From 5e587be27a7c3bd854b686952a5c9637a2432ff0 Mon Sep 17 00:00:00 2001 From: Marti Bolivar Date: Sun, 28 Nov 2010 11:23:33 -0500 Subject: reorganized all the arduino/ docs into a lang/ subdirectory since they're properly CC attributed now. --- docs/source/lang/variables.rst | 170 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 170 insertions(+) create mode 100644 docs/source/lang/variables.rst (limited to 'docs/source/lang/variables.rst') diff --git a/docs/source/lang/variables.rst b/docs/source/lang/variables.rst new file mode 100644 index 0000000..05a4f96 --- /dev/null +++ b/docs/source/lang/variables.rst @@ -0,0 +1,170 @@ +.. highlight:: cpp + +.. _lang-variables: + +Variables +========= + +A variable is a way of naming and storing a value for later use by +the program, such as data from a sensor or an intermediate value +used in a calculation. + +.. contents:: Contents + :local: + +.. _lang-variables-declaring: + +Declaring Variables +------------------- + +Before they are used, all variables have to be *declared*. Declaring a +variable means defining its type, giving it a name, and (optionally) +giving it an initial value (this is often referred to as +*initializing* the variable). Variables do not have to be initialized +(given a value) when they are declared, but it is good style to give +them an initial value whenever possible. + +Here is an example of declaring a variable named ``inputVariable1`` +with type :ref:`int ` (the ``int`` type is used to store +integers, like -2, -1, 0, 1, etc.):: + + int inputVariable1; + +In the above declaration, we did not give the variable an initial +value. Here is another example, where we declare an ``int`` variable +named ``inputVariable2``, with an initial value of ``0``:: + + int inputVariable2 = 0; + +The Maple environment comes ready to use with many useful types of +variables. See the :ref:`built-in types ` page for more +information. + +Here are a few examples of declaring variables of different types:: + + int lightSensVal; + char currentLetter; + unsigned long speedOfLight = 186000UL; + char errorMessage = {"choose another option"}; // see string + +Naming Variables +---------------- + +The rules for naming a variable are simple. Names for variables can +contain letters, numbers, and underscores (the underscore is the +:kbd:`_` character), and cannot begin with a number. So +``temperature_reading``, ``tempReading``, ``tempReading1``, and +``tempReading2`` are all valid variable names, but ``4_temp_readings`` +is not, because it begins with a number. + +You cannot choose a name for a variable that is one of the C++ +:ref:`keywords `. + +Variable names are case-sensitive, so "tempreading" and "tempReading" +are different variables. However, it is very bad style to write code +that chooses variables which are the same up to case. + +You should give your variables descriptive names, so as to make your +code more readable. Variable names like ``tiltSensor`` or +``pushButton`` help you (and anyone else reading your code) understand +what the variable represents. Variable names like ``var`` or +``value``, on the other hand, do little to make your code readable. + +.. _lang-variables-scope: + +Variable Scope +-------------- + +An important choice that programmers face is where (in the program +text) to declare variables. The specific place that variables are +declared influences how various functions in a program will "see" the +variable. This is called variable *scope*. See the :ref:`scope +reference ` for more information. + +.. _lang-variables-initializing: + +Initializing Variables +---------------------- + +Variables may be *initialized* (assigned a starting value) when they +are declared or not. It is always good programming practice however to +double check that a variable has valid data in it before it is used. +Using a variable before you give it a value is a common source of +bugs. + +.. _lang-variables-rollover: + +Variable Rollover +----------------- + +Every (numeric) type has a valid *range*. The range of a type is the +smallest and largest value that a variable of that type can store. +For example, the :ref:`int ` type has a range of +-2,147,483,648 to 2,147,483,647 [#frange]_. + +When variables are made to exceed their range's maximum value, they +"roll over" back to their minimum value. Note that this happens in +both directions. It's like in the game *Pac-Man* -- when Pac-Man goes +past the right edge of the screen, he reappears on the left, and when +he goes past the left side of the screen, he reappears on the right:: + + int x; + x = -2,147,483,648; + x = x - 1; // x now contains -2,147,483,647; rolled over "left to right" + + x = 2,147,483,647; + x = x + 1; // x now contains -2,147,483,648; rolled over "right to left" + +Each numeric type's reference page includes its range. See the +:ref:`built-in types ` reference for links to each type's +reference page. + +Using Variables +--------------- + +Once variables have been declared, they are given values using the +:ref:`assignment operator `, which is a single equals +sign, ``=``. The assignment operator tells the program to store the +value on the right side of the equals sign into the variable on the +left side:: + + inputVariable1 = 7; // sets variable named inputVariable1 to 7 + inputVariable2 = analogRead(2); // sets variable named inputVariable2 to + // the (digitized) input voltage read from + // analog pin #2 + +Once a variable has been set (assigned a value), you can test its +value to see if it meets certain conditions, or you can use its value +directly. For instance, the following code tests whether the +inputVariable2 is less than 100, then sets a delay based on +inputVariable2 (which, at that point, is at least 100):: + + if (inputVariable2 < 100) { + inputVariable2 = 100; + } + + delay(inputVariable2); + +See Also +-------- + +- :ref:`lang-scope` +- :ref:`lang-types` + +.. rubric:: Footnotes + +.. [#frange] This range might seem a little weird at first. The + reasons for this range of values have to do with the fact that an + ``int`` occupies 32 bits of memory, and the facts :: + + 2^31 = -2,147,483,648 + 2^31 - 1 = 2,147,483,647 + + + Why 2^31 instead of 2^32? Well, that has to do with `how ints are + (usually) stored + `_ on computers. + + +.. include:: cc-attribution.txt + -- cgit v1.2.3