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I have a boolean expression field, where the user can enter a variable name and then perform one of 12 boolean operations on it. For reasons related to the rest of the UI, I cannot make the field very wide.

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As such, I have designed a field that only shows a symbol - but when the user expands the drop-down, he can see the explanations for all the symbols.

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But now, my boss wants to add new boolean operations - and I have difficulty coming up with symbols for them. I proposed an alternative solution, but he refused and wanted to keep the tiny dropdown with symbols.

Two of the boolean expressions are "empty value" and "not empty value." The symbol for empty value is easy - ∅. But what is the opposite?

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    You might also find an answer on the math StackExchange community.
    – BrettFromLA
    Commented Mar 3, 2017 at 22:57
  • I am a bit confused with the premise of the question. The center dropdown compares left with right (equal, greater, etc.), but the empty non-empty doesn't compare left with right, but it is right itself. So an example would be: $variable = empty; $variable ≠ empty; no?
    – Alvaro
    Commented Mar 4, 2017 at 0:39
  • @Alvaro "Empty" is not a value, but the absence of value (null). It's not the same thing as an empty box, because an empty box is "" - a string of length 0. Rather than trying to explain to users what "null" is we decided to go with this.
    – SPavel
    Commented Mar 4, 2017 at 2:12
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    @BrettFromLA I'm less interested in making an interface for math enthusiasts, and more interested in making an interface with broad appeal. I don't care if it's "technically" correct as long as people get it.
    – SPavel
    Commented Mar 4, 2017 at 2:16
  • @SPavel: "I'm less interested in making an interface for math enthusiasts"; "The symbol for empty value is easy - ∅." - to interpret that symbol as "empty", you already have to be quite a math enthusiast. To anyone else, the circle with a slash means either "diameter" or "average".
    – O. R. Mapper
    Commented Mar 6, 2017 at 10:08

4 Answers 4

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Seeing that ∅ is your choice for an empty value, for non-empty one you can just prepend it with the negation symbol: ¬∅, or !∅.

As far as UX goes, this will only be instantly clear to people who are familiar with logic symbols (or most developers in the case of !). Luckily, as each dropdown item has text to elucidate what each symbol means, and as it may be fair to assume that most users will be familiar with most other symbols, having a few cryptic symbols is not such a big deal - users will be aware of the contextual help served by the dropdown.

Given that, you may just as well make up something slightly more memorable, like ○ for empty, and ● for non-empty. It may not be based on logic notation, but will work all the same.

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If the user is selecting this themselves/would naturally be going to the dropdown as a part of the user flow then I don't think the icon really matters at all. (~∅)? If they choose it then they already know what they are attempting to do.

Other suggestions "ev"/"nev".

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Not NULL seems to be the semantics you want to convey. != NULL could work (or replace NULL with empty symbol)

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The symbol you've suggested is "empty set" which is still quite mathematical. I think I'd go with an unfilled circle for empty ⃝ (Circle (U+20DD)) and then a filled circle for not-empty ⦿ (Circled Bullet (U+29BF)).

The most universally recognised full/empty symbols these days are an battery symbols, as everyone dreads seeing the empty one! These probably don't apply to your situation though.

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  • Circled bullet looks sort of like a radio button...
    – SPavel
    Commented Mar 4, 2017 at 19:56
  • Maybe if there were a number of them, but one of a number of symbols in a drop down list, I don't think anyone would confuse it for one.
    – Rik Lewis
    Commented Mar 5, 2017 at 7:24

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