Numerical precision when using colon operator

Question

Xingwang Yong 2022-4-2

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https://ww2.mathworks.cn/matlabcentral/answers/1686739-numerical-precision-when-using-colon-operator

评论： Stephen23 2022-4-2

I am doing simulation, which basically is iteration over time. Given the delta time dt and total time inter_pulse_space*num_pulse, the full time frame time1 can be easily calculated. I want to observe signal at time2 only. Surprisingly, time2 is not a subset of time1. Is this due to numerical precision?

clear
dt = 0.1/1e3; % 0.1 millisecond
num_pulse = 100;
inter_pulse_space = 40*dt;
time1 = dt:dt:inter_pulse_space*num_pulse;
time2 = inter_pulse_space/2 + (0:num_pulse-1)*inter_pulse_space;
ismember(time2,time1)
format long
time1(180)
time2(5)

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Answer 1

KSSV 2022-4-2

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https://ww2.mathworks.cn/matlabcentral/answers/1686739-numerical-precision-when-using-colon-operator#answer_932714

You cannot use ismember for floating point numbers. If you want to have values at time2, you can use interp1. Read about interp1.

If you want to comapre floating point numbers refer:

https://www.geeksforgeeks.org/problem-in-comparing-floating-point-numbers-and-how-to-compare-them-correctly/

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Stephen23 2022-4-2

You certainly can use ISMEMBER for floating point numbers.

If you understand the finite precision of binary floating point numbers and expect/require exact matches only, then there is absolutely nothing stopping you from using ISMEMBER.

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