How to check if several variables exist and assign a value to non-existing variables?

Question

Carlos HM 2018-6-23

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/407046-how-to-check-if-several-variables-exist-and-assign-a-value-to-non-existing-variables

评论： Stephen23 2020-4-18

I am trying to check if several variables exist, and assign a value to those of them who do not exist.

a=7;
e=4;
nums=["a", "b", "c", "d", "e", "f"];
i=1;
while i<=length(nums)
  if exist(nums(i),'var')==0
      eval(nums(i))=i;
  end
  i=i+1;
end

b, c, d and f do not exist before the while loop, so they should get assigned a value. But it doesn't work, I get the following error:

Function 'subsindex' is not defined for values of class 'string'.

Thank you for your help.

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Stephen23 2018-6-23

How would this situation ever arise?

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Rik 2018-6-23

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/407046-how-to-check-if-several-variables-exist-and-assign-a-value-to-non-existing-variables#answer_325925

在 MATLAB Online 中打开

Replacing bij chars arrays makes it work, although I wonder about eval. It's generally a bad idea, but I don't see a good alternative without knowing the context that you plan to use this in.

a=7;
e=4;
nums=['a', 'b', 'c', 'd', 'e', 'f'];
i=1;
while i<=length(nums)
  if exist(nums(i),'var')==0
      eval([nums(i) '=i;'])
  end
  i=i+1;
end

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Walter Roberson 2018-6-23

在 MATLAB Online 中打开

a=7;
e=4;
nums={'a', 'b', 'c', 'd', 'e', 'f'};
i=1;
while i<=length(nums)
  if exist(nums{i},'var')==0
      eval([nums{i} '=i;'])
  end
  i=i+1;
end

or

a=7;
e=4;
nums=["a", "b", "c", "d", "e", "f"];
i=1;
while i<=length(nums)
  if exist(nums(i),'var')==0
      eval([nums(i) + '=' + i]);
  end
  i=i+1;
end