Loops (for, while)

With states, we transitioned from pure functional programming to combination of functional and imperative programming.

One feature often associated with imperative programming are loops.Turns out that loops, as a feature are nonessential for imperative languages. With just variables we can already model all imperative programs.A loop can be emulated by using a function.

while loop

while(condition){
   statement(s);
}

• Note: The condition and the command must be passed by name so that they are reevaluated in each iteration.
• Note: while is tail recursive, so it can operate with a constant stack size.

for loop

In Scala there is a kind of for loop similar to Java’s extended for loop:

for (i <- 1 until 3) { 
    System.out.print(i + " ") 
}

The classical for loop in Java can not be modeled simply by a higher-order function. The reason is that in a Java program like:

for (int i = 1; i < 3; i = i + 1) { 
    System.out.print(i + " "); 
}

the arguments of for contain the declaration of the variable i (int i = 1), which is visible in other arguments and in the body.

for-expressions vs for-Loops

for-loops translate similarly to for-expressions, but using the foreach combinator instead of map and flatMap.

foreach is defined on collections with elements of type T as follows:

def foreach(f: T => Unit): Unit =
// apply ‘f‘ to each element of the collection

Example

for (i <- 1 until 3; j <- "abc") println(i + " " + j)

translates to:

(1 until 3) foreach (i => "abc" foreach (j => println(i + " " + j)))

output:

1a
1b
1c
2a
2b
2c