find the position of en element if you have the increment

1 次查看（过去 30 天）
F.O
F.O 2017-12-11
编辑： F.O 2017-12-11
How i will find mathematically what is the position of the element correspounding z=-2 it is 21 but I cant find it mathematically
p=[4.4 5.5 6.6 8];
h=[2 5 3 6];
x=[0:0.4:40];
z=[0:-0.1:-16]
vel=zeros(numel(z),numel(x));
vel(1:21,:)=4.4;
vel(22:71,:)=5.5;
vel(72:101,:)=6.6;
vel(102:161,:)=8;
  3 个评论
显示 1更早的评论
F.O
F.O 2017-12-11
编辑：F.O 2017-12-11
Hei Adam ,Actually ,sometimes I am very bad in asking and sorry for that. the thing is that every value in p corresspound to the values in h and I wanted to know how to get the position when z equals minus every value in h vector

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Stephen23
Stephen23 2017-12-11
编辑：Stephen23 2017-12-11
You need to compare the difference against a (carefully selected) tolerance:
>> z = 0:-0.1:-16; % do not use square brackets!
>> tol = 0.00001;
>> val = -4.4;
>> idx = find(abs(z-val)<=tol)
idx = 45
>>
Read more here:
https://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero
https://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0

更多回答（1 个）

James Tursa
James Tursa 2017-12-11
编辑：James Tursa 2017-12-11
You need to be careful how you search for this element. Because of the way the colon operator works with floating point fractions, you cannot guarantee that values you think might obviously be in the vector are actually there exactly. E.g.,
>> z=[0:-0.1:-16];
>> find(z==-4.4)
ans =
45
>> num2strexact(-4.4)
ans =
-4.4000000000000003552713678800500929355621337890625
>> num2strexact(z(45))
ans =
-4.4000000000000003552713678800500929355621337890625
>> find(z==-4.1)
ans =
Empty matrix: 1-by-0
>> num2strexact(-4.1)
ans =
-4.0999999999999996447286321199499070644378662109375
>> num2strexact(z(42))
ans =
-4.10000000000000053290705182007513940334320068359375
So, for the -4.4 value the colon operator happen to pick the same "closest" IEEE double value that you get when you type the value at the command line. But this didn't happen for the -4.1 value. So to do these types of operations in your code, you need to account for this effect and either use tolerances or use some type of integer range that you know can be searched exactly.

类别

在 Help Center 和 File Exchange 中查找有关 Manage Products 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by