# Re: map\_if, collect\_if?

**URL:** <https://www.ruby-forum.com/t/re-map-if-collect-if/57728>\
**Category:** Ruby\
**Created:** [April 13, 2006, 3:00pm UTC](https://www.ruby-forum.com/t/re-map-if-collect-if/57728 "2006-04-13T15:00:46Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![KroegerS\_Simon\_Sext](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/57b2e6/32.png) [@KroegerS\_Simon\_Sext](https://www.ruby-forum.com/u/KroegerS_Simon_Sext)\
**Post date:** [April 13, 2006, 3:00pm UTC](https://www.ruby-forum.com/t/re-map-if-collect-if/57728/1 "2006-04-13T15:00:46Z")

</div>

> > module Enumerable  
> > def map\_if( &block )  
> > find\_all(&block).map(&block)  
> > end  
> > end  
> > […]  
> > I will file away the find\_all {}.map {} idiom for future use  
> > […]

I would change that to:

```
map(&block).select{|e|e}

```

there can be surprising side effects if the user doesn’t  
expect the block to be called twice for each element.

cheers

Simon

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<div class="post-metadata">

**Author:** ![KroegerS\_Simon\_Sext](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/57b2e6/32.png) [@KroegerS\_Simon\_Sext](https://www.ruby-forum.com/u/KroegerS_Simon_Sext)\
**Post date:** [April 13, 2006, 3:25pm UTC](https://www.ruby-forum.com/t/re-map-if-collect-if/57728/2 "2006-04-13T15:25:18Z")

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On 4/13/06, Kroeger, Simon (ext) [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I would change that to:
> 
> ```
> map(&block).select{|e|e}
> 
> ```
> 
> there can be surprising side effects if the user doesn’t  
> expect the block to be called twice for each element.
> 
> cheers
> 
> Simon

Good point.  
This block twice thing felt elegant and wrong in the same place.  
I thaught that perfomance should not be a design issue, but Bruce’s  
point  
was well taken.  
The side effect thing might not strike often, but if it strikes, boy I  
would  
not want to debug that baby.

Nice discussion.

Robert

–  
Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui  
concerne l’univers, je n’en ai pas acquis la certitude absolue.

- Albert Einstein

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<div class="post-metadata">

**Author:** ![KroegerS\_Simon\_Sext](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/57b2e6/32.png) [@KroegerS\_Simon\_Sext](https://www.ruby-forum.com/u/KroegerS_Simon_Sext)\
**Post date:** [April 13, 2006, 3:56pm UTC](https://www.ruby-forum.com/t/re-map-if-collect-if/57728/3 "2006-04-13T15:56:04Z")

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On 4/13/06, Robert D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Ooops

module Enumerable  
def map\_if  
map{ |x| (r = yield(x)) ? r : nil}.compact  
end  
end

that one might do as a comprimise between speed, elegance and sanity?

–  
Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui  
concerne l’univers, je n’en ai pas acquis la certitude absolue.

- Albert Einstein

---

<div class="post-metadata">

**Author:** ![KroegerS\_Simon\_Sext](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/57b2e6/32.png) [@KroegerS\_Simon\_Sext](https://www.ruby-forum.com/u/KroegerS_Simon_Sext)\
**Post date:** [April 13, 2006, 4:33pm UTC](https://www.ruby-forum.com/t/re-map-if-collect-if/57728/4 "2006-04-13T16:33:09Z")

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On Thu, Apr 13, 2006 at 10:53:24PM +0900, Robert D. wrote:

> On 4/13/06, Robert D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:  
> module Enumerable  
> def map\_if  
> map{ |x| (r = yield(x)) ? r : nil}.compact  
> end  
> end
> 
> that one might do as a comprimise between speed, elegance and sanity?

It’s quite slow (two blocks in use…).

If you care a bit about performance (but not enough to use C),

def map\_if(&b)  
a = map(&b)  
a.delete(false) # to get the same semantics as select{|e| e}  
a.compact!  
a  
end

requires half as much mem in the worst case as, and runs ~60% faster  
than  
map(&b).select{|e| e}:

RUBY\_VERSION # =\> “1.8.4”  
module Enumerable  
def map\_if(&b)  
a = map(&b)  
a.delete(false) # to get the same semantics as select{|e| e}  
a.compact!  
a  
end

def map\_if2(&b)  
map(&b).select{|e| e}  
end

def map\_if3  
inject([]){|s,x| (v = yield(x)) ? s \<\< v : s }  
end

def map\_if4  
map{|x| (r = yield(x)) ? r : nil}.compact  
end  
end

require ‘benchmark’

TIMES = 100  
Benchmark.bmbm(10) do |bm|  
arr = (1…10000).to\_a  
bm.report(“compact!”){ TIMES.times{ arr.map\_if{true} } }  
bm.report(“select”){ TIMES.times{ arr.map\_if2{ true} } }  
bm.report(“inject”){ TIMES.times{ arr.map\_if3{ true} } }  
bm.report(“? : test”){ TIMES.times{ arr.map\_if4{ true} } }

# the block wouldn’t let us measure the actual performance:

#bm.report(“compact!”){ TIMES.times{ arr.map\_if{|x| x % 7 == 0 and x}  
} }  
#bm.report(“select”){ TIMES.times{ arr.map\_if2{|x| x % 7 == 0 and x} }  
}  
#bm.report(“inject”){ TIMES.times{ arr.map\_if3{|x| x % 7 == 0 and x} }  
}  
#bm.report(“? : test”){ TIMES.times{ arr.map\_if4{|x| x % 7 == 0 and x}  
} }  
end

# \>\> Rehearsal ---------------------------------------------

# \>\> compact! 0.380000 0.000000 0.380000 ( 0.431046)

# \>\> select 0.650000 0.000000 0.650000 ( 0.725197)

# \>\> inject 4.300000 0.020000 4.320000 ( 4.647794)

# \>\> ? : test 2.400000 0.010000 2.410000 ( 2.551757)

# \>\> ------------------------------------ total: 7.760000sec

# \>\>

# \>\> user system total real

# \>\> compact! 0.390000 0.000000 0.390000 ( 0.408975)

# \>\> select 0.640000 0.010000 0.650000 ( 0.747967)

# \>\> inject 3.880000 0.010000 3.890000 ( 4.211830)

# \>\> ? : test 2.070000 0.000000 2.070000 ( 2.199076)

---

<div class="post-metadata">

**Author:** ![KroegerS\_Simon\_Sext](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/57b2e6/32.png) [@KroegerS\_Simon\_Sext](https://www.ruby-forum.com/u/KroegerS_Simon_Sext)\
**Post date:** [April 13, 2006, 7:08pm UTC](https://www.ruby-forum.com/t/re-map-if-collect-if/57728/5 "2006-04-13T19:08:49Z")

</div>

On 4/13/06, Mauricio F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> end  
> map{|x| (r = yield(x)) ? r : nil}.compact  
> bm.report(“inject”){ TIMES.times{ arr.map\_if3{ true} } }
> 
> # \>\> inject 3.880000 0.010000 3.890000 ( 4.211830)
> 
> # \>\> ? : test 2.070000 0.000000 2.070000 ( 2.199076)
> 
> –  
> Mauricio F. - [http://eigenclass.org](http://eigenclass.org) - singular Ruby
> 
> Very nice work, I just added my original plan to the benchmark, that one  
> with the side effects  
> def map\_if5(&block)  
> find\_all(&block).map(&block)  
> end  
> it is labeled as “2 yields” below. I spare you the rest of the code, I  
> just  
> stole from Mauricio, thx 😉  
> and look at that, it is much faster than I thaught,

but the difference in the relative values also show that these BM are  
not  
really very accurate, I love them though  
as they give some general ideas.

```
            user system total real

```

compact! 0.330000 0.000000 0.330000 ( 0.328067)  
select 0.510000 0.000000 0.510000 ( 0.525555)  
inject 3.160000 0.020000 3.180000 ( 3.461044)  
? : test 1.590000 0.020000 1.610000 ( 1.707898)  
2 yields 0.880000 0.000000 0.880000 ( 0.897847)

–  
Deux choses sont infinies : l’univers et la bêtise humaine ; en ce qui  
concerne l’univers, je n’en ai pas acquis la certitude absolue.

- Albert Einstein