# Multiple assignment in conditional **URL:** \ **Category:** ruby-talk\ **Created:** [31 January 2012 06:46 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737 "2012-01-31T06:46:07Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![GS1](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@GS1](https://rubytalk.org/u/GS1)\ **Post date:** [31 January 2012 06:46 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/1 "2012-01-31T06:46:07Z")
I find this a strange Ruby error. foo = [1,2] # The following is fine. if (a = foo)   puts a end # The following causes a SyntaxError: multiple assignment in conditional. if (a, b = foo)   puts a end Why would that be a syntax error? Surely the grammar is   if EXPR     ... and (a, b = foo) is an expression, isn't it? Just curious, though my curiosity was prompted by a failing piece of real code. Gavin ---
**Author:** ![Josh\_Cheek](https://avatars.discourse-cdn.com/v4/letter/j/e79b87/32.png) [@Josh\_Cheek](https://rubytalk.org/u/Josh_Cheek)\ **Post date:** [31 January 2012 07:22 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/2 "2012-01-31T07:22:31Z")
What did you expect your real code to do? It seems to me that it should raise an error unless it's an array of size 2, and if it is, then it should return an array of size two, making the conditional necessarily true. So I don't know what you were expecting to happen. > **路路路** > > On Tue, Jan 31, 2012 at 12:46 AM, Gavin Sinclair \wrote: > > > I find this a strange Ruby error. > > > > foo = [1,2] > > > > # The following is fine. > > if (a = foo) > > puts a > > end > > > > # The following causes a SyntaxError: multiple assignment in conditional. > > if (a, b = foo) > > puts a > > end > > > > Why would that be a syntax error? Surely the grammar is > > > > if EXPR > >    ... > > > > and (a, b = foo) is an expression, isn't it? > > > > Just curious, though my curiosity was prompted by a failing piece of real > > code. > > > > Gavin ---
**Author:** ![GD1](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@GD1](https://rubytalk.org/u/GD1)\ **Post date:** [31 January 2012 07:57 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/3 "2012-01-31T07:57:17Z")
Hi Gavin, > I find this a strange Ruby error. > > foo = [1,2] > > # The following is fine. > if (a = foo) >    puts a > end > > # The following causes a SyntaxError: multiple assignment in conditional. > if (a, b = foo) >    puts a > end > > Why would that be a syntax error? Surely the grammar is > >    if EXPR >      ... > > and (a, b = foo) is an expression, isn't it? > > Just curious, though my curiosity was prompted by a failing piece of real > code. I can understand why the language rejects that construct (I've never tried it myself- an interesting find!). The method itself would be returning an array (correct?), and an array would always be treated as true in the expression, regardless of the results. Thus, it is likely to be some sort of error (why put an always-statically-true expression in a conditional?), and hence knocks it back with an error. I'm wondering as to the best behaviour if it were actually handled directly by the language (which I'm not specifically suggesting). True if any of the multiple assignments evaluate to true? Or just the first? Or last? Certainly not always-true. I can't say I'd know a sensible default. 馃槙 Garth > **路路路** > > On 31/01/12 17:16, Gavin Sinclair wrote: ---
**Author:** ![botp1](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@botp1](https://rubytalk.org/u/botp1)\ **Post date:** [31 January 2012 12:38 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/4 "2012-01-31T12:38:48Z")
because it's a parallel assignment and ruby would not know which to check, a or b, no? try eg,   a,b=true or   a,b,c=1 kind regards -botp > **路路路** > > On Tue, Jan 31, 2012 at 2:46 PM, Gavin Sinclair \ wrote: > > > # The following causes a SyntaxError: multiple assignment in conditional. > > if (a, b = foo) > > puts a > > end ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [31 January 2012 22:16 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/5 "2012-01-31T22:16:54Z")
> I find this a strange Ruby error. > > foo = [1,2] > > # The following is fine. > if (a = foo) > puts a > end > > # The following causes a SyntaxError: multiple assignment in conditional. > if (a, b = foo) > puts a > end > > Why would that be a syntax error? Surely the grammar is > > if EXPR > ... > > and (a, b = foo) is an expression, isn't it? Yeah, I'd agree. But why write such obfuscated code? > Just curious, though my curiosity was prompted by a failing piece of real > code. Well, there is no point in having assignments in conditionals - unless it's a loop in which case the code often becomes more elegant, e.g.: while ( line = gets )   puts line end For a simple if and unless there is really no advantage in having an assignment in the condition. Kind regards robert > **路路路** > > On Tue, Jan 31, 2012 at 7:46 AM, Gavin Sinclair \ wrote: > > -- > remember.guy do |as, often| as.you\_can - without end > [http://blog.rubybestpractices.com/](http://blog.rubybestpractices.com/) ---
**Author:** ![Nikolai\_Weibull2](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@Nikolai\_Weibull2](https://rubytalk.org/u/Nikolai_Weibull2)\ **Post date:** [31 January 2012 08:27 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/6 "2012-01-31T08:27:25Z")
> > # The following causes a SyntaxError: multiple assignment in conditional. > > if (a, b = foo) > I can understand why the language rejects that construct (I've never tried > it myself- an interesting find!). The method itself would be returning an > array (correct?), and an array would always be treated as true in the > expression, regardless of the results. Thus, it is likely to be some sort of > error (why put an always-statically-true expression in a conditional?), and > hence knocks it back with an error. if true   puts 'well that鈥檚 a relief' end > I'm wondering as to the best behaviour if it were actually handled directly > by the language (which I'm not specifically suggesting). True if any of the > multiple assignments evaluate to true? Or just the first? Or last? Certainly > not always-true. I can't say I'd know a sensible default. 馃槙 [a, b, 鈥.any? > **路路路** > > On Tue, Jan 31, 2012 at 08:57, Garthy D \ wrote: > > > On 31/01/12 17:16, Gavin Sinclair wrote: ---
**Author:** ![botp1](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@botp1](https://rubytalk.org/u/botp1)\ **Post date:** [31 January 2012 12:55 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/7 "2012-01-31T12:55:25Z")
ok, ignore. i get gavin's point regarding expression > **路路路** > > On Tue, Jan 31, 2012 at 8:38 PM, botp \ wrote: > > > try eg, > > > > a,b=true > > or > > a,b,c=1 ---
**Author:** ![botp1](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@botp1](https://rubytalk.org/u/botp1)\ **Post date:** [1 February 2012 00:22 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/8 "2012-02-01T00:22:35Z")
i dont know, Robert, since i also find   while ( line,x,y = get\_something) also elegant. if ruby would support that, wouldn't it be elegant? best regards -botp > **路路路** > > On Wed, Feb 1, 2012 at 6:16 AM, Robert Klemme \ wrote: > > > it's a loop in which case the code often becomes more elegant, e.g.: > > > > while ( line = gets ) ---
**Author:** ![GS1](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@GS1](https://rubytalk.org/u/GS1)\ **Post date:** [1 February 2012 11:49 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/9 "2012-02-01T11:49:47Z")
> > # The following causes a SyntaxError: multiple assignment in conditional. > > if (a, b = foo) > > puts a > > end > What did you expect your real code to do? It seems to me that it should > raise an error unless it's an array of size 2 Nope. It can return anything at all and Ruby will deal with it. Parallel assignment is (thankfully) flexible. def foo; 1; end a, b = foo # returns 1; a == 1, b == nil def bar; [1,2,3,4,5]; end a, b = bar # returns [1,2,3,4,5]; a == 1, b == 2 > and if it is, then it should > return an array of size two, making the conditional necessarily true. Nope. The conditional can be false. def foo; nil; end a, b = foo # returns nil; a == nil, b == nil > So I don't know what you were expecting to happen. def foo; nil; end if (a, b = foo)   puts "outcome 1" else   puts "outcome 2" # I was expecting this. end Of course I don't have a function that returns nil unconditionally! Just showing the structure. > **路路路** > > On Tue, Jan 31, 2012 at 6:22 PM, Josh Cheek \ wrote: ---
**Author:** ![GS1](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@GS1](https://rubytalk.org/u/GS1)\ **Post date:** [1 February 2012 11:57 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/10 "2012-02-01T11:57:55Z")
Here's the code with the thing that doesn't work. Of course you don't know the context, but I hope this is enlightening.   # week\_and\_day(date) -\> [17,1] week 17, day 1 (Mon)   # Returns nil if the given date is not in this term.   def week\_and\_day(date)     if not self.include? date       return nil     elsif (week, day = @t1.week\_and\_day(date))       return [week, day]     elsif (week, day = @t2.week\_and\_day(date))       week += @t1.number\_of\_weeks       return [week, day]     end   end I don't see anything obfuscated about that. @t1.week\_and\_day(date) will return nil if the date is outside the range of @t1, so we will go to the next part of the conditional. Here it is in acceptable Ruby.   # week\_and\_day(date) -\> [17,1] week 17, day 1 (Mon)   # Returns nil if the given date is not in this term.   def week\_and\_day(date)     if not self.include? date       return nil     elsif (week\_and\_day = @t1.week\_and\_day(date))       return week\_and\_day     elsif (week\_and\_day = @t2.week\_and\_day(date))       week, day = week\_and\_day       week += @t1.number\_of\_weeks       return [week, day]     end   end This one has a clumsy variable name "week\_and\_day" and an extra line to unpack it into "week" and "day". I'm not crying mercy over one extra line, but I definitely prefer the first version. It seems more intentional. Gavin > **路路路** > > On Wed, Feb 1, 2012 at 9:16 AM, Robert Klemme \ wrote: > > > Yeah, I'd agree. But why write such obfuscated code? ---
**Author:** ![GD1](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@GD1](https://rubytalk.org/u/GD1)\ **Post date:** [31 January 2012 08:41 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/11 "2012-01-31T08:41:22Z")
Hi Nikolai, > > > # The following causes a SyntaxError: multiple assignment in conditional. > > > if (a, b = foo) > > > I can understand why the language rejects that construct (I've never tried > > it myself- an interesting find!). The method itself would be returning an > > array (correct?), and an array would always be treated as true in the > > expression, regardless of the results. Thus, it is likely to be some sort of > > error (why put an always-statically-true expression in a conditional?), and > > hence knocks it back with an error. > > if true >    puts 'well that鈥檚 a relief' > end Haha, I \*knew\* someone would call me out on that one if I wasn't more specific. What I didn't expect was how little time it'd take. 馃槈 To clarify: If "if (a, b = foo)" would always evaluate to true, it might cause confusion for someone expecting it to base the evaluation on one of the assigned variables; whereas the intent for "if true" is pretty clear; it's an expression with less than two assignments. Still, it's not necessarily a strong argument or one I'd get behind. I'm just saying that I understand why it might have turned out that way. Maybe the potential for confusion led to that particular design decision? > > I'm wondering as to the best behaviour if it were actually handled directly > > by the language (which I'm not specifically suggesting). True if any of the > > multiple assignments evaluate to true? Or just the first? Or last? Certainly > > not always-true. I can't say I'd know a sensible default. 馃槙 > > [a, b, 鈥.any? Indeed, that's another possibility, although I'll point out that it's inconsistent with an empty array normally being treated as true in an ordinary expression. But hey, we are working with an odd case here, so why not? 馃檪 Garth > **路路路** > > On 31/01/12 18:57, Nikolai Weibull wrote: > > > On Tue, Jan 31, 2012 at 08:57, Garthy D \> \ wrote: > > > > > On 31/01/12 17:16, Gavin Sinclair wrote: ---
**Author:** ![Josh\_Cheek](https://avatars.discourse-cdn.com/v4/letter/j/e79b87/32.png) [@Josh\_Cheek](https://rubytalk.org/u/Josh_Cheek)\ **Post date:** [1 February 2012 00:28 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/12 "2012-02-01T00:28:23Z")
I don't think it would be elegant, because the relationship between the condition and the assignment is obscure. I would assume it's a shorthand for `loop { line, x, y = get_something; ... }` but would have to look at it for a while and then run some experiments to see. > **路路路** > > On Tue, Jan 31, 2012 at 6:22 PM, botp \ wrote: > > > On Wed, Feb 1, 2012 at 6:16 AM, Robert Klemme \> \ wrote: > > \> it's a loop in which case the code often becomes more elegant, e.g.: > > \> > > \> while ( line = gets ) > > > > i dont know, Robert, since i also find > > > > while ( line,x,y = get\_something) > > > > also elegant. > > if ruby would support that, wouldn't it be elegant? > > > > best regards -botp ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [1 February 2012 07:48 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/13 "2012-02-01T07:48:14Z")
I find something.each do |line, x, y|   ... end more elegant and idiomatic Ruby. That's why I typically use ARGF.each do |line|   ... end instead of while ( line = gets ). Kind regards robert > **路路路** > > On Wed, Feb 1, 2012 at 1:22 AM, botp \ wrote: > > > On Wed, Feb 1, 2012 at 6:16 AM, Robert Klemme \> \ wrote: > > > > > it's a loop in which case the code often becomes more elegant, e.g.: > > > > > > while ( line = gets ) > > > > i dont know, Robert, since i also find > > > > while ( line,x,y = get\_something) > > > > also elegant. > > if ruby would support that, wouldn't it be elegant? > > -- > remember.guy do |as, often| as.you\_can - without end > [http://blog.rubybestpractices.com/](http://blog.rubybestpractices.com/) ---
**Author:** ![GS1](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@GS1](https://rubytalk.org/u/GS1)\ **Post date:** [1 February 2012 12:05 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/14 "2012-02-01T12:05:57Z")
Sorry to reply to my own message, but I thought of something I should have included. Here is the forbidden version I would like to write. Obviously I'm combining assignment and conditional, which may not win plaudits as a general rule. > def week\_and\_day(date) > if not self.include? date > return nil > elsif (week, day = @t1.week\_and\_day(date)) > return [week, day] > elsif (week, day = @t2.week\_and\_day(date)) > week += @t1.number\_of\_weeks > return [week, day] > end > end But consider this alternative, where I separate the assignment and the conditional.   def week\_and\_day(date)     if not self.include? date       return nil     else       week, day = @t1.week\_and\_day(date)       if week         return [week, day]       else         week, day = @t2.week\_and\_day(date)         week += @t1.number\_of\_weeks         return [week, day]       end     end   end I consider this very clumsy. By \_not\_ combining assignment with conditional, I cannot use "elsif". I must split the "else" and the "if", and turn the second conditional into a child instead of a sibling. Definitely less intentional and unattractive code. Gavin > **路路路** > > On Wed, Feb 1, 2012 at 10:57 PM, Gavin Sinclair \ wrote: ---
**Author:** ![Joel\_VanderWerf](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Joel\_VanderWerf](https://rubytalk.org/u/Joel_VanderWerf)\ **Post date:** [1 February 2012 20:15 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/15 "2012-02-01T20:15:13Z")
You can trick ruby: def foo; nil; end if (\_ = (a, b = foo))    puts "outcome 1" else    puts "outcome 2" end So, the question is parsing, not semantics. > **路路路** > > On 02/01/2012 03:49 AM, Gavin Sinclair wrote: > > > def foo; nil; end > > > > if (a, b = foo) > >    puts "outcome 1" > > else > >    puts "outcome 2" # I was expecting this. > > end ---
**Author:** ![Josh\_Cheek](https://avatars.discourse-cdn.com/v4/letter/j/e79b87/32.png) [@Josh\_Cheek](https://rubytalk.org/u/Josh_Cheek)\ **Post date:** [1 February 2012 21:03 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/16 "2012-02-01T21:03:28Z")
Sorry, but I find this incredibly opaque. Could you just write what you expect the evaluation to be in each of these cases? a, b = 1, 1 a, b = 1, nil a, b = nil, 1 a, b = nil, nil I'd expect return values of [1, 1], [1, nil], [nil, 1], [nil, nil] which are all true. Now does this change when the RHS is an array instead of two values? For example, would `a, b = [1, 1]` return a different value than `a, b = [1, 1]`? Your example seems particularly devious in that it only has one value on the RHS of the assignment. This means that the second argument will be assigned nil (because there is no value for it), and because assignment operators always return the RHS, it will then return whatever single value was over there. This causes it to look like you are assigning multiple values to the variables and having it return nil, when in fact you are only assigning one value (the second variable is along for the ride and will always be nil). There are so many nuanced ruby edge cases one must know (and then must also know the implementation of how it is being returned) to understand that example that I think it has more overhead than it is worth. > **路路路** > > On Wed, Feb 1, 2012 at 5:49 AM, Gavin Sinclair \ wrote: > > > On Tue, Jan 31, 2012 at 6:22 PM, Josh Cheek \ wrote: > > > > \>\> # The following causes a SyntaxError: multiple assignment in > > conditional. > > \>\> if (a, b = foo) > > \>\> puts a > > \>\> end > > > > \> What did you expect your real code to do? It seems to me that it should > > \> raise an error unless it's an array of size 2 > > > > Nope. It can return anything at all and Ruby will deal with it. > > Parallel assignment is (thankfully) flexible. > > > > def foo; 1; end > > a, b = foo # returns 1; a == 1, b == nil > > > > def bar; [1,2,3,4,5]; end > > a, b = bar # returns [1,2,3,4,5]; a == 1, b == 2 > > > > \> and if it is, then it should > > \> return an array of size two, making the conditional necessarily true. > > > > Nope. The conditional can be false. > > > > def foo; nil; end > > a, b = foo # returns nil; a == nil, b == nil > > > > \> So I don't know what you were expecting to happen. > > > > def foo; nil; end > > > > if (a, b = foo) > >   puts "outcome 1" > > else > > puts "outcome 2" # I was expecting this. > > end > > > > Of course I don't have a function that returns nil unconditionally! > > Just showing the structure. ---
**Author:** ![Robert\_K1](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/robert_k1/32/1830_2.png) [@Robert\_K1](https://rubytalk.org/u/Robert_K1)\ **Post date:** [1 February 2012 12:33 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/17 "2012-02-01T12:33:42Z")
There are plenty more solutions, for example: def week\_and\_day(date)    return nil unless include? date    week\_and\_day = @t1.week\_and\_day(date) and return week\_and\_day    week\_and\_day = @t2.week\_and\_day(date)    week\_and\_day[0] += @t1.number\_of\_weeks if week\_and\_day    week\_and\_day end Maybe also def week\_and\_day(date)    return nil unless include? date    case    when week\_and\_day = @t1.week\_and\_day(date)      nil    when week\_and\_day = @t2.week\_and\_day(date)      week\_and\_day[0] += @t1.number\_of\_weeks    end    week\_and\_day end Kind regards robert > **路路路** > > On Wed, Feb 1, 2012 at 1:05 PM, Gavin Sinclair \ wrote: > > > On Wed, Feb 1, 2012 at 10:57 PM, Gavin Sinclair \ wrote: > > > > Sorry to reply to my own message, but I thought of something I should > > have included. > > > > Here is the forbidden version I would like to write. Obviously I'm > > combining assignment and conditional, which may not win plaudits as a > > general rule. > > > > > def week\_and\_day(date) > > > if not self.include? date > > > return nil > > > elsif (week, day = @t1.week\_and\_day(date)) > > > return [week, day] > > > elsif (week, day = @t2.week\_and\_day(date)) > > > week += @t1.number\_of\_weeks > > > return [week, day] > > > end > > > end > > > > But consider this alternative, where I separate the assignment and the > > conditional. > > > > def week\_and\_day(date) > > if not self.include? date > > return nil > > else > > week, day = @t1.week\_and\_day(date) > > if week > > return [week, day] > > else > > week, day = @t2.week\_and\_day(date) > > week += @t1.number\_of\_weeks > > return [week, day] > > end > > end > > end > > > > I consider this very clumsy. By \_not\_ combining assignment with > > conditional, I cannot use "elsif". I must split the "else" and the > > "if", and turn the second conditional into a child instead of a > > sibling. > > > > Definitely less intentional and unattractive code. > > -- > remember.guy do |as, often| as.you\_can - without end > [http://blog.rubybestpractices.com/](http://blog.rubybestpractices.com/) ---
**Author:** ![botp1](https://avatars.discourse-cdn.com/v4/letter/b/6de8d8/32.png) [@botp1](https://rubytalk.org/u/botp1)\ **Post date:** [2 February 2012 02:56 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/18 "2012-02-02T02:56:00Z")
> You can trick ruby: > > def foo; nil; end > > if (\_ = (a, b = foo)) > > puts "outcome 1" > else > puts "outcome 2" > end > > So, the question is parsing, not semantics. indeed. ruby just checks the last assignment. > if (x,y=(a, b = foo)) > p 1 > end SyntaxError: (irb):17: multiple assignment in conditional but i do not know why it has to do that checking. too much work. and could probably slow down ruby. kind regards -botp > **路路路** > > On Thu, Feb 2, 2012 at 4:15 AM, Joel VanderWerf \ wrote: ---
**Author:** ![GS1](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@GS1](https://rubytalk.org/u/GS1)\ **Post date:** [2 February 2012 11:45 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/19 "2012-02-02T11:45:55Z")
> Could you just write what you expect the evaluation to be in each of these > cases? > > a, b = 1, 1 > a, b = 1, nil > a, b = nil, 1 > a, b = nil, nil > > I'd expect return values of [1, 1], [1, nil], [nil, 1], [nil, nil] which > are all true. Yes, I expect what you expect. Also,   a, b = nil # a and b both get the value nil > Now does this change when the RHS is an array instead of two values? > For example, would `a, b = [1, 1]` return a different value than `a, b = > [1, 1]`? No, because you wrote them exactly the same! Assuming you meant to compare these two:   a, b = 1, 1   a, b = [1, 1] then I expect them to behave exactly the same. I can see why you expect it to set a to [1,1] and b to nil, but that's just not the way it is. If I'm not wrong, Ruby actually converts the first line above into the second line behind the scenes. After all, that's precisely how you return multiple values from a method, or at least give the appearance of doing so.   def two\_values(x)     [x\*2, x^2]   end   a, b = two\_values(5) From faint memory, to achieve the same thing in Perl you must wrap the LHS in an array. Ruby's equivalent would be   [a, b] = two\_values(5) # This doesn't work in Ruby, and it's not missed. > Your example seems particularly devious in that it only has one value on > the RHS of the assignment. No it doesn't. That's not a value, per se, it's a method call. The method call returns either an array, which assigns the two variables and activates the "if" branch, or nil, which activates the "else" branch. I didn't approach this being devious or thinking it would be at all controversial. I simply have a good intuition for assignment in Ruby. > This means that the second argument will be > assigned nil (because there is no value for it), No. Try this in irb:   if \_ = (a, b = [5,2,16,100])     puts a + b   end And this.   a, b = nil   a, b = 1   a, b = 1, 2   a, b = [1, 2]   a, b = [1,2,3,4,5]   a, \*b = nil   a, \*b = 1   a, \*b = 1, 2   a, \*b = [1, 2]   a, \*b = [1,2,3,4,5] and see what these assignment operators return and what values get set to a and b. > [...] > > There are so many nuanced ruby edge cases one must know (and then must also > know the implementation of how it is being returned) to understand that > example that I think it has more overhead than it is worth. There isn't a single nuance [1]; it all follows from understanding parallel assignment in Ruby. I didn't go out of my way to learn that. It must have been in one of the early Ruby books I read, or something, and something I use all the time (especially multiple returns from methods). [1] in 1.9.3. The earlier versions have the nuance of   a, b = nil # Returns [nil] instead of nil. That pre-1.9.3 behaviour is contrary to what I've always expected and I'm glad to see it corrected. I can only surmise that the syntax error "multiple assignment in conditional" is a hangover from those days as a (fair) warning that it's not going to do what you expect, and no good can come of it. They've changed the behaviour, but left the completely unnecessary syntax error. Gavin > **路路路** > > On Thu, Feb 2, 2012 at 8:03 AM, Josh Cheek \ wrote: ---
**Author:** ![GS1](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@GS1](https://rubytalk.org/u/GS1)\ **Post date:** [2 February 2012 11:55 UTC](https://rubytalk.org/t/multiple-assignment-in-conditional/64737/20 "2012-02-02T11:55:40Z")
Done and done!   def week\_and\_day(date)     if not self.include? date       nil     elsif \_ = (week, day = @t1.week\_and\_day(date))       [week, day]     elsif \_= (week, day = @t2.week\_and\_day(date))       [week + @t1.number\_of\_weeks, day]     end   end I guess I need to document a Ruby 1.9.3+ requirement 馃檪 > **路路路** > > On Thu, Feb 2, 2012 at 7:15 AM, Joel VanderWerf \ wrote: > > > You can trick ruby: > > > > def foo; nil; end > > > > if (\_ = (a, b = foo)) [...] [Next page](https://rubytalk.org/t/multiple-assignment-in-conditional/64737.md?page=2)