In my previous post I've explained how to use while and do-while loops. In today's post I will show you how to use a for loop. For loop allows you to do quite a lot. It was designed to iterate through a collection, do range iterations, producing a new collection, perform nested iterations easily, filter and mid-stream variable bindings. I will skip mid-stream variable binding and producing new collections since these are advanced topics. I will explain them in the future.
Iterate through a collection
Let's say that you want to perform some operations on each element of the collection. In the following example I want to print out each element of my list. I will use a for loop to do so.
val yourLuckyNumbers = List(23, 0, 512, 3, 4)
println("Your lucky numbers: ")
for (number <- yourLuckyNumbers) {
println(number)
}
Output would be:
Your lucky numbers:
23
0
512
3
4
Another example would be to execute an operation on each argument of Scala script (in this case the printout is reversed):
for (arg <- args) {
println(arg.reverse)
}
If you called the sample script: reverse.bat as:
reverse.bat abc efg
the output would be:
cba
gfe
Ranges: to and until
You can use a for loop also if you want to iterate over some range of values. Let's say that you want to iterate from 1 to 12:
for (i <- 1 to 12) {
println(i)
}
This one would generate the following output:
1
2
...
10
11
12
You can exclude the last value from iteration by replacing to with until. Let's say that you want to iterate from 0 to 59 using until:
for (i <- 0 until 60) {
println(i)
}
The output is obvious:
0
1
2
...
59
Filtering
If you want to iterate over a subset of collection you can use filters. Let's use a range and filters to generate even numbers from a range of 0 to 10:
for (i <- 0 to 10 if i % 2 == 0) {
println(i)
}
it's a shorter form of:
for (i <- 0 to 10) {
if (i % 2 == 0) {
println(i)
}
}
You can apply multiple filters. Let me generate numbers from 0 to 10 which are even and divisible by 3:
for (i <- 0 to 10
if i % 2 == 0
if i % 3 == 0) {
println(i)
}
Nested iterations
You can put multiple <- clauses separated with semicolons. This will tell Scala that loops are nested. Let's see the example below:
val multiDim = List(List(11, 12, 13),
List(21, 22, 23),
List(31, 32, 33))
for (
row <- multiDim;
element <- row
) println(element)
This actually behaves the same as:
val multiDim = List(List(11, 12, 13),
List(21, 22, 23),
List(31, 32, 33))
for ( row <- multiDim )
for (element <- row)
println(element)
Filtering and nested loops
You can use filtering in nested loops just like in normal loops. Let's see the following example:
val multiDim = List(List(11, 12, 13, 14),
List(21, 22, 23),
List(31, 32))
for (
row <- multiDim
if row.length > 2;
element <- row
if element % 2 == 0
) println(element)
In this example we have nested a loop which iterates over multiDim list. The outer loop will return only these lists which have more than two elements:
if row.length > 2;
For the inner loop it will return only even elements:
if element % 2 == 0;
We could implement this loop in a normal way as follows:
for (row <- multiDim if row.length > 2)
for (element <- row if element % 2 == 0)
println(element)
or:
for (row <- multiDim)
if (row.length > 2)
for (element <- row)
if (element % 2 == 0)
println(element)
As you can see you can save a lot of time if you know what is the best way to implement your code.
Comments and questions
I encourage you to comment and write questions regarding this post or loops in general at michaelflowersky at geekingspree.com. Thanks for passing by!
References
- Programming in Scala: A Comprehensive Step-by-Step Guide, 2nd Edition by Martin Odersky, Lex Spoon and Bill Venners
My recommendations!
Looking for something compact about Scala? Check out Scala for the Impatient by Cay Horstmann.
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