An if statement is used to execute code when certain conditions are met. The general syntax for an if statement is:
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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 curly braces. If the condition is false, the body is not executed. Here’s a more concrete example:
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 comment 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.
An if statement’s condition (which is inside the parentheses after if) often uses one or more boolean or comparison operators.
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:
if (x > 120) digitalWrite(pin, HIGH);
if (x > 120)
digitalWrite(pin, HIGH);
if (x > 120) {
digitalWrite(pin, HIGH);
}
However, the following two examples are different:
// example 1: two lines of code in the if body
if (x > 120) {
digitalWrite(pin1, HIGH);
digitalWrite(pin2, HIGH);
}
// example 2: one line of code in the if body, and
// another line of code after the if statement
if (x > 120)
digitalWrite(pin1, HIGH); // this is in the if body
digitalWrite(pin2, 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.
if/else allows greater control over the flow of code than the basic if statement, by allowing multiple tests to be grouped together. For example, an analog input could be tested, with one action taken if the input was less than 500, and another action taken if the input was 500 or greater. The code would look like this:
if (pinFiveInput < 500) {
// action A
} else {
// action B
}
else can precede another if test, so that multiple, mutually exclusive tests can be run at the same time.
Each test will proceed to the next one until a true test is encountered. When a true test is found, its associated block of code is run, and the program then skips to the line following the entire if/else construction. If no test proves to be true, the default else block is executed, if one is present, and sets the default behavior.
Note that an else if block may be used with or without a terminating else block, and vice-versa. An unlimited number of such else if branches is allowed. Here is a code example:
if (pinFiveInput < 500) {
// do Thing A
} else if (pinFiveInput >= 1000) {
// do Thing B
} else {
// do Thing C
}
Another way to express branching, mutually exclusive tests, is with a switch/case statement.
License and Attribution
Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.