R: if, if-else, and ifelse functions
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I. Introduction if, if-else, and ifelse functions are extremely powerful, and useful, in programming. In general, they allow a condition or collection of conditions to be checked, and depending on the result, certain code can then be run or not run. |
II. if function
The if function uses the following general format: if(condition) {code}
The if function works by testing a condition. If the condition is true, the code is run. If the condition is false, the code is not run.
Example
> x = 10
> y = 1
>
> # CONDITION IS FALSE
> if(x > 10) {y = 2}
> y
[1] 1
>
> # CONDITION IS TRUE
> if(x == 10) {y = 4}
> y
[1] 4
II. if-else function
The if-else function uses the following general format: if(condition) {code1} else {code2}
The if-else function works by testing a condition. If the condition is true, the code1 is run. If the condition is false, the code2 is run.
Example
> a = 1
> b = 1
>
> # CONDITION IS TRUE
> if(a == 1) {b = 2} else {b = 3}
> b
[1] 2
>
> # CONDITION IS FALSE
> if(a != 1) {b = 2} else {b = 3}
> b
[1] 3
III. ifelse function
The ifelse function uses the following general format: ifelse(condition, true.value, false.value)
The ifelse function works by testing a condition. If the condition is true, true.value is returned. If the condition is false, then false.value is returned.
Example
> x = 1
>
> # CONDITION TRUE
> ifelse(x == 1, 2, 4)
[1] 2
>
> # CONDITION FALSE
> ifelse(x > 1, 2, 4)
[1] 4
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