matlab cat（）函数：索引超出矩阵维度

Question

I am trying to concatenate an array of numbers from 1->(a-1) + (a+1)->n.

I was using the cat function

cat(2, 1:a-1, a+1:n)

but I am getting the error

Index exceeds matrix dimensions.

Unless I am completely mistaken, I am just trying to concatenate two matrices of numbers so I'm not quite sure why I'm getting this error.

I'm trying to accomplish this:

>> a = 3;
>> n = 10;
>> cat(2, 1:a-1, a+1:n)
ans = 
    [1,2,4,5,6,7,8,9,10]

Is this the wrong way to do it? Any idea why this error is coming up?

Answer 1

Do you have a variable called cat in your workspace?

>> cat(2, 2:3, 4:6)                   # this works fine
ans =
     2     3     4     5     6
>> cat = 1:3;                         # introduce the variable 'cat'
>> cat(2, 2:3, 4:6)                   # now it breaks
??? Index exceeds matrix dimensions.

Answer 2

It looks like you have a variable named cat in the workspace. The clean way is, of course, to rename the variable: If you have a sufficiently recent version of Matlab (R2012x, I think), you can replace cat in the first line it gets assigned (select the variable to see the gray ticks to the right of the window, indicating where the variable occurs in the function), and use shift+enter to replace all occurrences. Or you can use the Find/Replace all function (make sure you only replace words, not substrings, though).

If you cannot replace the existing variable name, you can use square brackets for catenation along the first and/or second dimension:

cat(2,a,b)

is equivalent to

[a,b]

Answer 3

Just for completeness, the concatenation you're trying to accomplish can also be achieved like so:

R = 1:n;
R = R(R ~= a)

I personally think this looks cleaner than

R = [1:a-1 a+1:n]

but that's personal; I always feel a little confusion towards something like 1:a-1>5 (is it ((1:a)-1)>5 or (1:(a-1))>5 or (1:a)-(1>5) or ...). I just always have to think for a second, whereas I understand my solution instantly.