Ruby conditions

Question

I want to take any truthy value from two expressions, for example 2, or take nothing. It can be done as:

if exp1
  a = exp1
elsif exp2
  a = exp2
end

I tried to make it short, and have the following:

a = 1 if exp1|| 2 if exp2

However ruby returns 1 in this case. What is the correct syntax in ruby to do this?

You mean you have one condition and you assign value to `a` according to it? — Marek Lipka, Jul 30 '14 at 13:42
No I have it as I wrote it - 2 'ifs' in the same line , it might be that it can't be done like that ? — Joel Blum, Jul 30 '14 at 13:44
No I need a to take either truthy value from them , or else a to be null and take nothing — Joel Blum, Jul 30 '14 at 13:45
@Joel_Blum are both examples supposed to be equivalent? Where do `1` and `2` come from? — Stefan, Jul 30 '14 at 14:26

Stefan · Answer 1 · 2014-07-30T14:39:58.243

8

This should work:

a = exp1 || exp2 || a

edited Jul 30 '14 at 14:39

answered Jul 30 '14 at 14:18

Stefan

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1

This checks the validity of `a` at first not `exp1`. It's nothing equivalent to `if exp1 a = exp1 elsif exp2 a = exp2 end` – konsolebox Jul 30 '14 at 14:31
@konsolebox you're right, didn't see that. – Stefan Jul 30 '14 at 14:38
1

@konsolebox I've updated my answer, I hope I got it right this time :-) – Stefan Jul 30 '14 at 14:40
Yes. It's simpler than mine. +1 – konsolebox Jul 30 '14 at 14:41

score 1 · Answer 2 · answered Jul 30 '14 at 14:20

1

a = exp1 ? exp1 : exp2 ? exp2 : a

Equivalent to:

if exp1,
    a is set to exp1
else if exp2,
    a is set to exp2
else
    a is set to a, which is virtually equivalent to doing nothing

answered Jul 30 '14 at 14:20

konsolebox

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score 0 · Answer 3 · answered Jul 30 '14 at 13:52

0

The correct syntax:

a = (1 if false) || (2 if true)

answered Jul 30 '14 at 13:52

hypersolid

56
4

I accepted it by mistake... it's not what I needed . I want my variable to keep it's original value if both conditions are falsy but that doesn't happen here.the only way that I can see is the long way I wrote in my question. – Joel Blum Jul 30 '14 at 14:05

score 0 · Answer 4 · answered Jul 30 '14 at 14:54

if exp1
  a = exp1
elsif exp2
  a = exp2
end

Can be shortened to

a = if exp1
  exp1
elsif exp2
  exp2
end

Or, if you prefer one-liners:

a = if exp1 then exp1 elsif exp2 then exp2 end

Any attempt to shorten it even further will change the semantics. For example:

a = exp1 || exp2 || nil

will evaluate exp1 exactly once and exp2 at most once, whereas the original snippet will evaluate exp1 once or twice and exp2 either twice or never.

(To be fair: my example will also change the meaning IFF a appears in exp1. In the OP's original code, an occurrence of a in exp1 will be interpreted as a method call, in my example as a local variable dereference which will evaluate to nil.)

Ruby conditions

4 Answers4