Why does MATLAB not finding a element from my matrix?

Question

0 个投票

x = 0.4:0.01:0.5
x = 1x11
    0.4000    0.4100    0.4200    0.4300    0.4400    0.4500    0.4600    0.4700    0.4800    0.4900    0.5000
idx = find(x==0.41)
idx =

  1x0 empty double row vector

3 个评论
显示 1更早的评论隐藏 1更早的评论

Tshikaya 2024-3-27

在 MATLAB Online 中打开

Thank you for the tip. My problem was actually the following:

A = zeros(1,100);
x = zeros(1,100);
for i = 1:100
    x(i) = 0.01*randi([38 45],1,1);
    if ismember(0.41,x(i));
        A(i) = 10;
    else
        A(i) = -1;
    end
end

So I was shoked to see that x was sometimes 0.41 but still not picking from the code. I have just changed ismember by ismembertol and now everything is fine.

Walter Roberson 2025-6-24

Note that the colon operator is handled as repeated addition, so 0.4, 0.4+0.1, 0.4+0.1+0.1, 0.4+0.1+0.1+0.1 and so on. Because double precision numbers operate in binary instead of in decimal, 0.1 cannot be exactly represented, and instead each 0.1 addition adds 0.1000000000000000055511151231257827021181583404541015625

Mathworks could have chosen an impletation of (initial value) + increment * (position in the sequence minus 1), so 0.4 + [0, 1, 2, 3, 4 ...] * 0.1 ... but Mathworks did not chose that (possibly for efficiency reasons.)

请先登录，再进行评论。

请先登录，再回答此问题。

Follow Question

Answer 1

Stephen23 2024-3-27

移动：Dyuman Joshi 2024-3-27

在 MATLAB Online 中打开

1 个投票

Testing for exact equivalence of binary floating point numbers should be avoided.

Compare the values, are they actually the same? (hint: no):

x = 0.4:0.01:0.5;
fprintf('%.40f\n', x(2), 0.41)
0.4100000000000000310862446895043831318617
0.4099999999999999755750934582465561106801

The recommended approach is to compare the absolute difference against a tolerance:

tol = 1e-5;
abs(x(2)-0.41)<tol
ans = logical
   1

2 个评论
显示无隐藏无

Tshikaya 2024-3-27

Thank you for your answer

Steven Lord 2025-6-24

If you're using release R2024b or later, you could use the isapprox function to determine if two numbers are approximately equal.

请先登录，再进行评论。

Why does MATLAB not finding a element from my matrix?

3 个评论
显示 1更早的评论隐藏 1更早的评论

采纳的回答

2 个评论
显示无隐藏无

更多回答（0 个）

类别

产品

标签

Community Treasure Hunt

Why does MATLAB not finding a element from my matrix?

3 个评论 显示 1更早的评论 隐藏 1更早的评论

采纳的回答

2 个评论 显示 无 隐藏 无

更多回答（0 个）

类别

产品

标签

另请参阅

Community Treasure Hunt

3 个评论
显示 1更早的评论隐藏 1更早的评论

2 个评论
显示无隐藏无