식의 횡단 순서를 따라서
Echo와 EchoFunction은 패턴 매칭시 식의 요소가 어떤 순서로 되짚어졌는지 보고를 가능하게합니다.
Cases가 식의 1레벨의 각 부분을 찾는 순서를 관찰합니다.
In[1]:=
![Click for copyable input](assets.ko/follow-expression-traversals/In_24.png)
Cases[{a, 2, {7, {1, b}}, 0}, x_ /; NumberQ[Echo[x]]]
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![](assets.ko/follow-expression-traversals/O_21.png)
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![](assets.ko/follow-expression-traversals/O_22.png)
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![](assets.ko/follow-expression-traversals/O_23.png)
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![](assets.ko/follow-expression-traversals/O_24.png)
Out[1]=
![](assets.ko/follow-expression-traversals/O_25.png)
같은 식의 모든 레벨을 찾습니다.
In[2]:=
![Click for copyable input](assets.ko/follow-expression-traversals/In_25.png)
Cases[{a, 2, {7, {1, b}}, 0}, x_ /; NumberQ[Echo[x]], All]
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![](assets.ko/follow-expression-traversals/O_26.png)
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![](assets.ko/follow-expression-traversals/O_27.png)
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![](assets.ko/follow-expression-traversals/O_28.png)
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![](assets.ko/follow-expression-traversals/O_29.png)
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![](assets.ko/follow-expression-traversals/O_30.png)
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![](assets.ko/follow-expression-traversals/O_31.png)
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![](assets.ko/follow-expression-traversals/O_32.png)
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![](assets.ko/follow-expression-traversals/O_33.png)
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![](assets.ko/follow-expression-traversals/O_34.png)
Out[2]=
![](assets.ko/follow-expression-traversals/O_35.png)