# This content is archived!

For the 2018-2019 school year, we have switched to using the WLMOJ judge for all MCPT related content. This is an archive of our old website and will not be updated.

# Introduction

Primitive arrays map the key type (int) to a value (the type of the array). In this lesson, we will explore mapping with letters as keys.

Notice the 0 as the accessing index. This is the key. With arrays, the key is always an int.

However, we can take advantage of the fact that a char can be easily cast to an int.

# Example

Let’s try a problem:

Find the number of occurrences of each letter in a word.

We need to map each letter to an index in the array. Let’s use a to 0, b to 1, c to 2, … , z to 25. Conveniently, ASCII values of letters are in the same order, just with a different starting number.

Mapping isn’t limited to only letters though. Just use a bigger array to support more characters.

Array mapping can also be used with other keys such as digits: counting the occurrences of digits. Later on, we will learn mapping with other data types, such as double, String, and even custom classes.