Reverse an array in ColdFusion and other tricks using slice syntax

In ColdFusion there is a dedicated function to reverse a string but there isn’t one to reverse an array. With the slice functionality added in ColdFusion 2018 you can easily reverse an array:

foo = [10,20,30,40,50];
bar = foo[::-1];

writeDump(foo); // outputs [10,20,30,40,50]
writeDump(bar); // outputs [50,40,30,20,10]

As you can see foo is unchanged and we have a new array bar which is in the reverse order to foo.

With this syntax we can also do things like picking the value of every element with an odd index starting at position 1.

foo = [10,20,30,40,50,60,70,80,90,100];
oddElements = foo[1::2];
writeDump(oddElements); // outputs [10,30,50,70,90]

As the start position defaults to 1 we could have written the above example as:

foo = [10,20,30,40,50,60,70,80,90,100];
oddElements = foo[::2];
writeDump(oddElements); // outputs [10,30,50,70,90]

If you want to pick the value of every element with an even index starting at position 2.

foo = [10,20,30,40,50,60,70,80,90,100];
evenElements = foo[2::2];
writeDump(evenElements); // outputs [20,40,60,80,100]

This one will return every 3rd element starting at position 2.

foo = [10,20,30,40,50,60,70,80,90,100];
thirdElementsStartingFromPositionTwo = foo[2::3];
writeDump(thirdElementsStartingFromPositionTwo); // outputs [20,50,80]

This one will return every 3rd element starting at position 2 up to and including position 5.

foo = [10,20,30,40,50,60,70,80,90,100];
bar = foo[2:5:3];
writeDump(bar); // outputs [20,50]

You can also use it to clone an array:

foo = [10,20,30,40,50];
bar = foo[:];
bar.deleteAt(1);
writeDump(foo); // outputs [10,20,30,40,50]
writeDump(bar); // outputs [20,30,40,50]

Get the first 3 elements of an array

foo = [10,20,30,40,50];
writeDump(foo[:3]); // outputs [10,20,30]

Get all elements from position 5 (including the value at position 5)

foo = [10,20,30,40,50,60,70,80,90,100];
writeDump(foo[5:]); // outputs [50,60,70,80,90,100]

Get all elements from position 5 (including the value at position 5) excluding the last 4 elements.

foo = [10,20,30,40,50,60,70,80,90,100];
writeDump(foo[5:-5]); // outputs [50,60]

The fun doesn’t have to stop with number arrays, we can do the same with sentences. For example we can split the sentence up into an array of words using a space as the delimiter, reverse it, and then join it back together again.

message = "am I as old as you are";
words = message.split(" ");
reversed = words[::-1].toList(" ");
writeDump(reversed); // are you as old as I am

I think this is great addition to CFML. There are some more examples in the docs:
https://helpx.adobe.com/coldfusion/developing-applications/the-cfml-programming-language/using-arrays-and-structures/populating-arrays-with-data.html#slicing

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