How to index two vectors together

Question

Guillem 2023-11-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2051637-how-to-index-two-vectors-together

编辑： Bruno Luong 2023-11-24

Hi!

I'd like to know how i can index two vectors together. By that I mean that, if for example I have the following two vectors:

vec_A = [1 11 21 31];

vec_B = [4 14 24 34];

How can I get the following vector

vec_C = [1:4 11:14 21:24 31:34];

I know I can do it with a for loop, but is there a way just to do it with vectors?

Thank you very much in advance,

Guillem

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Stephen23 2023-11-24

在 MATLAB Online 中打开

"is there a way just to do it with vectors?"

Not really in a general way. But you can certainly hide the loop:

A = [1,11,21,31];
B = [4,14,24,34];
C = arrayfun(@colon,A,B,'uni',0);
C = [C{:}]
C = 1×16
     1     2     3     4    11    12    13    14    21    22    23    24    31    32    33    34

Dyuman Joshi 2023-11-24

在 MATLAB Online 中打开

I sometimes forget that such functions exist. Using colon seems to be faster.

But, as I mentioned earlier, for loop would be the best method here.

%Bigger data
vec_A = 10.*(1:1e5)+1;
vec_B = vec_A+3;
tic
out1 = arrayfun(@(x, y) x:y, vec_A, vec_B, 'uni', 0);
toc
Elapsed time is 0.212617 seconds.
tic
C = arrayfun(@colon,vec_A,vec_B,'uni',0);
toc
Elapsed time is 0.076981 seconds.

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Answer 1

Dyuman Joshi 2023-11-24

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https://ww2.mathworks.cn/matlabcentral/answers/2051637-how-to-index-two-vectors-together#answer_1359182

在 MATLAB Online 中打开

One way is to use arrayfun() but that is just a for loop in disguise.

%Bigger data
vec_A = 10.*(1:1e5)+1;
vec_B = vec_A+3;
tic
out1 = arrayfun(@(x, y) x:y, vec_A, vec_B, 'uni', 0);
toc
Elapsed time is 0.214490 seconds.
out1 = [out1{:}]
out1 = 1×400000
    11    12    13    14    21    22    23    24    31    32    33    34    41    42    43    44    51    52    53    54    61    62    63    64    71    72    73    74    81    82

This is same as -

n = numel(vec_A);
out2 = cell(1,n);
tic
for k=1:n
    out2{k} = vec_A(k):vec_B(k);
end
toc
Elapsed time is 0.037649 seconds.
out2 = [out2{:}]
out2 = 1×400000
    11    12    13    14    21    22    23    24    31    32    33    34    41    42    43    44    51    52    53    54    61    62    63    64    71    72    73    74    81    82

However, you can see that the for loop is approximately 5.5 times faster than the arrayfun. So, performance wise for loop is the best option.

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Answer 2

Bruno Luong 2023-11-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2051637-how-to-index-two-vectors-together#answer_1359197

编辑：Bruno Luong 2023-11-24

在 MATLAB Online 中打开

There is FEX, and fast if you have compiler and compile the mex file.

https://www.mathworks.com/matlabcentral/fileexchange/29854-multiple-colon?s_tid=srchtitle

>> A = [1,11,21,31];
>> B = [4,14,24,34];
>> mcolon(A,B)
ans =
     1     2     3     4    11    12    13    14    21    22    23    24    31    32    33    34
>> A:B
ans =
     1     2     3     4

You can also overload MATLAB colon operator

>> mcolonops on
>> A:B
ans =
     1     2     3     4    11    12    13    14    21    22    23    24    31    32    33    34
>> mcolonops off
>> A:B
ans =
     1     2     3     4