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Groups > comp.soft-sys.math.mathematica > #2343

Replace unevaluated symbols

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Given an expression like

{x+y,2*x-y}

I would like to replace x and y with some values even if these
variables have some value assigned. For example, the code

expr = Hold[x+y,2x-y]
expr/.x->3

produces

Hold[3 + y, 2*3 - y]

but the code

x=2.5;
expr = Hold[x+y,2x-y]
expr/.x->3

produces the wrong result

Hold[x + y, 2 x - y]

I know this can be done using Block; however, I don't know the
variables to be replaced beforehand. I would like to have a function
with attribute HoldAll like

fun[expr,var1,value1,var2,value2,...]

that replaces the variables var1, var2, ... by the corresponding
values value1, value2,..., even if any of the variables var1, var2,...
has a preassigned value.

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Thread

Replace unevaluated symbols Antonio De Juárez <adejuarez@gmail.com> - 2011-05-13 10:29 +0000
  Re: Replace unevaluated symbols David Reiss <dbreiss@gmail.com> - 2011-05-14 07:10 +0000
  Re: Replace unevaluated symbols Albert Retey <awnl@gmx-topmail.de> - 2011-05-14 07:15 +0000
    Re: Replace unevaluated symbols Antonio De Juárez <adejuarez@gmail.com> - 2011-05-15 11:05 +0000

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