Summary

In this lesson, you learned that:

• An array stores several values of the same type, its elements, under one name: int scores[5] = {80, 92, 75, 60, 88};. With int scores[] = {80, 92, 75};, the compiler counts the elements itself. The size of an array is fixed and can't change.
• Each element has an index, counted from 0, so the last index is the size − 1. scores[0] reads the first element, scores[2] = 79; changes the third one. An element behaves like a variable of its type, so scores[1] += 5; works too.
• An index can be a variable, so a for loop can go through all the elements: for (int i = 0; i < count; i++), with <, not <=. You can't print a whole array with printf: print it element by element, in a loop.
• An index outside the array is a mistake. When it's a number in the code, the program doesn't compile (array index 5 is past the end of the array). When it's a variable, the program stops with RuntimeError: unreachable. C itself doesn't check indexes.
• The number of elements can be kept in a constant, const int COUNT = 5;, or calculated from the array: int count = sizeof(scores) / sizeof(scores[0]);. With sizeof, the loops stay right when the array changes.
• Patterns with arrays: the sum (a variable that starts at 0 before the loop, += in the loop, the result printed after the loop), the average ((double)total / count), counting (a counter with ++ inside an if) and the largest value (start with the first element, not with 0, and keep the index too if you need to know where it is).
• int squares[10]; creates an array without values, which a loop can fill. int votes[4] = {0}; sets all the elements to zero. An array can't be assigned or copied with =: copy it element by element.
• A two-dimensional array has rows and columns: int seats[3][4]. seats[1][2] is the element in the row with index 1 and the column with index 2. Two nested loops go through all the elements.

In the next lesson, on strings, you'll find out that text in C is an array too: an array of characters. But first, practice what you know on the temperatures of one week.