41. The if Statement

Few programs consist of a set of statements read one after another from top-to-bottom. At various points we need the code to evaluate one set of statements rather than another - depending on the current context in which the program is running. The if statement is key to directing which route to take.

As an example, let’s say we have some code that displays the result of a division. It’s very important that we don’t try to divide a number by 0 as this causes an error. In the code below we use the if statement to check if the denominator is 0 before we perform the division:

def numerator = 0
def denominator = 0

// ... various statements

if (denominator != 0) {
    println numerator / denominator
}

if evaluates the conditional expression between the parentheses - (..) - and will only process the statement block if the conditional result is true.

if - else

An else section can be provided where you want to process statements when the conditional result is false.

def numerator = 0
def denominator = 0

// ... various statements

if (denominator != 0) {
    println numerator / denominator
} else {
    println 'I refuse to divide by zero'
}

if - else if - else

if and else let you deal with situations where you have two possible outcomes but sometimes you might have a few conditions that you want to check for:

def game = 'tennis'

if (game == 'chess') {
    println 'I like to play chess'
} else {
    if (game == 'tennis') {
       println 'I can play tennis if you want'
    } else {
        println "Sorry, I don't know how to play $game"
    }
}

The code above places another if check within the else block and checks if the player is asking for a different game (“tennis”). This looks (sort of) clean but start to picture a larger set of checks and the code gets confusing. Instead of nesting the second if within the else block, Groovy lets you use else if:

def game = 'tennis'

if (game == 'chess') {
    println 'I like to play chess'
} else if (game == 'tennis') {
    println 'I can play tennis if you want - just let me warm up'
} else if (game == 'golf') {
    println 'I can play golf if you want but I get very angry'
} else {
    println "Sorry, I don't know how to play $game"
}

The code above tidies up the nesting by allowing the second if to be part of the else block. Essentially you can provide a long series of checks in a single if-else if set and Groovy will evaluate each until one resolves to true. You can optionally provide an else block at the end to ensure that can all other outcomes (defined or otherwise) be dealt with.

It’s important to note that once an if evaluates to true, no other if expressions are evaluated. It’s a case of the first positive match wins. In the code below, the conditional (game=='chess' || game == 'tennis') is redundant because an earlier condition would have returned true ((game == 'chess'))

if (game == 'chess') {
    println 'I like to play chess'
} else if (game == 'tennis') {
    println 'I can play tennis if you want'
} else if (game=='chess' || game == 'tennis') {
    println 'Can you really play both of these at once?'
} else {
    println "Sorry, I don't know how to play $game"
}

That last point is also important as Groovy will also not evaluate any expressions used in further if expressions. In the example below, --i is never evaluated as the first if expression resolves to true:

def i = 10

if (++i > 10) {
	//do something
} else if (--i < 10) {
	//do something
}

assert i == 11