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I want to export my products to another marketing platform.

My products have different prices and types.

I can only export a maximum of 100 and I want to make sure I export 100 exactly if possible.

I have a set of criteria rules against which I select products, and I have a rough target for the amount of each I want to select (like a percentage of the 1,000) because I know at any one time that one type of product is more popular than others - there's no point marketing more winter clothing in summer. The targets don't add up to 100 because the admin would be horrendous when I make changes to a rule.

  • RULE 1 - I want 30 yellow widgets between 10 and 20 dollars
  • RULE 2 - I want 50 red doodahs between 30 and 40 dollars
  • RULE 3 - I want 40 blue thingies between 100 and 200 dollars

I asked another question here recently about how to use ratios to calculate how to adjust these quantities so that I could select more or less products to reach my maximum. It works.

However, the problem I have is that perhaps there are only 20 yellow widgets between 10 and 20 dollars and I've told the system I want 30. This means I need to:

  • work out the ratios
  • the calc says I need to select 30
  • if there are only 20 yellow widgets then I select all of them
  • then I loop through the remaining rules calculate the ratios again and do the same check, ad nauseum

I have struggled with this for several days trying to make a system work, and to understand what's happening in my code when looping through. I wondered if there was a mathematical principle and method for this kind of problem.

At the moment this is what I do:

  • allocation limit: 100 products
  • I separate out the RULES where my target is greater than its available products into a list of VALID_RULES
  • I sum the number of products matching these rules and subtract this from the max 100 to find the remaining allocation limit (say, there's 70 left of the 100)
  • loop through the remaining rules where number of products is more than my chosen target
  • sum all the targets, then work out the ratio for each one to find the number of products I should allocate (say, the first one says instead of 50 red doodahs I should select 55)
  • check if the new allocation is greater than the products I have (say there are 52 red doodahs in my products database)
  • if the new allocation is greater than the available products, I move the rule to VALID_RULES with with the max products as the target number otherwise I set it to the new_allocation
  • I delete the rule from the RULES, move to the next rule and do all this again

But I am consistently getting more than my max of 100.

user2470281
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  • How is this "ad nauseum"? After you've bought the yellow and recalculated ratios, you should be doing it just among the red and blue, for a total of (100 - yellow). Once you buy your red, you should attempt the buy the remainder (100 - yellow - red) of blue. And that is the end. – Paul Sinclair Dec 18 '19 at 23:10
  • When you say you are consistently getting more than 100, how many more are you getting? Is it just a few? In particular, on that last pass where the remaining rules all have meetable allocations, it the amount of excess over 100 always less than the number of rules at the end? If so, then your problem is rounding. If not, then your problem is a bug in your programming. If the problem is rounding, the solution is simple, when doing the re-allocation, you only calculate $n-1$ values using ratios. The final value is the desired total minus the sum of the others. – Paul Sinclair Dec 19 '19 at 00:12

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