[Questions] Logical Indexing Problem

“Write a function called trim10 that takes two vectors of the same length as input arguments (it does not have to check the format of the input) and returns two row vectors of the same length as the input vectors. If it is called like this, [v_trimmed,trimmings] = trim10(v1,v2), then v_trimmed is identical to v1 except that every element v1(ii) that is greater than v2(ii)+10 must be trimmed, which means that it must be replaced by v2(ii)+10. Each element of trimmings is equal to the amount by which each element has been trimmed. The function must use logical indexing instead of explicit looping. Here is an example of the function being used:

>>

v1

v1 =

   36    26     4    17  -100    90

>> v2

v2 =

      34    15   -20     0     6    80

>> [v_trimmed,trimming] = trim10(v1,v2)

v_trimmed =

      36    25   -10    10  -100    90

trimmings =

       0     1    14     7     0     0

Here's my code

function [v_trimmed,trimmings] = trim10(v1,v2)

% Return v_trimmed

v_trimmed = v1;

v_trimmed(v_trimmed>(v2+10)) = v2(v_trimmed>(v2+10))+10;

% Return trimmings

trimmings = zeros(1,length(v1));

for ii = 1:length(trimmings)

      if v_trimmed(ii) == v1(ii)

          trimmings(ii) = 0;

      else

          trimmings(ii) = v1(ii)-(v2(ii)+10);

     end

end

end

I can't use logical indexing to find trimmings because it would return a 1x3 vector, without the 0s like the problem wants. Does any one have any suggestion on how to use logical indexing to find trimmings, or we can't use logical indexing in this case?

Thanks!

Answer 1

[Stephen23]

The task states clearly that the "function must use logical indexing instead of explicit looping", and using logical indexing actually makes the code much simpler than using loops anyway:

function [v_trimmed,trimming] = trim10(v1,v2)
v_trimmed = v1;
idx = v1>(v2+10);
v_trimmed(idx) = v2(idx)+10;
trimming = v1 - v_trimmed;
end

And tested using those vectors:

>> v1 = [36,26,4,17,-100,90]
v1 =
    36    26     4    17  -100    90
>> v2 = [34,15,-20,0,6,80]
v2 =
    34    15   -20     0     6    80
>> [v_trimmed,trimming] = trim10(v1,v2)
v_trimmed =
    36    25   -10    10  -100    90
trimming =
     0     1    14     7     0     0

Answer 2

[Adam]

trimmings = max( 0, v1 - ( v2 + 10 ) )

seems to do the job.

[Huy Truong]

I think if this problem were just to ask for finding trimmings, then your answer would be brilliant. It really shows me how a little analyzing before writing codes can make huge difference.