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I'm writing a script that requires a variable number of symbolic variables and am struggling to understand how to evaluate the resulting expressions

MWE:

(%i1)   foo:makelist(f[i],i,3);
(foo)   [f[1],f[2],f[3]]
(%i2)   bar:lreduce("*",foo);
(bar)   f[1]*f[2]*f[3]
(%i3)   vals:[1,2,3];
(vals)  [1,2,3]
(%i4)   
    ev(bar,foo:vals);
(%o4)   f[1]*f[2]*f[3]

Here, I wanted to evaluate the product for f[1]:1, f[2]:2 and f[3]:3.

The following works:

ev(bar,lambda([L], for i thru length(L) do f[i]:L[i])(vals));

however, I thought there was likely a more direct / practical method; perhaps a different way of declaring symbolic variables, altogether?

Rax Adaam
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1 Answers1

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In this case, since the free variables are just subscripted f[1], f[2], f[3], you can assign a list to f temporarily and then it will fish out the appropriate values:

(%i6) e: f[1]*f[2]*f[3];
(%o6)                       f  f  f
                             1  2  3
(%i7) ev(e, f = [1, 2, 3]);
(%o7)                           6
(%i8) ev(e, f = [a, b, 7]);
(%o8)                         7 a b

More generally, the most straightforward way to handle problems like this one is to form a list of equations and then substitute into the expression. E.g.:

(%i12) e: x*y*z;
(%o12)                        x y z
(%i13) l: map ("=", [x, y, z], [11, 22, 33]);
(%o13)              [x = 11, y = 22, z = 33]
(%i14) subst (l, e);
(%o14)                        7986
Robert Dodier
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