Find the first element that satisfy a condition in a vector

Question

paul mary 2019-3-19

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/451048-find-the-first-element-that-satisfy-a-condition-in-a-vector

回答： Sritama Mitra 2020-9-6

Hello! I have a homework: if v is [1 2 3 4 5 4 3 2 1] and n is 3, it will find 4 5 and 4 because their sum of 13 is the largest of any 3 consecutive elements of v. The function returns summa, the sum as the first output argument and index, the index of the first element of the n consecutive ones as the second output.

for example

[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],3)
summa = 13
index = 4
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],1)
summa = 5
index = 5
[summa, index] = max_sum([1 2 3 4 5 4 3 2 1],9)
summa = 25
index = 1

Then I have written the code. The summa Part is correct but the index part ist wrong. index is always equal to v(k-n+1). Could someone give me some idea?

Here is the result of my code:

max_sum([1 2 3 4 5 4 3 2 1],1)
summa =5
index =1

function [summa,index] = max_sum(v,n)
k = length(v);
a = zeros(1,k-n+1);
if isscalar(n) == 1 && n > 0 && n == round(n)
    if n <= k
        for ii = 1:(k-n+1)
            b = sum(v(ii:ii+(n-1))); 
            a(ii) = b;
            summa = max(a);
        end
        index = v(ii);
    else
        summa = 0;
        index = -1;
    end
else
    fprintf('wrong');
end

6 个评论
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Stephen23 2019-3-20

编辑：Stephen23 2019-3-20

See also earlier question (different OP):

https://www.mathworks.com/matlabcentral/answers/444784-write-a-function-called-max_sum-that-takes-v-a-row-vector-of-numbers-and-n-a-positive-integer-as

Eva Petrossova 2019-3-28

Hey, I can't manage with it, how did you do ?

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Answer 1

Andrei Bobrov 2019-3-20

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/451048-find-the-first-element-that-satisfy-a-condition-in-a-vector#answer_366343

编辑：Andrei Bobrov 2019-3-20

function [summa, index] = max_sum(A,n)
    B = movsum(A(:),[0 n-1]);
    summa = max(B);
    index = find(summa == B);
end

or without movsum:

function [summa, index] = max_sum(A,n)
    B = A((1:n) + (0:numel(A)-n)');
    summa = max(B);
    index = find(summa == B);
end

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dpb 2019-3-20

I'm guessing if OP turns this one in, the instructor eyebrows will raise -- or at least they should! :)

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Answer 2

Sritama Mitra 2020-9-6

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/451048-find-the-first-element-that-satisfy-a-condition-in-a-vector#answer_490681

在 MATLAB Online 中打开

function [summa,index]=max_sum(v,n)
k=length(v);
index=ones(1,1);
if n<=k
    for i=1:(k-n+1)
        b=sum(v(i:i+n-1));
        a(i)=b;
        summa=max(a);
        
    end
    x=find(a==summa);
    [o,p]=size(x);
   
    if p~=1
        index=x(1);
    else
        index=x;
    end
        
else
    summa=0;
    index=-1;
end
end