Question

我们需要在<条码>内使用<条码>If,例如<条码> 可使用[If[i<3, i],{i, 5}]。

但我希望结果为<代码>{1,2}。

这样做有什么办法?

EDIT:
What if we consider Table[If[i< 3, f[i]], {i, 5}] which gives {f[1], f[2], Null, Null, Null}

Answer 1

Concisely:

Table[If[i < 3, i, ## &[]], {i, 5}]

This works because the function ## & does not immediately evaluate.

## & is a "vanishing" function.

{1, 2, ## &[], 3, 4}

----> {1, 2, 3, 4}

See SlotSequence , for more information.

Answer 2

If you need to remove it from an existing list, you can use

DeleteCases[list, Null]

或

list /. Null -> Sequence[]

(a bit m或e advanced).

关于您的<代码>可下载,例如,首先指出,<代码>If中的第二 com是不必要的(甚至见pink):

list = Table[If[i < 3, i], {i, 5}]

To filter the table elements by a condition, you might want to use something similar to

list = Select[Table[i, {i, 5}], # < 3 &]

instead.

Finally, if you need to generate the list without ever adding rejected elements to it (to save mem或y), I suggest using Reap and Sow:

Reap@Do[If[i < 3, Sow[i]], {i, 5}]
list = %[[2, 1]]

I haven t actually verified the mem或y usage of this compared to a plain Table, and note that if you generate only numbers, which can be st或ed in a packed array, the Table construct may be m或e mem或y efficient. On the other hand if you generate a truly huge number of generic expressions, the maj或ity of which will be rejected in If, Sow / Reap may be better.

Answer 3

作为一种替代办法,您可使用这一回答,该回答专门针对有条件的表层建设。这里将如何研究:

In[12]:= tableGenAltMD[i,{i,5},#<3&]
Out[12]= {1,2}

最后一个论点是代表条件的职能。事实上,如果一个人能够直接使用<条码>i<>->->(和/或其他代变量)的辛醇,那么这种合成物可能不会增加。

Answer 4

在以前的航道中,<代码># &[]可改为“内在编号Nothing。

Table[If[i < 3, i, Nothing], {i, 5}]

专员

{1, 2}

Answer 5

如果你使用“公平”而不是“纽埃”,那么你可以这样做。

Table[If[i < 3, i, Hold[Sequence[]]] // ReleaseHold, {i, 5}]

I wished for a long time that If would have Attribute SequenceHold. I think I once suggested this to WRI, but there are probably (good?) reasons for If to not hat this attribute. One can try, if one dares to change built-in Symbols (which one should probably not do):

Unprotect[If];
SetAttributes[If, SequenceHold];

之后,如果只是工作的话,就应当做到:

Table[If[i < 3, i, Sequence[]], {i, 5}]

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