Question: Checking a condition

I need to do this with a sequence of a large number of elements but here is my question with a sequence of fewer elements.


I want to check each element of the sequence if it is negative. And if any element is negative I want to replace with 0 (or do something else). I tried the following code but it didn't work. 

for k from 1 to 7 do
  if (a[k]<0) then subs(a[k]=0, a) end if:
end do;

Please help me to figure it out. 

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