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arduino:control_structures:if [2013/02/15 20:45] (current)
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 +======if  (conditional)  and   ==,  !=,  <,  > (comparison operators)   ======
 +
 +**//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:
 +
 +<code arduino>if (someVariable > 50)
 +{
 +  // do something here
 +}</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.
 +
 +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.
 +<code arduino>
 +
 +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
 +</code>
 +
 +The statements being evaluated inside the parentheses require the use of one or more operators:
 +
 +===== Comparison Operators:   =====
 + **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)
 +
 +==== Warning:     ====
 +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 [[arduino:arithmetic operators:Assignment|assignment operator]]), 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 [[arduino:control structures:If...else]]] construction.
 +
 +
 +
 +Source: arduino.cc
  
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arduino/control_structures/if.txt · Last modified: 2013/02/15 20:45 (external edit)

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