MATLAB: Permutation of only x elements of a vector

MATLABrandomrandom number generatorvector

Imagine a vector of natural numbers between 1 and 20, say

    x=randperm(20)

I want to generate a vector identical to x, but changing the position of exactly 4 elements. How can I do this in general for a vector of m elements, when I want to change n elements?

Best Answer

Doesn't your question just reduce to selecting 4 random integers from 1 to numel(x) using either randperm if the elements have to swap or randi if you accept the positions being the same (as per Stephen's question). You can then swap them in a fixed order or using randperm (which runs the risk of not doing any swapping)

x = 1:20;
swapidx = randperm(numel(x), 4);  %or randi(n, 1, 4);
x(swapidx) = x(fliplr(swapidx))  %swap 1st index with fourth, 2nd with 3rd
x(swapidx) = x(swapidx(randperm(4)))  %swap randomly (may not swap)

Related Solutions

MATLAB: How to randomly swap 2 elements in an array

I'm going to assume at least one of the swap elements must be an number, and that the swapped numbers are allowed to have the same value (so you can some one 9 with another 9).

C=  [1     6     5    10     1     2    11     8     2     7     3     7     9     9];
bold=[1 5 6 9 10 12 13 14];
C_temp=cell(1,2*numel(C));
C_temp(2:2:end)=num2cell(C);
valid_idx=1:numel(C_temp);
valid_idx(bold*2)=[];
swap_idx=[1 1];%invalid output to enter the loop
while ~any(mod(swap_idx,2)==0) || ... %at least 1 needs to be a number
        swap_idx(1)==swap_idx(2) %the indices must be different
    swap_idx=valid_idx(randperm(numel(valid_idx),2));
end
%perform swap
C_temp(swap_idx)=C_temp(swap_idx(end:-1:1));
D=cell2mat(C_temp);
clc,disp(C),disp(D)

MATLAB: Swap only 2 elements in an array

This works:

a=[4 1 9 13 5 20 19]
a([1 7]) = a([7 1])
a =
     4     1     9    13     5    20    19
a =
    19     1     9    13     5    20     4

Best Answer

Related Solutions

MATLAB: How to randomly swap 2 elements in an array

MATLAB: Swap only 2 elements in an array

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