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For example of below Switch function.

In[]:= Module[{f}, f[x_] := Switch[x, 1, 2, 3, 4]; {f[1], f[3], f[x]} ] Out[]= {2, 4, Switch[x, 1, 2, 3, 4]}

f[1] and f[3] is evaluated as defined, but since it doesn't define a default case, the original input seems to be a more natural result, and more friendly one.

Is it possible to define the above f[x] so the output becomes:

Out[]= {2, 4, f[x]}

I tried to use Unevaluated[f[x]], but it doesn't seem to work.

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2 Answers 2

Reset to default
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ClearAll[f, x] Block[{f}, f[x_] := Switch[x, 1, 2, 3, 4, _, Defer[f][x]]; {f[1], f[3], f[5], f[x]}] 
{2, 4, f[5], f[x]}
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Block[{f}, f[x_] := With[{res = Switch[x, 1, 2, 3, 4, _, $Failed]}, res /; res =!= $Failed]; {f[1], f[3], f[x]} ] (*{2, 4, f[x]}*)
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