Vim Replace On Each Line With Different String - vim

Question

I've got a document from a client which has a GIANT table in it which looks something like this:
<table id="someid">
<tr>
<td>Product</td>
<td class="product1">Product 1</td>
<td class="product2">Product 2</td>
<td class="product3">Product 3</td>
<td class="product4">Product 4</td>
<td class="product5">Product 5</td>
</tr>
<tr>
<td>Boiling Point</td>
<td>72</td>
<td>91</td>
<td>38</td>
<td>21</td>
<td>41</td>
</tr>
[ 45 more rows here]
</table>
Only there are actually 15 products, and instead of "product1" and "product2" they have the actual name of the products as their preexisting classes.
The client has asked me to add classes to each of the appropriate td elements so that they are matched up with their product like class="product1" added to each 2nd td for every row.
Everything is static... I'm wondering if there's a quick way to do this in vim? Is it possible to tell vim to add a string to a certain position on every 18th line? Or am I stuck manually adding all the classes?

Answer 1

Suppose that the relation between your class name and the line number can be described as an expression, you can use :[range]substitute and s/\= to replace lines with any expression. For example,
1,10s/<td/\=submatch(0) . ' class="product' . (line('.') % 5 - 1) . '"'
will change
<tr>
<td></td>
<td></td>
<td></td>
</tr>
<tr>
<td></td>
<td></td>
<td></td>
</tr>
to
<tr>
<td class="product1"></td>
<td class="product2"></td>
<td class="product3"></td>
</tr>
<tr>
<td class="product1"></td>
<td class="product2"></td>
<td class="product3"></td>
</tr>
You can adapt those arguments in the above command to suit your needs.
For complex replacement, you can define a helper function in a temporary vim file such as foo.vim
function! GetClassName()
let order = line('.') % 5
if order == 1
return 'a'
endif
if order == 2
return 'b'
endif
endfunction
then source it by :source %.
Next, you can switch to your file and use it as follows
1,10s/^\s\+<td/\=submatch(0) . ' class="' . GetClassName() . '"'