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diff --git a/docs/source/lang/variables.rst b/docs/source/lang/variables.rst deleted file mode 100644 index 9094cd5..0000000 --- a/docs/source/lang/variables.rst +++ /dev/null @@ -1,170 +0,0 @@ -.. 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 <lang-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 <lang-numeric-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 <lang-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 <lang-scope>` 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 <lang-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 <lang-numeric-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 <lang-assignment>`, 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-numeric-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 - <http://en.wikipedia.org/wiki/Two%27s_complement>`_ on computers. - - -.. include:: cc-attribution.txt - |