Iteration

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Of course, once we’ve created an array of data, we need some way to access its elements easily in our code. Thankfully, we can use the loop structures we’ve learned in a previous chapter to iterate across our arrays. Iteration is the term we use to describe accessing each element in the array and possibly performing some repeated action using that element.

For example, consider the flowchart below, showing a simple program that uses loops and arrays:

This program begins by accepting a single input from the user, stored in the variable x. That input is used to determine the size of an array, denoted by the array(x) block. Each programming language has its own way of creating an array, but we’ll use this simplified form in these flowcharts.

Next, the program reaches a For loop which uses i as its iterator variable. That variable will include all values from 0 up to, but not including, the value stored in x. Notice that there is a parenthesis to the right of x, showing that it is not included in the sequence denoted by [0, x).

Inside of that loop, we receive another input from the user, stored in the variable y. Then, we store that value into the array, using our iterator variable i as the index in the array. So, the first input we receive and store in y will be stored in a[0], the first element of the array. In most programming languages, we use square brackets [] after an array variable to denote a specific element in the array, with the index of that element shown inside of the square brackets. When the loop repeats, the next value will be stored in a[1], and so on, until the array contains x values.

Once the first For loop terminates, we create a new variable named sum and set it initially equal to 0. We’ll use this variable to add up all of the numbers in the array in the next For loop. However, that loop is defined a bit differently. In the flowchart we see that the loop is defined as j : a, which means that we are using j as our iterator variable, but instead of getting values from a mathematical sequence such as [0, x) from the first loop, we are taking the items directly from our array, a, instead. These special For loops are sometimes known as For Each loops or Enhanced For loops, depending on the language. In essence, they repeat the loop one time for each element in the array given. So, we’d say “for each j stored in a, add j to sum” to describe this loop.

The first time we run the code inside of that loop, the variable j will be storing the value of the some element in array a. So, we could say that j has the same value as a[0]. We’ll add that value to the sum variable, then repeat the loop. In the next iteration, the variable j will then store a different value in a, or a[1]. We’ll continue to repeat the loop until each element in a has been used–the loop ensures we have seen all the elements exactly once.

At the end, when we output the sum variable, it should be the sum of all of the elements in the array.

Of course, we can easily rewrite these loops as While loops instead, or we could use the standard form of the For loop as the second loop, using the iterator variable to refer to the index of the element inside of the array instead of the element itself.