Arrays and Strings

The problem

You are given a string such as abracadabra. How many different letters does it contain?

That one question already needs the three habits of this lesson:

• Walk through the data in a controlled way.
• Count things in a small table instead of searching for them.
• Change data in place when memory or time is tight.

An array is a row of values stored one after another in memory, numbered from index 0 to n - 1. A string is an array of characters with a few extra tools. Reading a[i] costs the same for any i, which is called O(1), or constant time.