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
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, 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, and so on, until the array contains
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
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. 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. 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.
As a general rule:
- Use a While loop when the decision to “loop again” depends on the calculations and logic of the loop body.
- Use a For loop when the number of loops you are going to make is fixed.
- Use a For-Each loop when all the following are true:
- the language supports it;
- you do not care what order the data is looped through;
- you want to ensure every element of the aggregated variable is examined;
- you do not change any data in the aggregate variable (look but don’t touch).
Consider taking one egg at a time out of full egg carton.
- If you were going to crack eggs until you had at least one cup of egg-goop, that is a while loop.
- If you are going take 3-eggs for an omelet (a fixed number of eggs), that is a for loop.
- If you are going to swap the eggs around in the carton, or maybe draw faces on them, that is also a for loop (there are a dozen eggs). Here you are changing the “elements” in the carton.
- If you are going to weigh each egg, then put it back where it came from–that is a for-each loop,