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Where is documentation for colon range of the form 1:vector?

3 vues (au cours des 30 derniers jours)
Paul Hoffrichter
Paul Hoffrichter le 13 Juin 2023
Commenté : Stephen23 le 13 Juin 2023
I think the following code examples should result in a Matlab error and not be allowed to run. I think the code syntax should be disallowed by Matlab.
Where is the documentation that describes the use of the colon operator having the following form? All the examples in https://www.mathworks.com/help/matlab/ref/colon.html appear to use only scalar integers.
I was reviewing code that had code analogous to:
a = [ 3 9 44];
b = 1:a
b = 1×3
1 2 3
I didn't recognize this form. The results appear to show that only the first element of a is used. Where is the documentation that allows this?
A quick experiment shows that the first element sof both vectors are used when creating a range:
a = [2 12 11];
b = [ 13 19 144];
c = a:b
c = 1×12
2 3 4 5 6 7 8 9 10 11 12 13

Réponse acceptée

Steven Lord
Steven Lord le 13 Juin 2023
The third item in the Description section of the documentation page to which you link describes what the colon, : operator does when its inputs are non-scalar.
"x = j:i:k creates a regularly-spaced vector x using i as the increment between elements. The vector elements are roughly equal to [j,j+i,j+2*i,...,j+m*i] where m = fix((k-j)/i). However, if i is not an integer, then floating point arithmetic plays a role in determining whether colon includes the endpoint k in the vector, since k might not be exactly equal to j+m*i. If you specify nonscalar arrays, then MATLAB interprets j:i:k as j(1):i(1):k(1)." [Emphasis added.]
  4 commentaires
Stephen23 le 13 Juin 2023
This should definitely be deprecated. And also that counter-productive column-iteration behavior of FOR.

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