Binary Search - Descending order

Question

Queenie Chan 2020-4-8

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/516286-binary-search-descending-order

回答： Divya Gaddipati 2020-4-13

Hi! I would like to know how to modify the code below so that it can work for vectors that are sorted in descending order, rather than ascending order. Your function should not sort the vector.

This is the code:

% FILENAME: binsearch-2.m
% SUBMISSION TIME: 08/04/2020 07:13:38 pm
% --- Begin file ---
function [left, right] = binsearch(vec,targetVal,left,right)
% SEARCH Binary search algorithm step.
%   [left, right] = BINSEARCH(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.
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

3 个评论
显示 1更早的评论隐藏 1更早的评论

James Tursa 2020-4-8

@darova: More of your comments need to be answers ...

James Tursa 2020-4-8

编辑：James Tursa 2020-4-8

@Queenie: You realize that the posted code is only one step of a binary search, not the entire search algorithm, right? You would need to call this routine repeatedly.

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Divya Gaddipati 2020-4-13

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/516286-binary-search-descending-order#answer_425811

在 MATLAB Online 中打开

Hi,

You need to keep searching until you find the target. For this you need to add a while loop, which keeps reducing the search space.

Since, your vector is in descending order, if the middle value is less than the target, then you need to search on the left side, i.e, right = midPt - 1.

You need to modify your code as shown below:

function [left, right] = binsearch(vec, targetVal, left, right)
while (left < right)
    midPt = floor((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