find the position of en element if you have the increment

Question

F.O 2017-12-11

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/372337-find-the-position-of-en-element-if-you-have-the-increment

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更早的评论隐藏 1更早的评论

Stephen23 2017-12-11

What are p, h, and vel supposed to be used for?

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。

Answer 1

Stephen23 2017-12-11

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/372337-find-the-position-of-en-element-if-you-have-the-increment#answer_295731

编辑：Stephen23 2017-12-11

在 MATLAB Online 中打开

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
>>

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

Answer 2

James Tursa 2017-12-11

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/372337-find-the-position-of-en-element-if-you-have-the-increment#answer_295734

编辑：James Tursa 2017-12-11

在 MATLAB Online 中打开

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.