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authorMarti Bolivar <mbolivar@mit.edu>2010-11-28 11:23:33 -0500
committerMarti Bolivar <mbolivar@mit.edu>2010-11-28 11:23:33 -0500
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reorganized all the arduino/ docs into a lang/ subdirectory since
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+.. highlight:: cpp
+
+.. _lang-for:
+
+for Loops
+=========
+
+.. contents:: Contents
+ :local:
+
+Description
+-----------
+
+A ``for`` loop is used to repeat a block of statements enclosed in
+curly braces. ``for`` loops are useful for performing repetitive
+operations, and are often used in combination with :ref:`arrays
+<lang-array>` to operate on collections of data or multiple
+:ref:`pins <gpio>`. A ``for`` loop is composed of two parts: first, a
+*header*, which sets up the for loop, and then a *body*, which is made
+up of lines of code enclosed in curly braces.
+
+There are three parts to the ``for`` loop header: an *initialization*
+expression, *loop condition* expression, and a *post-loop*
+expression. The general syntax looks like this::
+
+ for (initialization; condition; post-loop) {
+ // all of these lines inside the curly braces are part
+ // of the loop body.
+ statement 1;
+ statement 2;
+ ...
+ }
+
+(Note that there is no semicolon after the post-loop). The
+initialization happens first and exactly once, before the loop begins.
+Each time through the loop, the condition is tested. The condition is
+a :ref:`boolean <lang-boolean>` expression. If it is true, then the
+list of statements inside the curly braces are executed. Next, the
+post-loop is executed. The loop then begins again by evaluating the
+condition again, entering the loop body if it is true. This proceeds
+until the condition becomes false.
+
+Examples
+--------
+
+Here's an example::
+
+ // Dim an LED using a PWM pin
+ int pwmPin = 9; // LED in series with 470 ohm resistor on pin 9
+
+ void setup() {
+ pinMode(pwmPin, PWM);
+ }
+
+ void loop() {
+ for (int i=0; i <= 65535; i++) {
+ pwmWrite(pwmPin, i);
+ delay(1);
+ }
+ }
+
+There is a ``for`` loop In the :ref:`loop() <lang-loop>` function of
+the above example. This loop starts by declaring an ``int`` variable
+named ``i``, whose value starts out at zero. The loop proceeds by
+checking if ``i`` is less than or equal to 65535. Since ``i`` is
+zero, this is true, and so the calls to :ref:`pwmWrite()
+<lang-pwmwrite>` and :ref:`delay() <lang-delay>` happen next. At this
+point, the post-loop expression ``i++`` is evaluated, which
+:ref:`increments <lang-increment>` ``i``, so that ``i`` becomes one.
+That concludes the first time through the loop. Each "time through
+the loop" is referred to as an *iteration*.
+
+The loop then jumps back to the beginning, checking the condition as
+the beginning of its second iteration (initialization is skipped,
+since this only happens once, before the first iteration). One is
+less than 65535, so the loop statements are executed again. This
+proceeds over and over until the iteration when ``i`` finally
+reaches 65536. At that point, the condition is no longer true, so the
+loop stops executing, and the ``loop()`` function returns.
+
+Here's another example, using a ``for`` loop to brighten and fade an
+LED (see the :ref:`pwmWrite() <lang-pwmwrite>` reference for more
+information)::
+
+ int pwmPin = 9; // hook up the LED to pin 9
+ void loop() {
+ int x = 1;
+ for (int i = 0; i >= 0; i += x) {
+ analogWrite(pwmPin, i); // controls the brightness of the LED
+ if (i == 65535) {
+ x = -1; // switch direction, so i starts decreasing
+ }
+ delay(1);
+ }
+ }
+
+Coding Tips
+-----------
+
+The C ``for`` loop is more flexible than ``for`` loops found in some
+other computer languages, including BASIC. Any or all of the three
+header elements may be left blank, although the semicolons are
+required. Also the statements for initialization, condition, and
+post-loop can be any valid C statements, and use any C datatypes,
+including :ref:`floating point numbers <lang-double>`. These types
+of unusual ``for`` loops sometimes provide solutions to less-common
+programming problems.
+
+For example, using a multiplication in the post-loop line will
+generate a `geometric progression
+<http://en.wikipedia.org/wiki/Geometric_progression>`_::
+
+ for(int x = 1; x <= 100; x = x * 2) {
+ SerialUSB.println(x);
+ }
+
+
+This loop prints out the numbers 1, 2, 4, 8, ..., 64. Check
+your understanding of ``for`` loops by answering the following two
+questions (answers are in footnote [#fanswers]_\ ):
+
+1. How many iterations occur before the loop finishes?
+
+2. Why does it stop at 64?
+
+See also
+--------
+
+- :ref:`while <lang-while>` loops
+- :ref:`do <lang-dowhile>` loops
+
+.. rubric:: Footnotes
+
+.. [#fanswers]
+ 1. Seven.
+
+ 2. After the seventh iteration, the post-loop causes ``x`` to
+ equal 128. This is larger than 100, so the loop condition is
+ false, and the loop stops.
+
+
+.. include:: cc-attribution.txt