Persistent assumption

[Sam Takoy]

Hi,

Is it possible to make an Assumption for the duration of a block of
code? In other words, to say "In all of the following code, Assume[x>0]".

I appreciate that it is possible to use,

Assuming[x>0,

a whole lot of code

]

but that's quite awkward. Looking for a better alternative.

Thanks,

Sam

[Leonid Shifrin]

Sam,

you can use the global variable $Assumptions - in particular, temporarily
redefine it as a logical And of what it is currently and the conditions you
wish to hold for your block of code. Here is one possible way to do this
using Block (reprinting from one of the past threads where a similar
question was asked)

ClearAll[
computeUnderAssumptions];
SetAttributes[computeUnderAssumptions, HoldFirst];
computeUnderAssumptions[expr_, assumptions_List] :=
Block[{$Assumptions = And[$Assumptions, Sequence @@ assumptions]},
expr];

Example:

In[1] =
computeUnderAssumptions[
FullSimplify[Im[(a + I*b)^2]] +
FullSimplify[Re[(a + I*c)^2],
ComplexityFunction -> (Count[#, _Complex,
Infinity] &)], {Element[{a, b, c}, Reals]}]

Out[1] = 2 a b + (a - c) (a + c)

Hope this helps.

Regards,
Leonid

[Bob Hanlon]

$Assumptions = {x > 0};

Simplify[Sqrt /@ {x^2, y^2}]

{x, Sqrt[y^2]}

$Assumptions =.

Bob Hanlon

---- Sam Takoy <samt...@yahoo.com> wrote:

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