5.1f:", 3.14)`` ``: 3.1:`` ``%0Nf`` Print at least N columns, align right, fill with 0 ``printf(":%05d:", 12)`` ``:00012:`` ``%-Nf`` Print at least N columns, align left ``printf("%-5d:", 12)`` ``:12 :`` ========== ================================================== =========================== ================ New line special character ========================== ``printf`` behaves like ``print`` in that it never starts a new line in the output by itself. So if you have the following code:: System.out.printf("hello %d", 123); System.out.printf("world"); the program will print “``hello 123world``” on a single line. It is possible to force new lines using a special character combination: “``\n``” (a backslash, then small letter ``n``). So you can reproduce the behavior of ``println`` for example as follows:: System.out.printf("hello\n"); It is also possible to use “``\n``” in the middle of a format string, for example:: System.out.println("hello,\nworld!\n"); will print “hello,” on a first line, then “world!” on a second line. .. tip:: “``\n``” is one of multiple special combinations that can be found in strings. This will be handled in a later lecture on `strings and character literals`__. .. __: stringchars.html Important concepts ================== - the difference between ``print``, ``println`` and ``printf``; - how ``printf`` interprets its *format string*; - the difference between *format conversion* (1 character ``s``, ``f``, etc.) and *format specifier* (the entire group from ``%`` to conversion); - the “basic” *format conversions*: ``s``, ``f``, ``d``, ``c``, ``%``; - how to extend format specifiers for alignment and changing the number of decimals; - using the special combination “``\n``”. Further reading =============== - Introduction to Programming, section 2.4.1 (pp. 38-39) - Absolute Java, section 2.1 (pp. 58-67), also exercises 1-7 ---- Copyright and licensing ======================= Copyright © 2014, `Raphael Poss `__. Permission is granted to distribute, reuse and modify this document according to the terms of the Creative Commons Attribution-ShareAlike 4.0 International License. To view a copy of this license, visit `http://creativecommons.org/licenses/by-sa/4.0/ `_.