replace elements in an Array with other elements

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

Answer 1

[Guillaume]

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]

Excellent! I need this. Thank you so much.

Answer 2

[Sean de Wolski]

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!

Answer 3

[Walter Roberson]

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]

This also fails on the example given, when there are consecutive elements to replace.

Answer 4

[Shashank]

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