Extracting every nth and (n+1)th element

Hi,

I have A = (1:101). How can I extract every nth and (n+1)th element?

For example, if n = 10, I want reate B = [10, 11, 20, 21, ...100, 101].

I know how to extract every nth element using: A(1:n:end).

Any help would be appreciated! Thank you.

Inna

Answer 1

[Cris LaPierre]

I'm not aware of any slick way to do this. I'd combine two arrays, one from 10:10:101, and one from 11:10:101.

n=10;
A = (1:101);
ind = sort([n:n:101 n+1:n:101]);

A(ind)

ans = 1×20

10 11 20 21 30 31 40 41 50 51 60 61 70 71 80 81 90 91 100 101

[Inna Pelloso]

Thank you!

Answer 2

[Nenad Mijatovic]

Not sure dose this do better then already offered answers but this might be more comact form:

a = [1:1000];      % array
N = 10;              % nth element
b = a(mod(1:length(a),N)==0);    %resulting array

I have been using this trick to preview large datasets.

[Stephen23]

The answer does not solve the original question (which requires not only the Nth but also the (N+1)th elements), as it returns only every Nth element:

a = 1:1000; % removed superfluous square brackets
N = 10;
b = a(mod(1:length(a),N)==0)

b = 1×100

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300

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The simpler standard MATLAB approach (i.e. an even "more comact form") for returning every Nth element:

b = a(N:N:end)

b = 1×100

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300

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However this answer could be modified to solve a slightly generalized version of the original question:

V = [0,1]; % => N, N+1
b = a(ismember(mod(1:numel(a),N),1+V))

b = 1×200

1 2 11 12 21 22 31 32 41 42 51 52 61 62 71 72 81 82 91 92 101 102 111 112 121 122 131 132 141 142

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V = [0,1,2,6]; % => N, N+1, N+2, N+6
b = a(ismember(mod(1:numel(a),N),1+V))

b = 1×400

1 2 3 7 11 12 13 17 21 22 23 27 31 32 33 37 41 42 43 47 51 52 53 57 61 62 63 67 71 72

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See also:

https://www.mathworks.com/company/technical-articles/matrix-indexing-in-matlab.html

[Nenad Mijatovic]

of course. I actially missed the n+1 part of the orriginal question.

thanks for the correction.