The difference between ? and /;

Question

I can do the following

Cases[Range@10, x_ /; OddQ[x]]

But why cannot I do x_?OddQ[x] instead of _?OddQ in the following?

Cases[Range@10, x_ ? OddQ[x]]

Generally speaking, I am confused with using ? and /;.

Edit

After reading the existing comments and answer, I understood now that ? needs function. However, it still confused me, when we prefer ? to /; and vice versa?

I don't really understand the question. Did you read the documentation? ? must be followed by a function and OddQ[x] is not a function. This question feels a bit like "I can compute a sum with Sum, but why can't I compute a sum with Product?" — Szabolcs, Sep 02 '20 at 08:00
PatternTest >> Properties and relations: "PatternTest applies test functions to patterns, which need not have names" and "Condition evaluates a Boolean expression on named parts of a pattern" — kglr, Sep 02 '20 at 08:10
possible duplicate: Using a PatternTest versus a Condition for pattern matching — kglr, Sep 02 '20 at 08:20

AsukaMinato · Accepted Answer · 2020-09-02T12:50:32.260

2

If you should use OddQ[x]

Cases[Range@10, x_?(x \[Function] OddQ[x])]

Edit

Thanks to the comment for pointing it out, the x in (x \[Function] OddQ[x]) is irrelevant to the first x, you can replace it with any other vars.

In other words, (x \[Function] f[x]) == Function[x,f[x]] == f[#]& == f

edited Sep 02 '20 at 12:50

answered Sep 02 '20 at 08:13

AsukaMinato

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This is basically identical to what @Montevideo showed (use a pure function), but I find this version to be potentially misleading: you use x for two entire separate an unrelated purposes. Someone who is already confused about the difference between /; and ? could very well end up being even more confused, and believing that there is some connection between those two xs. – Szabolcs Sep 02 '20 at 08:52

score 0 · Answer 2 · answered Sep 02 '20 at 08:00

0

You need to do this to solve this problem:

Cases[Range@10, x_?(OddQ[#] &)]
Cases[Range@10, x_?(OddQ)]

You may have confused pattern matching and conditional judgment functions.

answered Sep 02 '20 at 08:00

A little mouse on the pampas

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(OddQ[#] &) is OddQ , nearly no difference. – AsukaMinato Sep 02 '20 at 08:11
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Cases[Range[10], _?OddQ] – cvgmt Sep 02 '20 at 09:06

The difference between ? and /;

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2 Answers2

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