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diff --git a/source/arduino/if.rst b/source/arduino/if.rst index d75f6d3..00f1a4d 100644 --- a/source/arduino/if.rst +++ b/source/arduino/if.rst @@ -1,95 +1,78 @@ -.. _arduino-if: +.. highlight:: cpp -if (conditional) and ==, !=, <, > (comparison operators) -======================================================== +.. _arduino-if: -**``if``**, which is used in conjunction with a comparison -operator, tests whether a certain condition has been reached, such -as an input being above a certain number. The format for an if test -is: +if Statements +============= +An ``if`` statement is used to execute code when certain conditions +are met. The general syntax for an ``if`` statement is:: + if (condition) { + body + } -:: +An ``if`` statement first tests whether its *condition* is true (such +as an input being above a certain number). If the condition is true, +the ``if`` statement executes its *body*, which is made up of lines of +code inside :ref:`curly braces <arduino-braces>`. If the condition is +false, the body is not executed. Here's a more concrete example:: - if (someVariable > 50) - { + if (someVariable > 50) { // do something here } +The program tests to see if ``someVariable`` is greater than 50. If it +is, the program executes every line in the curly braces (which in the +above example does nothing, since the body is just the :ref:`comment +<arduino-comments>` line "``// do something here``"). +Put another way, if the statement in parentheses is true, the +statements inside the braces are run. If not, the program skips over +the code. -The program tests to see if someVariable is greater than 50. If it -is, the program takes a particular action. Put another way, if the -statement in parentheses is true, the statements inside the -brackets are run. If not, the program skips over the code. +An ``if`` statement's condition (which is inside the parentheses after +``if``) often uses one or more :ref:`boolean <arduino-boolean>` or +:ref:`comparison <arduino-comparison>` operators. +Writing the if Body +------------------- +The brackets may be omitted after an ``if`` statement's +conditional. If this is done, the next line (which ends in a +semicolon) becomes the only line in the body. The following three +``if`` statements all do the same thing:: -The brackets may be omitted after an *if* statement. If this is -done, the next line (defined by the semicolon) becomes the only -conditional statement. + if (x > 120) digitalWrite(ledPin, HIGH); -:: - - - if (x > 120) digitalWrite(LEDpin, HIGH); - if (x > 120) - digitalWrite(LEDpin, HIGH); - - if (x > 120){ digitalWrite(LEDpin, HIGH); } - - if (x > 120){ - digitalWrite(LEDpin1, HIGH); - digitalWrite(LEDpin2, HIGH); - } // all are correct - - - -The statements being evaluated inside the parentheses require the -use of one or more operators: - - + digitalWrite(ledPin, HIGH); -Comparison Operators: -~~~~~~~~~~~~~~~~~~~~~ + if (x > 120) { + digitalWrite(ledPin, HIGH); + } -:: +However, the following two examples are different:: - x == y (x is equal to y) - x != y (x is not equal to y) - x < y (x is less than y) - x > y (x is greater than y) - x <= y (x is less than or equal to y) - x >= y (x is greater than or equal to y) + // example 1: two lines of code in the if body + if (x > 120) { + digitalWrite(ledPin1, HIGH); + digitalWrite(ledPin2, HIGH); + } + // example 2: one line of code in the if body, and + // another line of code after the if statement + if (x > 120) + digitalWrite(ledPin1, HIGH); // this is in the if body + digitalWrite(ledPin2, HIGH); // this is NOT in the if body +In the first example, since the body is enclosed in curly braces, both +lines are included. In the second example, since the curly braces are +missing, only the first line is in the ``if`` body. -Warning: +See Also -------- -Beware of accidentally using the single equal sign -(e.g.``if (x = 10)`` ). The single equal sign is the assignment -operator, and sets x to 10 (puts the value 10 into the variable x). -Instead use the double equal sign (e.g.``if (x == 10)``), which is -the comparison operator, and tests *whether* x is equal to 10 or -not. The latter statement is only true if x equals 10, but the -former statement will always be true. - - - -This is because C evaluates the statement ``if (x=10)`` as follows: -10 is assigned to x (remember that the single equal sign is the -`assignment operator <http://arduino.cc/en/Reference/Assignment>`_), -so x now contains 10. Then the 'if' conditional evaluates 10, which -always evaluates to TRUE, since any non-zero number evaluates to -TRUE. Consequently, ``if (x = 10)`` will always evaluate to TRUE, -which is not the desired result when using an 'if' statement. -Additionally, the variable x will be set to 10, which is also not a -desired action. - - - -**if** can also be part of a branching control structure using the -`if...else <http://arduino.cc/en/Reference/Else>`_] construction. +- :ref:`boolean operators <arduino-boolean>` +- :ref:`comparison operators <arduino-comparison>` +- :ref:`else <arduino-else>` |