Determine the relational operator in an expression

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]'

[John D'Errico]

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]

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

Answer 1

[Pooja Kumari]

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

Answer 2

[Image Analyst]

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 <=.

[Geovane Gomes]

I need to catch the operator in the original expression exp3

[Steven Lord]

The two expressions a > b and b < a are mathematically equivalent. Symbolic Math Toolbox reserves the right to rewrite one of those expressions as the other if it feels it necessary.

Can you say more about what you're hoping to do that requires you to distinguish those two mathematically equivalent statements?

[Geovane Gomes]

Yes, I understand that both are equivalent.

I aim to obtain the coefficients of the expression.

It works when I am using "<=" or "==". But did not with ">=".

syms x y
exp1 = 2*x + 3*y <= 5;
exp2 = 2*x + 3*y == 5;
exp3 = 2*x + 3*y >= 5;
exp1Coef = flip(coeffs(lhs(exp1)))

exp1Coef = 

exp2Coef = flip(coeffs(lhs(exp2)))

exp2Coef = 

exp3Coef = flip(coeffs(lhs(exp3)))

exp3Coef = 

5

If I could get the operator, I would change lhs to rhs with, for example, an if statement.

I appreciate all the answers so far!

[Paul]

@Steven Lord

Will the Symbolic Math Toolbox ever keep >= in a symbolic expression? Function hasSymType doesn't list 'ge' (nor 'gt') as a valid symbolic type.

[Walter Roberson]

syms x y
exp1 = 2*x + 3*y <= 5

exp1 = 

exp2 = 2*x + 3*y == 5

exp2 = 

exp3 = 2*x + 3*y >= 5

exp3 = 

[A1,b1] = equationsToMatrix(lhs(exp1)==rhs(exp1))

A1 = 

b1 = 

[A2,b2] = equationsToMatrix(lhs(exp2)==rhs(exp2))

A2 = 

b2 = 

[A3,b3] = equationsToMatrix(lhs(exp3)==rhs(exp3))

A3 = 

b3 = 

[Image Analyst]

@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]

@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!

Answer 3

[Les Beckham]

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 = '>='