What is happening in the following code of a trie

Question

I was going over this example of a Trie and I am having difficulty understanding the following code

void Trie::addWord(string word)
{
    Node * currentNode = root;

    for (int i = 0; i < word.size(); ++i)
    {
        char currentChar = tolower(word.at(i));
        int index = currentChar - 'a';
        assert(index >= 0);     // Makes sure the character is between a-z
        if (currentNode->children[index] != NULL)
        {
            // check if the current node has the current character as one of its decendants
            currentNode = currentNode->children[index];
        }
        else
        {
            // the current node doesn't have the current character as one of its decendants
            Node * newNode = new Node(currentChar);
            currentNode->children[index] = newNode;
            currentNode = newNode;
        }
        if (i == word.size() - 1)
        {
            // the last character of the word has been reached
            currentNode->end = true;
        }
    }
}

My question is why is a being subtracted here

 int index = currentChar - 'a';

Answer 1

At the line int index = currentChar - 'a'; The currentChar (what ever it is) will be subtracted by 'a' character which has the ASCII value 97.
In this case you have two conditions:

1. First If currentChar is between a-z, the result of index will always be >= 0.

2. Otherwise currentChar is not between a-z because index will be negative number, currentChar can't be between A-Z because of the tolower() function.

You can refeare to this link to know more about ASCII values

Also you need to update the condition assert(index >= 0 && index < 26) because { ,} ,| and ~ will make index >=0