Excellent - many thanks, Sander!

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

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

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 return missing:

hash = <||>;
Do[AssociateTo[hash, i -> i], {i, 1, 10, 2}]
Table[If[MissingQ[hash[i]], "No value", hash[i]], {i, 10}]

returns the correct result. This might even be a faster and more flexible solution.

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

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 think the Hold interferes with Table, but I'm not 100% sure what the exact problem is. Reading a 'trace' is never easy:

Trace[Table[If[ValueQ[f[i]], f[i], "No value"], {i, 2}]]

gives:

{Table[If[ValueQ[f[i]],f[i],No value],{i,2}],{{ValueQ[f[i]],!Hold[Evaluate[f[i]]]===Hold[f[i]],{{{{i,1},f[1],1},Hold[1]},Hold[1]===Hold[f[i]],False},!False,True},If[True,f[i],No value],f[i],{i,1},f[1],1},{{ValueQ[f[i]],!Hold[Evaluate[f[i]]]===Hold[f[i]],{{{{i,2},f[2]},Hold[f[2]]},Hold[f[2]]===Hold[f[i]],False},!False,True},If[True,f[i],No value],f[i],{i,2},f[2]},{1,f[2]}}