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author | Perry Hung <iperry@gmail.com> | 2011-01-24 23:23:29 -0500 |
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committer | Perry Hung <iperry@gmail.com> | 2011-01-24 23:23:29 -0500 |
commit | c48689d34809943a5907884bd287cea9ae275352 (patch) | |
tree | d49ff06b0d4b81f6ab0eac8060d178ce7542476c /docs/source/lang/cpp/for.rst | |
parent | 64431fd4b59cb8656365f1fad5f679cd4d756239 (diff) | |
parent | a9b2d70bc7799ca96c1673b18fe3012b1a4dd329 (diff) | |
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diff --git a/docs/source/lang/cpp/for.rst b/docs/source/lang/cpp/for.rst new file mode 100644 index 0000000..71c5aca --- /dev/null +++ b/docs/source/lang/cpp/for.rst @@ -0,0 +1,142 @@ +.. highlight:: cpp + +.. _lang-for: + +``for`` +======= + +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. + +.. contents:: Contents + :local: + +Syntax +------ + +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 |