Question

I want to implement a VBA function to number Excel rows based upon the grouping depth of the row.

But I think a general algorithm for generating TOCs is more interesting.

The problem is:

Given a list of “indented” lines such as

One
 Two
  Three
   Four
 Five
Six

(the “indentation level” may be assumed to be known and part of the input data)

To generate the following output:

1.    One
1.1    Two
1.1.1   Three
1.1.1.1  Four
1.2    Five
2.    Six

Of course my code is up and running … and also hidden under THWoS (The Heavy Weight of Shame)

Answer 1

Use a stack for the numbers. Loop through each row, and check the indentation level of each row, with no indentation being level 1.

1. If the current indentation level is greater than the size of the stack push as many ones as the difference is onto the stack (the difference would usually be just one, but this works even if someone puts a level 3 heading under a level 1 heading, for instance)
2. If the current indentation level is less than the size of the stack, pop and discard as many numbers as the difference is and then increment the top number on the stack.
3. If the current indentation level is equal to the size of the stack, increment the top number on the stack

For each row, the current title number is the numbers on the stack concatenated together with a . to separate them.

Note how the size of the stack handily represents the previous line’s indentation level.

For people who find it easier to read code, here’s a JavaScript implementation for modern browsers:

const toc = `
One
 Two
  Three
   Four
 Five
  Six
  Seven
 Eight
Nine
Ten
`;

let stack = [];

toc.trim().split(/\n/g).forEach(line => {
  // Gets the identitation level with 1 being no indentation and so forth
  let level = line.match(/^\s*/)[0].length + 1;

  if (level > stack.length) {
    while (level > stack.length)
      stack.push(1);
  } else {
    while (level < stack.length)
      stack.pop();

    stack[stack.length - 1]++;
  }
  
  let title = stack.join(".") + ". " + line.trim();

  document.body.appendChild(document.createElement("div")).innerText = title;
});