CODEWITHSUNDEEP
scalascala-collections
Tom
Tom

Reputation: 4662

What's difference with these two in scala in print Set

I am learning Scala programming language, and got confused by this:

var set = Set("hello", "world")

set += "Tom"
println(set)

println(set += "Tom")

The first print will output 

Set(hello, world, Tom)

But the second will output

()

So, why they are different, I thought println(set += "Tom") will first do, set += "Tom", and then print its result? Isn't it?

Upvotes: 1

Views: 64

Answers (2)

elm
elm

Reputation: 20435

As already answered by @pedrofurla, the second print displays the result of evaluating the assignment, namely the Unit denoted with ().

In addition, note we can print the result of evaluating the following block delimited by curly brackets,

println( {set += "Tom" ; set} )
Set(hello, world, Tom)

namely, add "Tom" to set as the first expression, then deliver the updated set as the final result of the block evaluation.

Upvotes: 1

pedrofurla
pedrofurla

Reputation: 12783

In scala a += b dessugars to a = a + b. The type of assignment expression is Unit. So, unlike C or Java, the result of an assignment expression doesn't result in the left-hand side but in the Unit value.

Upvotes: 5

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