Error-check spacing of a vector

Question

Zheng Sen Ter 2021-9-29

0
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评论： Zheng Sen Ter 2021-9-29

Hi, I encountered a problem doing integration using simpson 3/8 or 1/3 or trapezoid rule on 2 sets of vectors x and y. It turns out that the error occur in my function file. For integration, i need to check the spacing of vector x to be the same first.

let me put it like this example:

range = [10,20];
p = 1e-3;
x = range(1):p:range(2);
% check spacing
n = length(x)
h = x(2)-x(1)
for i = 2:n-1
    if x(i+1) - x(i) ~= h
        error('x spacing not correct')
    end
end

Although each spacing of x should be the same, which is p=1e-3. However, the for loop gives error. (error: x spacing ot correct) . It doesnt give error if p = 1.

Is this because of the limitation of Matlab? Sorry if this is a pretty basic theory i should know.

Thanks for your help.

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Answer 1

Geoff Hayes 2021-9-29

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https://ww2.mathworks.cn/matlabcentral/answers/1462789-error-check-spacing-of-a-vector#answer_797289

Zheng - you are trying to compare two floating point numbers using (in this case) the inequality operand. When comparing two floats or doubles, you need to use a tolerance as described at Compare floating-point numbers as the equality (or inequality) operands will not necessarily give the desired result.