Colon-generated arrays with or without brackets

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Alec Poulin 2018-3-27

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https://ww2.mathworks.cn/matlabcentral/answers/391046-colon-generated-arrays-with-or-without-brackets

编辑： Stephen23 2021-3-31

采纳的回答： Rik

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From what I can tell, the arrays a:d:b and [a:d:b] are exactly the same thing. For example,

    >> [1:3] == 1:3
    ans =
      1×3 logical array
       1   1   1

Yet, these two expressions give different results:

    >> [1:3]' + 1:3
    ans =
         2     3
    >> [1:3]' + [1:3]
    ans =
         2     3     4
         3     4     5
         4     5     6

Why?

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Rik 2018-3-27

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The reason for this is the unexpected order in which this statement is evaluated:

[1:3]' + 1:3
([1:3]' + 1):3
([1;2;3]+1):3
[2;3;4]:3
2:3
[2,3]

Adding the brackets forces the grouping before and after the colon operator (parentheses would have worked as well).

An error might break poorly coded functions that rely on this behavior. I think an mlint warning and warning when running the code would be better. If you use it as a shortcut, it will still work, but users will be warned about possibly unintended consequences. (similar to how if a<b<c triggers an mlint warning, but will run if you actually meant to use it like that (e.g. with a custom class that overloads lt))

Stephen23 2021-3-31

编辑：Stephen23 2021-3-31

@Rik: I suspect there is more code broken by this "feature" and time wasted debugging this rather unintuitive behavior, than gained by a few instances where it has been used intentionally (I do not recall ever seeing this).

MATLAB has tweaked other syntaxes over the years, this one is overdue for its retirement too.

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