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Filter map for values of None

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty height:90px;width:728px;box-sizing:border-box;

33

5

I've searched around a bit, but haven't found a good answer yet on how to filter out any entries into a map that have a value of None. Say I have a map like this:

val map = Map[String, Option[Int]]("one" -> Some(1), 
 "two" -> Some(2), 
 "three" -> None)

I'd like to end up returning a map with just the ("one", Some(1)) and ("two", Some(2)) pair. I understand that this is done with flatten when you have a list, but I'm not sure how to achieve the effect on a map without splitting it up into keys and values, and then trying to rejoin them.

scala functional-programming hashmap filtering

share|improve this question

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

add a comment | 

33

5

I've searched around a bit, but haven't found a good answer yet on how to filter out any entries into a map that have a value of None. Say I have a map like this:

val map = Map[String, Option[Int]]("one" -> Some(1), 
 "two" -> Some(2), 
 "three" -> None)

I'd like to end up returning a map with just the ("one", Some(1)) and ("two", Some(2)) pair. I understand that this is done with flatten when you have a list, but I'm not sure how to achieve the effect on a map without splitting it up into keys and values, and then trying to rejoin them.

scala functional-programming hashmap filtering

share|improve this question

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

add a comment | 

I've searched around a bit, but haven't found a good answer yet on how to filter out any entries into a map that have a value of None. Say I have a map like this:

val map = Map[String, Option[Int]]("one" -> Some(1), 
 "two" -> Some(2), 
 "three" -> None)

I'd like to end up returning a map with just the ("one", Some(1)) and ("two", Some(2)) pair. I understand that this is done with flatten when you have a list, but I'm not sure how to achieve the effect on a map without splitting it up into keys and values, and then trying to rejoin them.

scala functional-programming hashmap filtering

share|improve this question

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

val map = Map[String, Option[Int]]("one" -> Some(1), 
 "two" -> Some(2), 
 "three" -> None)

share|improve this question

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

share|improve this question

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

asked Aug 7 '12 at 21:30

KChalouxKChaloux

2,22832640

3 Answers
3

active

oldest

votes

35

Like every collection type in the scala.collection namespace a Map has the filter method defined and Optionhas the isDefined method, which is true for Some and false for None. You can filter out the Nonevalues by combining these two:

scala> map.filter(_._2.isDefined)
res4: scala.collection.immutable.Map[String,Option[Int]] = Map(one -> Some(1), two -> Some(2))

share|improve this answer

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Neato. I wouldn't have guessed the syntax though. Still a little confused on _. From what I understand, we're telling it to go to the second item (the value) of the first (current) pair?
  
  – KChaloux
  Aug 7 '12 at 21:47
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @KChaloux In this case _ refers to the first argument of a function literal being passed to the filter method. It's a shorthand for x => x._2.isDefined
  
  – Nikita Volkov
  Aug 7 '12 at 22:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  And the _2 comes from the Tuple.
  
  – fracz
  Nov 1 '15 at 19:48
  

  
  
  
  

add a comment | 

46

If you're filtering out None values, you might as well extract the Some values at the same time to end up with a Map[String,Int]:

scala> map.collect case (key, Some(value)) => (key, value) 
res0: scala.collection.immutable.Map[String,Int] = Map(one -> 1, two -> 2)

share|improve this answer

edited Oct 29 '15 at 15:31

Zoltán

14k867112

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

add a comment | 

6

Also
map.filterKeys( map(_) != None)

or

for( (k,v) <- map if( v!= None)) yield (k,v)

This approach provides a general filterValues method that doesn't exist on maps.

I miss such a method, because none of the alternatives is perfect.

[Updated later] This is a better version that doesn't do a lookup on each entry and still reads reasonably clearly.

map.filter( case (x,y)=> y!=None)

share|improve this answer

edited Aug 16 '16 at 17:21

answered Nov 19 '13 at 20:05

ConorRConorR

33237

add a comment | 

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35

Like every collection type in the scala.collection namespace a Map has the filter method defined and Optionhas the isDefined method, which is true for Some and false for None. You can filter out the Nonevalues by combining these two:

scala> map.filter(_._2.isDefined)
res4: scala.collection.immutable.Map[String,Option[Int]] = Map(one -> Some(1), two -> Some(2))

share|improve this answer

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Neato. I wouldn't have guessed the syntax though. Still a little confused on _. From what I understand, we're telling it to go to the second item (the value) of the first (current) pair?
  
  – KChaloux
  Aug 7 '12 at 21:47
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @KChaloux In this case _ refers to the first argument of a function literal being passed to the filter method. It's a shorthand for x => x._2.isDefined
  
  – Nikita Volkov
  Aug 7 '12 at 22:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  And the _2 comes from the Tuple.
  
  – fracz
  Nov 1 '15 at 19:48
  

  
  
  
  

add a comment | 

35

Like every collection type in the scala.collection namespace a Map has the filter method defined and Optionhas the isDefined method, which is true for Some and false for None. You can filter out the Nonevalues by combining these two:

scala> map.filter(_._2.isDefined)
res4: scala.collection.immutable.Map[String,Option[Int]] = Map(one -> Some(1), two -> Some(2))

share|improve this answer

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Neato. I wouldn't have guessed the syntax though. Still a little confused on _. From what I understand, we're telling it to go to the second item (the value) of the first (current) pair?
  
  – KChaloux
  Aug 7 '12 at 21:47
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  @KChaloux In this case _ refers to the first argument of a function literal being passed to the filter method. It's a shorthand for x => x._2.isDefined
  
  – Nikita Volkov
  Aug 7 '12 at 22:13
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  And the _2 comes from the Tuple.
  
  – fracz
  Nov 1 '15 at 19:48
  

  
  
  
  

add a comment | 

Like every collection type in the scala.collection namespace a Map has the filter method defined and Optionhas the isDefined method, which is true for Some and false for None. You can filter out the Nonevalues by combining these two:

scala> map.filter(_._2.isDefined)
res4: scala.collection.immutable.Map[String,Option[Int]] = Map(one -> Some(1), two -> Some(2))

share|improve this answer

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

scala> map.filter(_._2.isDefined)
res4: scala.collection.immutable.Map[String,Option[Int]] = Map(one -> Some(1), two -> Some(2))

share|improve this answer

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

answered Aug 7 '12 at 21:34

drexindrexin

21.9k35876

46

If you're filtering out None values, you might as well extract the Some values at the same time to end up with a Map[String,Int]:

scala> map.collect case (key, Some(value)) => (key, value) 
res0: scala.collection.immutable.Map[String,Int] = Map(one -> 1, two -> 2)

share|improve this answer

edited Oct 29 '15 at 15:31

Zoltán

14k867112

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

add a comment | 

46

If you're filtering out None values, you might as well extract the Some values at the same time to end up with a Map[String,Int]:

scala> map.collect case (key, Some(value)) => (key, value) 
res0: scala.collection.immutable.Map[String,Int] = Map(one -> 1, two -> 2)

share|improve this answer

edited Oct 29 '15 at 15:31

Zoltán

14k867112

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

add a comment | 

If you're filtering out None values, you might as well extract the Some values at the same time to end up with a Map[String,Int]:

scala> map.collect case (key, Some(value)) => (key, value) 
res0: scala.collection.immutable.Map[String,Int] = Map(one -> 1, two -> 2)

share|improve this answer

edited Oct 29 '15 at 15:31

Zoltán

14k867112

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

scala> map.collect case (key, Some(value)) => (key, value) 
res0: scala.collection.immutable.Map[String,Int] = Map(one -> 1, two -> 2)

share|improve this answer

edited Oct 29 '15 at 15:31

Zoltán

14k867112

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

edited Oct 29 '15 at 15:31

Zoltán

14k867112

edited Oct 29 '15 at 15:31

Zoltán

14k867112

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

answered Aug 7 '12 at 23:01

Kristian DomagalaKristian Domagala

3,1701521

6

Also
map.filterKeys( map(_) != None)

or

for( (k,v) <- map if( v!= None)) yield (k,v)

This approach provides a general filterValues method that doesn't exist on maps.

I miss such a method, because none of the alternatives is perfect.

[Updated later] This is a better version that doesn't do a lookup on each entry and still reads reasonably clearly.

map.filter( case (x,y)=> y!=None)

share|improve this answer

edited Aug 16 '16 at 17:21

answered Nov 19 '13 at 20:05

ConorRConorR

33237

add a comment | 

6

Also
map.filterKeys( map(_) != None)

or

for( (k,v) <- map if( v!= None)) yield (k,v)

This approach provides a general filterValues method that doesn't exist on maps.

I miss such a method, because none of the alternatives is perfect.

[Updated later] This is a better version that doesn't do a lookup on each entry and still reads reasonably clearly.

map.filter( case (x,y)=> y!=None)

share|improve this answer

edited Aug 16 '16 at 17:21

answered Nov 19 '13 at 20:05

ConorRConorR

33237

add a comment | 

Also
map.filterKeys( map(_) != None)

or

for( (k,v) <- map if( v!= None)) yield (k,v)

This approach provides a general filterValues method that doesn't exist on maps.

I miss such a method, because none of the alternatives is perfect.

[Updated later] This is a better version that doesn't do a lookup on each entry and still reads reasonably clearly.

map.filter( case (x,y)=> y!=None)

share|improve this answer

edited Aug 16 '16 at 17:21

answered Nov 19 '13 at 20:05

ConorRConorR

33237

share|improve this answer

edited Aug 16 '16 at 17:21

answered Nov 19 '13 at 20:05

ConorRConorR

33237

answered Nov 19 '13 at 20:05

ConorRConorR

33237

answered Nov 19 '13 at 20:05

ConorRConorR

33237