One-dimensional arrays

Watch the loop walk the array

Four scores, one loop

The code is in simple pseudocode. ← means "store this in", LEN(scores) gives the number of values in scores, and FOR … TO counts up to and including its end value.

Before you press Next: scores holds 4 values. How many times will line 4 run, and what is the last value i takes?

 1  scores ← [7, 4, 9, 5]
 2  total ← 0
 3  FOR i ← 0 TO LEN(scores) - 1
 4      total ← total + scores[i]
 5  ENDFOR
 6  OUTPUT total

Predict, then check

Commit to an answer before you look. This is the question that catches most people out.

names ← ['Ali', 'Bea', 'Cal'] names[1] ← 'Ben' What does names hold now?

What you need to know

• A data structure is a way of organising and storing related data in a program so it can be used efficiently. A one-dimensional array is one data structure.
• An array is an ordered collection of elements, all of the same data type, stored under a single identifier.
• Each element is identified by its position, its index. Write the array name and then the index in square brackets to read or change it: scores[2].
• Arrays are commonly zero-indexed: the first element is at index 0, so an array of n elements has indexes 0 to n - 1.
• An array can be declared and given its values in one statement, such as names ← ['Ali', 'Bea', 'Cal']. Assigning to an index, such as names[1] ← 'Ben', changes only that element.
• A loop from index 0 to the length minus 1 processes every element in turn. That is how tasks such as totalling, finding the largest value and searching are done.
• The length of an array is the number of elements it holds. Trying to access an index outside 0 to length - 1 is an error.
• Using an array instead of many separate variables keeps related values together and lets one loop process them, so code is shorter and works for any number of values.

The big picture

A one-dimensional array stores a collection of values, all of the same data type, in order, under one name. Each value is an element, and each element sits at a numbered position called its index. You reach an element by writing the array name followed by the index in square brackets, like scores[2]. Arrays are usually zero-indexed, so an array of n elements has indexes 0 to n - 1, and using an index outside that range is an error. Because one name plus a changing index can reach every element, a single loop can process the whole array, however many values it holds.

⚠ Watch out

Counting positions from 1. The first element is at index 0, so the last one is at length - 1. A loop that runs up to the length itself asks for one element too many, and that index is outside the array.

Memory hook

Picture a row of lockers with one name painted over the whole row, numbered from locker 0. The name gets you to the row; the number gets you to one locker. Four lockers means lockers 0, 1, 2 and 3, so there is no locker 4 to open.

Check yourself

data holds 6 values. What are its first and last indexes? A loop runs FOR i ← 1 TO 6 reading data[i]. Which element does it miss, and which index goes wrong?