Iter rec - does it always have to be an integer?

Question

Tom 2012-1-7

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https://ww2.mathworks.cn/matlabcentral/answers/25543-iter-rec-does-it-always-have-to-be-an-integer

I'm just trying to set up some 'for' loops and I have a couple of arrays that don't go up in integer values. I've been using this kind of thing so far: -

iter=1;
for n=1:20;
  y=n^2
  yrec(iter)=y;
  iter=iter+1;
end

How do I make it so the iter value is less than one?

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Matt Fig 2012-11-2

在 MATLAB Online 中打开

I'm just trying to set up some 'for' loops and I have a couple of arrays that don't go up in integer values. I've been using this kind of thing so far: -

iter=1;
for n=1:20;
  y=n^2
  yrec(iter)=y;
  iter=iter+1;
end

How do I make it so the iter value is less than one?

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Answer 1

Walter Roberson 2012-1-7

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https://ww2.mathworks.cn/matlabcentral/answers/25543-iter-rec-does-it-always-have-to-be-an-integer#answer_33483

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You cannot use non-integer indices for arrays. Instead, (for example)

iter=1;
itervals = [];
for n=1:20;
  y = n^2
  yrec(n) = y;
  itervals(n) = iter;
  iter = iter + 0.123;
end

Then yrec(K) is the yrec that corresponds to location itervals(K)

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Tom 2012-1-9

Thanks for your time Walter. I understand your answer now, but I'm still unsure about your example code. If I wanted my intervals for n to be 0.1, how would the code read?

Walter Roberson 2012-1-9

iter=1;

itervals = [];

for n=1:20;

y = n^2

yrec(n) = y;

itervals(n) = iter;

iter = iter + 0.1;

end

The "for" loop controls how many times to execute, and when you choose to have that loop start from 1 and increment by 1 (the default) then the loop variable also gives you the array index you want to use. But the array index is not the same as the value of the point represented -- just like in the slope example, x(1) does not mean x=1 but rather means "the first x". The array itervals that I showed in my example code is recording the actual values for the coordinate.

For example, the array x = [2, 5, 7] is indexed at 1, 2, and 3; x(1) is 2, x(2) is 5, x(3) is 7. If you were using y=x.^2 then the corresponding y would be [4, 25, 49], so you would read off

x(1) is 2 and y(1) corresponds to the function value evaluated at x(1)

x(2) is 5 and y(2) corresponds to the function value evaluated at x(2)

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