Examples of for-loops iterating over Strings, Ranges and Lists.

The arguments to for(_)do(_)

It’s easy to become confused about what your for loop is iterating over, and what values the parameter to the do-block body takes on.

  • The first argument to for()do(_) must to be a Collection of some kind. Remember, strings are Collections, as are lists, sequences, sets, dictionaries and ranges.

  • The second argument must be a block with a single parameter: the do-block. The block parameter (the variable before the →) will be given a different value each time the do-block is executed.

  • How many times will be do-block be executed? As many times as there are elements in the collection.

  • What values will the parameter take on? The elements of the collection (and not the indices of those elements).

Lets look at some simple examples.

Iterating over a String

In the example above, the for loop is iterating over the string "wombat"; the values of x are the single-character strings "w", "o", "m", etc. So that’s what is printed, one character to a line. Change the body of the do-block to accumulate the characters of the string in reverse order, or to count the number of vowels.

Iterating over a Sequence or Range

In the example above, the first argument to the for loop is the sequence 1..6. So the values bound to x will be 1 on the first iteration, 2 on the second, 3 on the third, and so on. The print statement demonstrates this for you. Change the do-block to calculate the sum of the elements in the sequence.

Iterating over a List

In the example above, the first argument to the for loop is the list containing the thee elements 34, 67, 98. So the values bound to x will be 34 on the first iteration, 67 on the second, and 98 on the third. Once again, the print statement demonstrates this for you. Change the do-block to check that the elements are in sorted order.

Notice the difference between the above code and the final example:

In the example above, the first argument to the for loop is the value of a.indices. What is that? a.indices is the collection of valid indices into the list a — in this case the range 1..3, because a.size is 3.

Hence, the values bound to x will be 1 on the first iteration, 2 on the second, and 3 on the third. If we want to access the elements of a, then we need to use a.at(_), as demonstrated by the print statement, which prints out the element index, a colon, and then the element itself.

Now modify the loop body to replace each element of the list a by its square.