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How do I extract the condition from a conditional expression?

David Golber
Posted 1 year ago

I have a calculation which at a certain point produces a conditional expression like

CE= (expression involving x) if 1<= x <= 3.

I want to extract the fact that the variable which is conditional is x, and that the bounds are 1 and 3. Then I can continue the calculation by getting the values of the expression when x is 1 and when x is 3.

I guess I could do this by turning CE into a character string, searching for "if" and so on. But is there an easier (non-acrobatic) way?

Thanks
POSTED BY: David Golber
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David Golber
David Golber
Posted 1 year ago

Thanks for all these. Hans Milton's and Gianluca Gorni's suggestions certainly work. I'm not sure of Bill Nelson's. (In the meantime - not surprisingly - I've done what i needed in a completely different way.)

And here's a bit: If i write

e[x_] := ( Sqrt[1 - x^2] if - 1 <= x <= 1);

Then

e[.5] gives -1 + 0.866025 if <= 0.5

But if i write

f[x_] := ConditionalExpression[Sqrt[1 - x^2], -1 <= x <= 1];

Then

f[.5] gives 0.866025
POSTED BY: David Golber

If you write

 ( Sqrt[1 - x^2] if - 1 <= x <= 1)

the if is interpreted as a symbolic variable with no meaning, that multiplies Sqrt[1 - x^2].

The if that you see when you do this

Solve[x^2 + y^2 == 1, y, Reals]

is for display only. It is not valid input syntax.
POSTED BY: Gianluca Gorni
Hans Milton
Hans Milton
Posted 1 year ago

One could use Apply to convert the conditional expression to a list:

ce = ConditionalExpression[x^f, 1 <= x <= 3]

List @@ ce
POSTED BY: Hans Milton

A little more directly, the condition is the second element of the expression ConditionalExpression[expr,cond]. You can extract it the same way you would with a list:

ConditionalExpression[1/(1 + n), Re[n] > -1][[2]]
POSTED BY: Gianluca Gorni
Bill Nelson
Bill Nelson
Posted 1 year ago
POSTED BY: Bill Nelson
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