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











Your Answer





StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "387"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathematica.stackexchange.com%2fquestions%2f189270%2fwhy-cases-and-select-treat-the-list-differently%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























1 Answer
1






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


















draft saved

draft discarded




















































Thanks for contributing an answer to Mathematica Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid …



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmathematica.stackexchange.com%2fquestions%2f189270%2fwhy-cases-and-select-treat-the-list-differently%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







-

Popular posts from this blog

If I really need a card on my start hand, how many mulligans make sense? [duplicate]

Image
5 This question already has an answer here: How do you calculate the likelihood of drawing certain cards in your opening hand? 3 answers There is a funny modern deck where you play Treasure Hunt, but you really need to have it on the starting hand. How many mulligans should I take, if my deck contains 60 cards and contains 4 copies of Treasure Hunt? magic-the-gathering probability share | improve this question asked Nov 23 '18 at 12:12 dan dan 152 4
Read more

Alcedinidae

Image
Alcedinidae Martin-pêcheur à dos bleu ( Alcedo azurea ) Classification (COI) Règne Animalia Embranchement Chordata Sous-embr. Vertebrata Classe Aves Ordre Coraciiformes Famille Alcedinidae Rafinesque, 1815 Les Alcédinidés ou Alcedinidae forment une famille d'oiseaux plus connus sous les noms de martins-pêcheurs et martins-chasseurs. Sommaire 1 Description 2 Répartition et habitat 3 Systématique 3.1 Liste alphabétique des genres 3.2 Liste des espèces 4 Chez les Anciens : l'alcyon 4.1 Étymologie (Littré) 4.2 Oiseau « fabuleux » ? 5 Notes 6 Articles connexes 7 Liens externes 7.1 Bibliographie Description | Ce sont des oiseaux compacts de taille petite à moyenne (10 à 46 cm), au bec droit et long, en forme de poignard. Leurs pattes sont courtes, et ils portent un plumage aux couleurs vives. Répartition et habitat | Cosmopolites, ils fréquentent surtout
Read more

Can an atomic nucleus contain both particles and antiparticles? [duplicate]

Image
9 $begingroup$ This question already has an answer here: What happens if we put together a proton and an antineutron? 2 answers Is it theoretically possible to make a "deuterium" atom containing a proton and an antineutron in its nucleus? Would the strong nuclear force cause attraction between a proton and an antineutron? Would such a nucleus be stable, or would the proton somehow annihilate the antineutron when close enough? nuclear-physics antimatter share | cite | improve this question asked yesterday
Read more