Why Cases and Select treat the List differently?












3















Look at these two examples:



In[1]:=Select[f[i], MatchQ[_f]]

Out[1]:=f



In[2]:=Cases[f[i],_f]

Out[2]:={}


It seems that the Select take $f[i]$ as List, but Cases do not.



I also can not understand why the first gives that answer.



Thanks.






pattern-matching filtering







share|improve this question


















  • 1





    ref / Select / Generalizations&Extensions / #1.

    – Kuba♦
    2 days ago


















3















Look at these two examples:



In[1]:=Select[f[i], MatchQ[_f]]

Out[1]:=f



In[2]:=Cases[f[i],_f]

Out[2]:={}


It seems that the Select take $f[i]$ as List, but Cases do not.



I also can not understand why the first gives that answer.



Thanks.






pattern-matching filtering







share|improve this question


















  • 1





    ref / Select / Generalizations&Extensions / #1.

    – Kuba♦
    2 days ago
















3












3








3


1






Look at these two examples:



In[1]:=Select[f[i], MatchQ[_f]]

Out[1]:=f



In[2]:=Cases[f[i],_f]

Out[2]:={}


It seems that the Select take $f[i]$ as List, but Cases do not.



I also can not understand why the first gives that answer.



Thanks.






pattern-matching filtering







share|improve this question














Look at these two examples:



In[1]:=Select[f[i], MatchQ[_f]]

Out[1]:=f



In[2]:=Cases[f[i],_f]

Out[2]:={}


It seems that the Select take $f[i]$ as List, but Cases do not.



I also can not understand why the first gives that answer.



Thanks.






pattern-matching filtering




pattern-matching filtering






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 2 days ago









XiaoaiXXiaoaiX

1105




1105








  • 1





    ref / Select / Generalizations&Extensions / #1.

    – Kuba♦
    2 days ago
















  • 1





    ref / Select / Generalizations&Extensions / #1.

    – Kuba♦
    2 days ago










1




1





ref / Select / Generalizations&Extensions / #1.

– Kuba♦
2 days ago







ref / Select / Generalizations&Extensions / #1.

– Kuba♦
2 days ago












1 Answer
1






active

oldest

votes


















6















  1. Select preserves the head of the original expression, while Cases always returns the result in a List.


  2. Select operates only at level one, whereas Cases accepts a levelspec.



Specifically your first output is equivalent to these:



Part[f[i], {}]



Delete[f[i], 1]


The second can be made to match by expanding the levelspec to include level zero:



Cases[f[i], _f, {0, 1}]





{f[i]}



Recommended reading:




  • What best practices or performance considerations are there for choosing between Cases, Position, Pick and Select?

  • Head and everything except Head?

  • How to get Cases to wrap results in an arbitrary head?






share|improve this answer



















  • 2





    The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

    – Theo Tiger
    2 days ago











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1 Answer
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active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









6















  1. Select preserves the head of the original expression, while Cases always returns the result in a List.


  2. Select operates only at level one, whereas Cases accepts a levelspec.



Specifically your first output is equivalent to these:



Part[f[i], {}]



Delete[f[i], 1]


The second can be made to match by expanding the levelspec to include level zero:



Cases[f[i], _f, {0, 1}]





{f[i]}



Recommended reading:




  • What best practices or performance considerations are there for choosing between Cases, Position, Pick and Select?

  • Head and everything except Head?

  • How to get Cases to wrap results in an arbitrary head?






share|improve this answer



















  • 2





    The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

    – Theo Tiger
    2 days ago
















6















  1. Select preserves the head of the original expression, while Cases always returns the result in a List.


  2. Select operates only at level one, whereas Cases accepts a levelspec.



Specifically your first output is equivalent to these:



Part[f[i], {}]



Delete[f[i], 1]


The second can be made to match by expanding the levelspec to include level zero:



Cases[f[i], _f, {0, 1}]





{f[i]}



Recommended reading:




  • What best practices or performance considerations are there for choosing between Cases, Position, Pick and Select?

  • Head and everything except Head?

  • How to get Cases to wrap results in an arbitrary head?






share|improve this answer



















  • 2





    The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

    – Theo Tiger
    2 days ago














6












6








6








  1. Select preserves the head of the original expression, while Cases always returns the result in a List.


  2. Select operates only at level one, whereas Cases accepts a levelspec.



Specifically your first output is equivalent to these:



Part[f[i], {}]



Delete[f[i], 1]


The second can be made to match by expanding the levelspec to include level zero:



Cases[f[i], _f, {0, 1}]





{f[i]}



Recommended reading:




  • What best practices or performance considerations are there for choosing between Cases, Position, Pick and Select?

  • Head and everything except Head?

  • How to get Cases to wrap results in an arbitrary head?






share|improve this answer














  1. Select preserves the head of the original expression, while Cases always returns the result in a List.


  2. Select operates only at level one, whereas Cases accepts a levelspec.



Specifically your first output is equivalent to these:



Part[f[i], {}]



Delete[f[i], 1]


The second can be made to match by expanding the levelspec to include level zero:



Cases[f[i], _f, {0, 1}]





{f[i]}



Recommended reading:




  • What best practices or performance considerations are there for choosing between Cases, Position, Pick and Select?

  • Head and everything except Head?

  • How to get Cases to wrap results in an arbitrary head?







share|improve this answer












share|improve this answer



share|improve this answer










answered 2 days ago









Mr.Wizard♦Mr.Wizard

230k294741040




230k294741040








  • 2





    The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

    – Theo Tiger
    2 days ago














  • 2





    The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

    – Theo Tiger
    2 days ago








2




2





The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

– Theo Tiger
2 days ago





The essence of (1) is that Select[f[i], MatchQ[_f]] matches nothing and only returns an empty expression with head f. It is the same as Select[f[i], MatchQ[_somethingelse]]. The recommended answers are very informative btw.

– Theo Tiger
2 days ago


















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