Loop and Conditional Statements

Question

Paulo Eduardo Beiral 2021-6-24

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/864330-loop-and-conditional-statements

评论： Paulo Eduardo Beiral 2021-6-24

采纳的回答： Steven Lord

在 MATLAB Online 中打开

I've got two vectors 1x366 u and v, and I have to make a new 1x366 vector of the arctg of u and v.

But I've got a few conditions depending if the respective value of u and v are negative or positive.

Here's some of the code which I started writing, but I am stuck.

Thank you for any help.

for n = 1:length(u)
if u(n) <0 && v(n) < 0
  atan(v./u); 
elseif u(n) > 0 && v(n) < 0       
   atan(u./v) + 4.712;
    
elseif u(n) < 0 && v(n) > 0 
   atan(u./v) + 1.571;
   
elseif u(n) > 0 && v(n) > 0
    atan(v./u) + 3.142;
end
end

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Scott MacKenzie 2021-6-24

Have you looked at atan2? This is the four-quadrant version of atan -- might be what you need.

Paulo Eduardo Beiral 2021-6-24

Unfortunately atan2 wont help me in this case, I need exactly the atan with the conditions I entered in my code.

Thank you, mister.

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Answer 1

Steven Lord 2021-6-24

1
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https://ww2.mathworks.cn/matlabcentral/answers/864330-loop-and-conditional-statements#answer_732740

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As written your code calls atan at most once per element of u but then throws that result away. You also have magic numbers that look like approximations to multiples of pi.

Instead I recommend using logical indexing.

base1 = atan(v./u);
base2 = atan(u./v);
result = NaN(size(u)); % determine what you want result to be if none of the cases hold
case1 = (u < 0) & (v < 0);
result(case1) = base1(case1);
case2 = (u > 0) & (v < 0);
result(case2) = base2(case2) + (3*pi/2);
% and similar for case3 and case4