45. Branching statements

Groovy has three branching statements:

• break
• continue
• return

break

We first came across the use of break in the switch statement - it’s used to indicate the end of the set of statements for the case and signals that the switch statement has completed.

The break reserved word is also used to exit out of a loop - even if there are more iterations possible. In the code below I iterate through the list until I reach the value 'Earth' and then break out of the loop:

def planets = [
    'Mercury',
    'Venus',
    'Earth',
    'Mars'
]

for (planet in planets) {
    println planet
    if (planet == 'Earth') {
        break
    }
}

When we looked at the while loop I gave an example of setting a flag variable and checking that as the loop’s condition. This could have been refined using break and not using flag at all:

def num = 0

while (true) {
    num++
    if (num**2 == 64) {
        break
    }
}

println "The square root of 64 is $num"

I’ll refine that while loop just a little further:

def num = 0

while (++num) {
    if (num**2 == 64) {
        break
    }
}

println "The square root of 64 is $num"

As Groovy resolves a number other than 0 to be true, ++num will allow the loop to commence and we still rely on break to get us out of the loop.

continue

The continue reserved word will cause the next iteration of the loop - ignoring anything within the rest of the current iteration. In the loop below I use continue to ignore 'Earth'.

def planets = [
    'Mercury',
    'Venus',
    'Earth',
    'Mars'
]

for (planet in planets) {
    if (planet == 'Earth') {
        continue
    }
    println planet
}

Labelled branching

If you have a secret desire to create spaghetti code that quickly becomes unreadable then labels are for you!

Labels are used when you have nested loops - a loop within a loop. Both break and continue can be given a label that directs the program to another nesting level. In the example below I label the first loop flowerlist and, when I get to the colour 'Green' in the inner loop, my continue is directed not at the inner loop but at the outer one labelled flowerlist - this is called a “labelled continue”:

def colours = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet']

def flowers = ['daisy', 'rose', 'tulip']

flowerlist:
    for (flower in flowers) {
        for (colour in colours) {
            if (colour == 'Green') {
                continue flowerlist
            }
            println "$colour $flower"
        }
    }

As you can see, the label consists of a name, followed by a colon (:) that appears above the loop being labelled. The code above will display the following:

Red daisy
Orange daisy
Yellow daisy
Red rose
Orange rose
Yellow rose
Red tulip
Orange tulip
Yellow tulip

A labelled break works much the same way and defers control back to the loop designated by the label.

There are times when labels are useful but really think about what you need to do before resorting to them. For example, using a break in the code above would have done the job:

def colours = ['Red', 'Orange', 'Yellow', 'Green', 'Blue', 'Indigo', 'Violet']

def flowers = ['daisy', 'rose', 'tulip']

for (flower in flowers) {
    for (colour in colours) {
        if (colour == 'Green') {
            break
        }
        println "$colour $flower"
    }
}

return

The return statement is used to hand control back to the caller¹. In most cases, return is used in a method or closure to return a value. In the most basic usage, the keyword return just appears on a line by itself:

//some code
return

Any code that appears below the return is unreachable and is never evaluated:

//some code...
return
//unreachable code...

Return can be provided a value and this is returned to the caller:

//some code
return 3.14

You can use an expression in the return statement and the result is returned:

//some code
return circumference / diameter

Using return within the body of a script² will cause it to exit as the command-line/terminal is the caller. When exiting you can choose to return a value - usually 0 if the script succeeds, or an error number if it fails.