朱莉娅选择数组/矩阵中除一个元素外的所有元素 [英] julia select all but one element in array/matrix

问题描述

我想知道是否可以在julia数组中选择除一个元素(按索引)以外的所有元素.

I would like to know if it is possible to select all but one element (by index) in a julia array.

例如，以R语言为例，为了不选择矩阵中的特定行，可以这样写:

For example in R language in order to not select a particular row in a matrix one would write:

a = matrix(1:9, 3, 3)
a
1 4 7
2 5 8
3 6 9

然后:

a[-2, ]
1 4 7
3 6 9

现在我想在茱莉亚做同样的事情.我尝试使用逻辑运算符，但是找不到(取消)选择特定索引的方法.这是我尝试过的:

Now I would like to do the same thing in julia. I tried using logical operators, but I cant find a way to (un) select a particular index. Here is what I tried:

a = rand(3,3)
a[.!= 2, :]
ERROR: syntax "!=" is not a unary operator

或与R中相同:

a[-2, :]

和其他一些选项.茱莉亚的工作原理如下:

and a few other options. What is working in julia is the following:

a[a .>= .5, :] 

或

a[[2,3], :]

选择秒和第三行.无论如何，我真的很想知道如何在julia数组中选择一个元素(例如行)中的一个元素.

to select the sec and third row. Anyway I would really like to know how to select all but one of a particular element (row for example) in a julia array.

推荐答案

以下是一种选择:

A = rand(3,3)
B = A[1:end .!= 2,:]

1:end获取行索引的完整列表(您也可以使用1:size(A,1))，然后.!=(请注意.表示逐元素比较)选择不等于2的索引.

1:end gets a complete list of row indices (you could also use 1:size(A,1)) and then .!= (note the . indicating element-wise comparison) selects the indices not equal to 2.

如果要以这种方式选择列，则可以使用:

If you wanted to select columns in this way you would use:

C = A[:, 1:end .!= 2]

请注意，end关键字将自动等于您引用的行，列或其他维的最后一个索引值.

Note that the end keyword automatically will equal the last index value of the row, column, or other dimension you are referencing.

注意:答案已更新，以反映@Matt B在评论中建议的改进(使用end而不是size()).

Note: answer updated to reflect improvements (using end instead of size()) suggested by @Matt B in comments.

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