for loop with vectors of different dimensions help

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soloby 2015-6-23

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评论： Walter Roberson 2015-6-23

x = -10:0.1:10;
y = 0:.005:1;
f1 = trapmf(x,[-2 0 0 2]);
f2 = trapmf(x,[3 6 6 9]);
for i = 1:201
    y(i) = [x(i),f1(i)]
end

this doesn't seem to work but I dont know why??

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Answer 1

Walter Roberson 2015-6-23

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x = -10:0.1:10;
f1 = trapmf(x,[-2 0 0 2]);
f2 = trapmf(x,[3 6 6 9]);
y = [x(:), f1(:)];

this would produce two columns; you could also use rows like

y = [x; f1];

If you need to use a loop then

x = -10:0.1:10;
f1 = trapmf(x,[-2 0 0 2]);
f2 = trapmf(x,[3 6 6 9]);
y = zeros(length(x), 2);
for k = 1 : length(x)
  y(k,:) = [x(k), f1(k)];
end

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soloby 2015-6-23

在 MATLAB Online 中打开

Also, if I were to divide my f1 into 2 pieces, and my first half is all the way up to right before the peak, do you know of a way to make the second half? sorry for bombarding you with a lot of questions, but here's what I mean.

 [a,ix]=max(f1);
 for k = 1 :ix
  lower_1(k,:) = [x(k), f1(k)];
 end

would the second half be something like

 for k1 = ix:100
     upper_1(k,:) = [x(k), f1(k)];
 end

again, thank you for your help

Walter Roberson 2015-6-23

y(k,:) means to take the k'th row (e.g., k=3 would be row #3), and all columns. So all of row #k.

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Answer 2

Geoff Hayes 2015-6-23

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Soloby - please clarify what you mean by this doesn't seem to work. What is the problem? Are you observing any errors, and if so, what are they? Could it be that the error is In an assignment A(I) = B, the number of elements in B and I must be the same.

Note that you initialize y to be an array of 201 elements and then overwrite each element of y in the for loop. Why? Are you trying to populate y so that it is a 201x1 array. If so, it may be that you want to be doing the following instead

 for k = 1:201
    y(k,:) = [x(k),f1(k)];
 end

Note the use of k as the index variable rather than i since MATLAB uses the latter to represent the imaginary number.

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soloby 2015-6-23

Subscripted assignment dimension mismatch.

Error in practice (line 14) y(k,:) = [x(k),f1(k)];

I don't know why it gives me a dimension error since both x and f1 are 1x201?

Walter Roberson 2015-6-23

It would depend how you initialized y. In your initial code you initialized y as a row vector, so it would be 1 x 201, so when k was 1 you would be trying to assign a two element vector into y(1,:) which is 1 x 141. You need to either not initialize y or initialize y to 141 x 2 like I did in my version.

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