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Let's say I want to define a function indexed by an natural number $g$, which is given by $$f_g(x)=g+x$$

I could (I guess naively) write the following code to define it up until $g=19$

For[g = 1, g < 20, g++, f[g] = g + # &]

But, for some reason, this doesn't work. If you try to evaluate, let's say

f[3]
f[3][10]

you see that the use of # delays the replacement of $g$ until you give the function an argument. $f_3(10)$ above returns $30$ for example, since that's the value of $g$ at that point in the code.

Now, I am aware I could just define a two-variable function (I think that's actually doable in the original problem I'm working on too) but I would still like to understand what is going on here and how I can get around this without actually avoiding the problem altogether.

Thank you all.

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  • $\begingroup$ Looking at your first equation: f3(10) should be 3+10=13? But later on you want it evaluated as 30? $\endgroup$
    – Syed
    Apr 3, 2022 at 16:43
  • $\begingroup$ I didn't want it evaluated as $30$, but it does so anyway. I wanted $13$, which is not what happens with the above code. This specific question got solved in the answer below, but the solution doesn't quite work for my original code, so I asked a new question here $\endgroup$ Apr 3, 2022 at 16:56

2 Answers 2

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It's somewhat interesting what exactly gets held here (as in Hold). The anonymous function holds its arguments unevaluated, so:

For[g = 1, g < 20, g++, f[g] = g + # &]

Assigns g + # & to f[1] through f[19], in exactly the form.

You could change this to:

For[g = 1, g < 20, g++, f[g] = Evaluate[g + #] &]

And it will subsequently work, as f[1] becomes 1 + # &, f[2] becomes 2 + # & and so on. Note that operator precedence around Evaluate and & can be somewhat non-intuitive. For example, (Evaluate[g] + #) & gives a somewhat unexpected form which holds the Evaluate term in its entirety. I suspect this may be explained somewhere, but I've not personally encountered where.

More generically, you may also want to consider one of the following:

f[g_] := g + # &
f[g_?IntegerQ] := g + # &
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  • $\begingroup$ This exactly what I was looking for, some way to "force" the evaluation to take precedence, but I couldn't figure it out myself. Thank you! $\endgroup$ Apr 3, 2022 at 16:21
  • $\begingroup$ Never mind, it still doesn't work for my original problem. Evaluate just stays there and doesn't do anything. I've asked a new question here with exactly the problem that I'm facing. $\endgroup$ Apr 3, 2022 at 16:55
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Let me know if this is what you intended?

enter image description here

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  • $\begingroup$ Yes that works too. And thank you for that neat "indexed argument" notation, which I did not know :) $\endgroup$ Apr 3, 2022 at 17:09
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    $\begingroup$ Nice and new to me too. Is there any hidden problem with this as I read that people try to avoid subscript whenever you can? $\endgroup$
    – hana
    Apr 3, 2022 at 18:13
  • $\begingroup$ Subscript[f, 1][x] // Head returns Plus. And should it not have returned f1? $\endgroup$
    – Syed
    Apr 3, 2022 at 18:27

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