The sidenum function appends a sign to a number based on a side. The name sidenum was chosen due to it's similarities with signum in spelling and behavior (dealing with signs).

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The sidenum function of a side x and a number y is defined as follows:
note: 'bid' can be replaced with 'buy', and 'ask' can be replaced with 'sell' or 'offer'. The string representation or type of side varies by trading system.

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The sidenum function appears in most trading applications, though not always named 'sidenum'. It can be helpful for both position related math and conserving screen real-estate by displaying 2 concepts (side & quantity) as 1 (a signed quantity).

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Java

    public static int sidenum(String side, int quantity) {
        if (side.equals("ask"))
            return -quantity;
        return quantity;
    }

Clojure

    (defn sidenum [side quantity]
      (if (= side "ask")
        (- quantity)
        quantity))

Ruby

    def sidenum(side, quantity)
      if side == "ask"
        -quantity
      else
        quantity
      end
    end