Arrays: The Element Count Is Not an Index — Read

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The 5 in scores[5] is a slot count, not a usable index

Consider this declaration: int scores[5] = {12, 7, 19, 4, 10}; It creates exactly five int elements. The 5 tells C++ how many slots to allocate for scores. It does not name one of those slots. To access a stored value, you use an index, and the first index is 0.

CPPThe declaration creates five slots and fills them with values.

int scores[5] = {12, 7, 19, 4, 10};

Because counting starts at 0, five elements need the indices 0, 1, 2, 3, and 4. The declaration's number and the largest usable index are different: the element count is 5, while the final valid index is one less, or 4.

CHECKPOINT 1Not answered

What does the 5 mean in int scores[5] = {12, 7, 19, 4, 10};?

Checkpoints are not graded. They are here so you catch yourself before the quiz does — stuck, ask the tutor on the right.

Five slots have the valid indices 0 through 4

Each index identifies one position in the same array. For scores, scores[0] is 12, scores[1] is 7, scores[2] is 19, scores[3] is 4, and scores[4] is 10. There is no sixth slot belonging to scores. Writing scores[5] asks for a position immediately beyond the five allocated elements.

INDEX	VALUE
0	12
1	7
2	19
3	4
4	10
5	Outside the array

The element count is 5, but the final valid index is 4.

Five elements end at index 4, not index 5.

An assignment changes one named slot and leaves the other four alone

In this assignment, the right-hand side is evaluated first: scores[1] = scores[0] + scores[3]. The program reads scores[0], which is 12, and scores[3], which is 4. Their sum is 16. The left-hand index, 1, identifies the only slot that receives the new value.

CPPThe right-hand indices are read; only the left-hand index is updated.

int scores[5] = {12, 7, 19, 4, 10};
scores[1] = scores[0] + scores[3];

// scores is now {12, 16, 19, 4, 10}

The values at indices 0 and 3 are used as inputs, not changed by the expression. Index 1 changes from 7 to 16, while indices 0, 2, 3, and 4 keep their original values.

CHECKPOINT 2Not answered

Enter the complete array after scores[1] = scores[0] + scores[3].

scores[1] = scores[0] + scores[3];

Checkpoints are not graded. They are here so you catch yourself before the quiz does — stuck, ask the tutor on the right.

A complete indexed traversal stops when i reaches 5

For this array, the condition i < 5 permits i to be 0, 1, 2, 3, and 4. Those are exactly the five valid indices, so every element is visited once. When i becomes 5, the condition is false and the loop stops before trying to access scores[5].

CPPThe loop visits indices 0 through 4 and stops before index 5.

for (int i = 0; i < 5; i++) {
    cout << scores[i] << " ";
}

Changing the condition to i <= 5 adds one unwanted iteration. The loop still visits indices 0 through 4, then allows i to equal 5 and attempts scores[5]. The problem is not the number of valid values already printed. The problem is that the condition permits an index outside the array.