My script won't run (error) how do I fix it?

My script is below. When it runs (using [left, right] = search(a, 367, 1, 100)) it shows this message: Index exceeds matrix dimensions.

Error in search (line 12) if vec(midPt) == targetVal

It should output the index 73 - how can I fix it?

My Script:

function [left, right] = search(vec,targetVal,left,right)
  % SEARCH Binary search algorithm step.
  %  [left, right] = SEARCH(vec,targetVal,left,right) returns the end points
  %   produced by one step of the binary search algorithm with
  %   target value targetVal. The elements of vec must be in ascending order.

    % Compute the mid-point by halving the distance between
    % the end points and rounding to the nearest integer.
    while left < right
        midPt = round((left + right)/2)

        if vec(midPt) == targetVal
      % targetVal has been found

        left = midPt;
        right = midPt;

        elseif vec(midPt) < targetVal
        % targetVal must be before midPt

        right = midPt - 1;

        else
        % targetVal must be after midPt

        left =  midPt + 1;
        end

    end

Answer 1

[KSSV]

For [left, right] = search(a, 367, 1, 100) ;

left = 1; right = 100 ; midPt = round((left + right)/2) ; midpt = 51

Your size of a would be less then 51, so the error popped out. Check your size of a

[Charlotte Davies]

Thank you! I entered in the vector a = primes(541) which fixed this issue, but then I only received midpoints 51, 26, 13, 7, 4 and 2 and both left and right equal to 1 which makes no sense as the index for the target value is 73 (which should be output as the final value of left and right, and the midpoints should approach 73 rather than 1). What should I do?

Answer 2

[Roger Stafford]

The error message probably occurs when your vector ‘vec’ does not contain the value ‘targetVal’. In that case you repeatedly either reduce ‘right’ or else repeatedly increase ‘left’ until one of them goes below 1 or the other above 100, at which time you will be beyond the index limits in ‘vec’. You need to avoid getting right < left.