Combine If statement and for loop?!

Question

Christian 2017-3-2

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评论： Christian 2017-3-2

Hello everybody,

I would like to combine the for loop with an if Statement:

if  x<0
    for i=1:length(gradient);
        delta_x_{i} = x(i)-sqrt(r^2/(1+gradient(i).^2));
        delta_x=[cell2mat(delta_x_)]';
        delta_y_{i} = gradient(i)*delta_x(i); 
        delta_y=[cell2mat(delta_y_)]';
    end
else
    for j=1:length(gradient);
        delta_x_{j} = x(j)+sqrt(r^2/(1+gradient(j).^2));
        delta_x=[cell2mat(delta_x_)]';
        delta_y_{j} = gradient(j)*delta_x(j); 
        delta_y=[cell2mat(delta_y_)]';
    end
end

But as far as I can see, it is not working? What am I doing wrong? I guess I'm not seeing the wood for the trees...

Appreciate any Help! Christian

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Stephen23 2017-3-2

编辑：Stephen23 2017-3-2

Hmm... not totally clear to me, but someone may be able to make sense of it. Perhaps you should skip the if altogether and use logical indexing:

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

https://www.mathworks.com/help/matlab/math/matrix-indexing.html

Christian 2017-3-2

在 MATLAB Online 中打开

I got it!!!

      for i=1:length(gradient1);
          if x(i) > 0
              delta_x(i) = x(i)+sqrt(r^2/(1+gradient1(i).^2));
              delta_y(i) = gradient1(i)*delta_x_(i);
          end
          if x(i) < 0
              delta_x_(i) = x(i)-sqrt(r^2/(1+gradient1(i).^2));
              delta_y_(i) = gradient1(i)*delta_x_(i);
          end
      end

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Answer 1

Andrei Bobrov 2017-3-2

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编辑：Andrei Bobrov 2017-3-2

在 MATLAB Online 中打开

delta_x = x(:)+sign(x(:)).*sqrt(r^2./(1+gradient1(:).^2));
delta_y = gradient1(:).*delta_x;

We use gradient1 instead gradient. gradient - it's function from MATLAB

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Christian 2017-3-2

the "Gradient" is just an example. in my script I use german words :)

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Answer 2

Steven Lord 2017-3-2

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在 MATLAB Online 中打开

x is a vector. From the documentation for the if keyword, "if expression, statements, end evaluates an expression, and executes a group of statements when the expression is true. An expression is true when its result is nonempty and contains only nonzero elements (logical or real numeric). Otherwise, the expression is false."

So the body of your if statement:

if x<0

is only executed if ALL the elements of x are less than 0. Otherwise you drop into the else section.

Your corrected code is closer, using a logical scalar as the if expression, but you may be missing a couple cases. If you've preallocated delta_x and delta_y, having 0 as the last element in x won't leave those two arrays shorter than you expect. You may also want to consider what happens if x is Not-a-Number, better known as NaN.