Dear members of the community I trying to improve an algorithm where data is stored in a hash table and where ValueQ is going to be used to operate only over defined elements. The case can be easily exemplified as follows: Let us tabulate some data Do[f[i] = i, {i, 1, 10, 2}] clearly if one…

Hi Jesus, This is indeed some ‘unexpected’ behavior, as an alternative you can use (2 methods): If[ValueQ[f[#]], f[#], "No value"] & /@ Range[10] Table[If[Head[f[i]] =!= f, f[i], "No value"], {i, 10}] I think it has to do with ValueQ[f[i]] that is converted to: !Hold[Evaluate[f[i]]]===Hold[f[i]]…

I noticed the same thing–either way, it is a bug. Moderators, please take note and pass on to the developers.

This is not a bug. ValueQ is HoldAll, meaning the expression that is evaluated is ValueQ[f[i]], which becomes f[1], f[2], etc, which is obviously different from f[i]. (Recall that Table is a dynamic scoping construct, effectively using Block[{i=val},…] to evaluate the expression in its first argum…

Thanks Guys Your answers are all useful. Itai-- Thanks for your very precise explanation. It is educational. I am going to adopt ‘map with a pure function’ since it seems to me that it is more straightforward and easier to read. Regards Jesus

Thanks for the explanation. I find writing these things using Map easier anyhow. That’s why I also suggested above to use Map over Table in such example. I didn’t know about the nested behaviour of these scopic constructs, thanks for showing.

This is not a bug. ValueQ is a funny function and it’s good to be aware what it actually does before attempting to use it. To say that it checks if the expression has a value is inaccurate and can be misleading. ValueQ[x] will check the OwnValues of x is x is a symbol. But in all other cases Val…

As a side note: you could use an association to store your data: Associations provide generalizations of symbolically indexed lists, associative arrays, dictionaries, hashmaps, structs, and a variety of other powerful data structures. Then when you ask for a value that is not yet defined it will…

Sander Huisman, Tanks, this is also a good implementation. I tend to think that pure functions are the fastest way to proceed when writing algorithms… but that is more than a feeling than anything else. It will keep in mind this alternative way of working things out. Thanks again Jesus

In principle - according to Sanders remark - Trace is already perfectly showing what is going on. One can improve the readability simply by wrapping the output in a “repeated frame” like so: Framed //@ Trace[Table[If[ValueQ[f[i]], f[i], "No value"], {i, 2}]] Regards – Henrik

That’s a real neat tip! Thanks! A small improvement is to color the frames differently: Framed[#, FrameStyle -> RandomColor[]] & //@ Thanks Henrik!

Avoid unexpected behavior with ValueQ?

Questions

jerico (J Jesus Rico-Melgoza) November 7, 2016, 7:59pm 13

Excellent - many thanks, Sander!

(As an off topic remark concerning readability: See this discussion ).

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Avoid unexpected behavior with ValueQ?

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