why is my vector bigger than how I created

Question

Khang Nguyen 2020-11-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/656393-why-is-my-vector-bigger-than-how-i-created

评论： Stephen23 2020-11-22

采纳的回答： Setsuna Yuuki.

在 MATLAB Online 中打开

I tried to do a remain sum using matlab and graph it,

why is my sumvec vector is 101? should the while loop runs from 10 to 100

%equ = sqrt(x+1)
% calculating remain sum of sqrt(x+1) from with 10 to 100 sub interval
sum = 0
sumvec = []
for n = 10:100
    sum = 0
    width = (3 - 0)/n
    
    for c = 0:n
        base = width * c
        height = sqrt(base+1)
        sum = sum + (width * height)
    end
    sumvec(n+1) = sum
end
figure;
i = 0:101
scatter(i,sumvec)

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Stephen23 2020-11-22

在 MATLAB Online 中打开

"why is my sumvec vector is 101? should the while loop runs from 10 to 100"

When n==100, which element do you expect this indexing to refer to?:

sumvec(n+1) = ..

Important: do NOT call any variable sum, as this shadows the very important inbuilt sum function.

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Answer 1

Setsuna Yuuki. 2020-11-21

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/656393-why-is-my-vector-bigger-than-how-i-created#answer_552048

编辑：Setsuna Yuuki. 2020-11-22

在 MATLAB Online 中打开

Because the values are saved from space 11 onwards.

You must change n+1 in sumvec()

n+1 = 11 -- n-9 = 1

sum = 0
sumvec = []
for n = 10:100 
    sum = 0;
    width = (3 - 0)/n;
    
    for c = 0:n
        base = width * c;
        height = sqrt(base+1);
        sum = sum + (width * height);
    end
    sumvec(n-9) = sum; %% change n+1 -- n-9
end
figure;
i = 1:length(sumvec) %i vector
scatter(i,sumvec)