Indexed Loops and Array Traversal

A for-each loop looks like it hands you each position of an array, one after another. Because of that, it is tempting to think that writing to the loop variable will write into the array. Many learners expect that a line such as item = 17 inside the loop will fill the array with 17s. This article tests that belief against what the loop actually does, and it shows where the for-each loop is a good fit and where it is not.

Follow the loop one pass at a time. On the first pass, the for-each loop reads the value 4 from the first slot of arr and gives that value to the name item. The statement item = 17 then associates the name item with 17. The array slot that supplied the 4 was never named in that statement, so nothing in the array is reached. The diagram shows the two things side by side: the array has its own slots, and the loop variable is a separate name. Only the name on the left changes.

The next pass gives item the next value from the array, 8, and the earlier assignment is simply replaced. The three array cells shown stand in for the full array, and the same reasoning applies to every slot. Only the loop variable is reassigned. The array cells keep the values they had before the loop started.

The key to this topic is a distinction between two words that sound alike in everyday speech. A value is the piece of data stored in a slot. An element is the actual memory location that is part of the array and that holds the value. The array is the collection of these elements, and each element carries one value at a time.

This distinction explains the whole behavior. A for-each loop works with values. It supplies the value of each element to the loop variable, not the element itself. Because the loop never gives you a handle on the slot, writing to the loop variable has no route back into the array.

When the goal is to change the array, the loop has to name the slot being changed. An indexed loop does this by using the index to refer to the element directly. The example below is an illustration written for this article, not an excerpt from the source. It doubles every value in the same array, first with a for-each loop and then with an indexed loop.

On each pass, the index i selects one slot, and the assignment writes a new value into that slot. Because the write goes to the element itself, the change remains after the loop ends. The loop variable item in the for-each version was only a copy, so the same arithmetic never reached the array.

The for-each loop is not always the right tool. There is no simple way to use it to process only part of an array, and it cannot walk the elements in reverse order. Its strength is the opposite case: when you want to process all the values in order, it makes the code a little simpler, because it removes the need for array indices.