1

I have an array sorted in ascended order. I want to replace the biggest m numbers found even positions in the array with 0.

My algorithm that I thought looks like this:

k=1;
for j=n:1  %%starting from last position to first
    if(rem(j,2)==0 && (k<=m))  %%checking if the position is even & not getting over m numbers
        B(j) = 0;
        k = k + 1;
    end
end

Can anyone point out why it is not working? Thank you!

Stewie Griffin
  • 14,889
  • 11
  • 39
  • 70
Shury
  • 468
  • 3
  • 15
  • 49

2 Answers2

1

I believe this should do the trick. This works for vectors with both odd and even number of elements.

n = numel(B);
B((n-mod(n,2)):-2:(n-mod(n,2)-2*M)) = 0

or 

n = mod(numel(B),2);
B((end-n:-2:end-n-2*M)) = 0

I prefer Shai's solution, but if your vector is huge, and M is relatively small, I would go with this approach, as it avoids creating a vector of length numel(B)/2

Community
  • 1
  • 1
Stewie Griffin
  • 14,889
  • 11
  • 39
  • 70
1

A bit more complex

even = (n-rem(n,2)) : -2 : 1; % even indices in descending order
B( even(1:m) ) = 0; % set to zero

Note how n-rem(n,2) ensures that we start from the last even index into B.

PS,
It is best not to use j as a variable name in Matlab.

Community
  • 1
  • 1
Shai
  • 111,146
  • 38
  • 238
  • 371