Determine the relational operator in an expression

Question

Geovane Gomes 2023-11-15

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https://ww2.mathworks.cn/matlabcentral/answers/2047817-determine-the-relational-operator-in-an-expression

回答： Les Beckham 2023-11-16

I need to find out the operator in an expression.

I tried to convert the symbolic expression using char and then find the position of the operator ("<=","==",">="). However, when using the char function, Matlab always puts it in the form "<=" when it is an inequality. Therefore I never find the gt operator.

syms x y
exp1 = x <= y;
exp2 = x == y;
exp3 = x >= y;
charExp = char([exp1,exp2,exp3])
charExp = '[x <= y, x == y, y <= x]'

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John D'Errico 2023-11-15

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syms x y

exp1 = x <= y

exp1 =

exp2 = x == y

exp2 =

exp3 = x >= y

exp3 =

But it also flipped the left and right hand sides. So the two are equivalent. All that matters is what is on each side.

Torsten 2023-11-15

Your expressions already have to be of type "char", not of type "sym", to extract the operator used.

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Answer 1

Pooja Kumari 2023-11-15

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编辑：Pooja Kumari 2023-11-15

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Dear Geovane,

It is my understanding that you are facing issues with converting symbolic expression using "char".

It seems that the "char" function in MATLAB always represents inequalities using the "<=" operator, regardless of the original operator. Unfortunately, there is no direct way to obtain the original ">=" operator using the "char" function. It is clear that "exp3" will give "x>=y" but it switched to "<=" which is correct expression as shown below:

syms x y
exp3 = x >= y;
char(exp3)
ans = 'y <= x'

Regards,

Pooja

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Answer 2

Image Analyst 2023-11-15

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Then you can use contains to run down the various math operators you might possibly encounter, and then take appropriate action

syms x y
exp3 = x >= y;
% Cast to string.
thisExpression = char(exp3)
thisExpression = 'y <= x'
if contains(thisExpression, '>=')
    % Operator is >= so do something with that knowledge.
    fprintf('Operator is >=.\n');
elseif contains(thisExpression, '>')
    % Operator is > so do something with that knowledge.
    fprintf('Operator is >.\n');
elseif contains(thisExpression, '<=')
    % Operator is <= so do something with that knowledge.
    fprintf('Operator is <=.\n');
elseif contains(thisExpression, '<')
    % Operator is > so do something with that knowledge.
    fprintf('Operator is <.\n');
elseif contains(thisExpression, '~=')
    % Operator is ~= so do something with that knowledge.
    fprintf('Operator is ~=.\n');
    % etc for the other possible operators.
end
Operator is <=.

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Image Analyst 2023-11-16

编辑：Image Analyst 2023-11-16

@Geovane Gomes not sure why you didn't yet, but could you explicitly answer @Steven Lord's question: "Can you say more about what you're hoping to do that requires you to distinguish those two mathematically equivalent statements?" Do you just want to get the coefficients? But if you have the expression, you already know the coefficients. Or is your expressions comcing to you somehow in some mysterious method where you don't know the coefficients yet? If so, what exactly might be an input that you need to inspect to find the coefficients?

Geovane Gomes 2023-11-16

@Image Analyst,

In order to make it more "friendly", I would like the expressions as input, not only its coefficients.

@Walter Roberson's solution is enough for me.

Thanks!

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Answer 3

Les Beckham 2023-11-16

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If you can rely on x always being on the left in your "test" expressions, you can test for y being on the left in the result returned by char (to detect that the symbolic toolbox reversed the order of the comparison). For example:

syms x y
exp1 = x <= y;
exp2 = x == y;
exp3 = x >= y;
charExp3 = char(exp3)
charExp3 = 'y <= x'
if contains(charExp3, '<=')
    if charExp3(1) == 'x'
        operator = '<='
    else
        operator = '>=' % if symbolic toolbox swapped the comparison, swap the result
    end
end
operator = '>='