Arrays: Counts and Zero-Based Indices

The number in an array declaration is an element count

Look at this declaration: it creates one array named scores with room for four int elements. The element type is int, the array name is scores, the element count is 4, and the initializer list supplies the starting values. The 4 does not identify an element. It tells C++ how many elements the array contains.

int scores[4] = {7, 4, 9, 2};

Initializer values fill the array from left to right

The initializer list places its values in order. The first value, 7, goes into the first position, followed by 4, then 9, then 2. Every position has the declared type int. The declaration fixes this array at four elements, so the list describes four values for four positions.

The number in an access expression is a zero-based index

When you read an element, the number in brackets is its index. Index 0 means the first element, index 1 means the second, index 2 means the third, and index 3 means the fourth. Therefore, an array with four elements has valid indices 0-3. Its final value, 2, is read with scores[3], not scores[4].

scores[0]  // reads 7
scores[1]  // reads 4
scores[2]  // reads 9
scores[3]  // reads 2

Brackets mean count in a declaration and index in an expression

The same brackets have different meanings depending on where they appear. In int scores[4], the 4 declares how many elements scores contains. In scores[3], the 3 selects one element from that array. The declaration uses a count, while the access expression uses a zero-based index.

cout << scores[0] << endl;  // prints 7
cout << scores[2] << endl;  // prints 9
cout << scores[3] << endl;  // prints 2