Insert End

Section 23.10 Insert End

We will always keep the values in our ArrayList packed together at the start of the array. That means if we have 10, 20, 30 in an ArrayList with m_capacity of 5, the contents of the array will always look like [10, 20, 30, ???, ???]. We will never skip a space in the array by storing the items like [10, ???, 20, 30, ???] .

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Keeping the data compacted like this serves two important roles:

It makes it easy to locate elements in the ArrayList. When we want to access the element at index 2 of the ArrayList (the third item), we can just look at m_arr[2]. If we allowed a gap in the array, we would not know that the element at index 2 is actually the third item.

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It makes it easy to find the next available location in the array. When m_size is 3, that means we have items at indexes 0-2 (size - 1). The next available index would be 3.

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Insight 23.10.1.

m_size is always the first location that is NOT currently in use. m_size - 1 is the index of the last element.

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Given this design, we generally will insert items at the end of the ArrayList (similar to how in a vector we generally use push_back() to add items). A basic version of it might look like:

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template<typename T>
void ArrayList<T>::insertEnd(const T& newItem) {
    m_arr[m_size] = newItem;  // place the new item in first unused location
    m_size++;                 // increase size to reflect new item
}

Given memory that looks like:

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The intList variable contains an m_size of 3 and m_capacity of 5. It also has a pointer to an array named m_arr. That array contains [10, 20, 30, ???, ???] — Figure 23.10.1. `intList` currently has an logical size of 3.
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Let us imagine what would happen if insertEnd(40) was called on it. First, insertEnd would copy the new item into the location indicated by m_size (3). Then it will increase m_size to 4, telling us that elements 0-3 of the array are now valid indexes in the ArrayList.

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The intList variable contains an m_size of 4 and m_capacity of 5. It also has a pointer to an array named m_arr. That array contains [10, 20, 30, 40, ???] — Figure 23.10.2. `intList` after inserting 40.
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