Print Each Array Element with Its Index

Every array slot has an index and a value

The array [4, 2, 7, 1] has four slots. Each slot has a position called its index and a piece of stored data called its element value. Array indices start at 0, so the first slot has index 0 and the last slot has index 3. The value 7 is stored in the slot at index 2.

The index and the element value can be different numbers. In this array, index 0 locates the value 4, and index 2 locates the value 7. The number below a slot tells you where the slot is. The number inside the slot is the data you can read.

The loop variable names the current index, not the current value

When a loop variable i moves through this array, it takes the index values 0, 1, 2, and 3. For each value of i, arr[i] reads the element value in the slot at that index. The pair changes together as the loop moves: i = 0 gives arr[0] = 4, i = 1 gives arr[1] = 2, i = 2 gives arr[2] = 7, and i = 3 gives arr[3] = 1.

At i = 2, the loop variable is not the element value 7. It is the location used inside arr[i]. The expression arr[2] evaluates to 7 because index 2 points to the slot where 7 is stored. Printing i would print 2, while printing arr[i] would print 7.

The condition i < n visits every valid slot exactly once

For this array, n is 4, so the loop starts with i = 0 and tests i < 4. The test is true, the loop reads index 0, and then i increases by 1. The same steps read indices 1, 2, and 3. After index 3, the increment makes i equal to 4, and the test i < 4 becomes false.

The loop stops before trying to read arr[4]. The valid indices end at 3 because the array has four slots and indexing starts at 0. This boundary gives exactly four iterations, one for each slot.

for (int i = 0; i <= n; i++) {
    print(arr[i]);
}

One loop iteration prints one index-value pair

The loop prints two expressions on each iteration. The first expression is i, which supplies the current index. The second expression is arr[i], which supplies the element value stored at that index. Combining them with the required text produces one line for each slot.

int arr[] = {4, 2, 7, 1};
int n = 4;

for (int i = 0; i < n; i++) {
    cout << "Index " << i << ": " << arr[i] << endl;
}

int[] arr = {4, 2, 7, 1};
int n = 4;

for (int i = 0; i < n; i++) {
    System.out.println("Index " + i + ": " + arr[i]);
}

Index 0: 4
Index 1: 2
Index 2: 7
Index 3: 1