# replace elements in an Array with other elements

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Rica on 20 Jul 2015
Commented: Juan Jiménez on 22 Jun 2020
hi all,
i have the folwing situation:
i have an array:
A=[1 2 3 4 4 4 5 8 7 4 6 4]
i want to find the element equal to the value 4.
I do this:
index=find(A==4)
i want now to replace the element with this index with the previous value. it means i want to get:
A_new=[1 2 3 3 3 3 5 8 7 7 6 6].
i did it with a loop. is there any method without loop?
thank you

Guillaume on 20 Jul 2015
While it may be possible to do it without a loop, with a combination of diff, find and possibly cumsum it's going to be a lot more obscure than a simple loop and probably not any more efficient.
The simplest loop would be:
for idx = find(A == 4)
A(idx) = A(idx-1);
end
which also works for consecutive elements to replace.
Juan Jiménez on 22 Jun 2020
Excellent! I need this. Thank you so much.

Walter Roberson on 20 Jul 2015
A_new = A;
A_new(index) = A(index-1);
This will fail if the very first thing was a 4, as there is no "previous" for the first element.
Guillaume on 20 Jul 2015
This also fails on the example given, when there are consecutive elements to replace.

Sean de Wolski on 20 Jul 2015
Edited: Sean de Wolski on 20 Jul 2015
A=[1 2 3 4 4 4 5 8 7 4 6 4]
idx = find(A~=4);
B = interp1(idx,A(idx),1:numel(A),'previous','extrap')
Gotta love the 'previous' and 'next' options added to interp1 a few releases ago!

Shashank on 14 Jun 2018
How can I copy paste 2nd element to 3rd, 3rd to 4th and so on.. A=[1,2,3,4,5,6,7]; A_New=[1,2,2,3,3,4,4,5,5,6,6,7]; ---> Required