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I am trying to find the way how to represent characteristic function of a single-point set. My idea was to write the points by set:

Ainput = {{2/5, 1/4}};

ListPlot[Ainput, PlotRange -> {{0, 1}, {0, 1}}]

enter image description here

Now I want to find a y coordinates for points

{{x -> 2/5}, {x -> 3/5}}

but because I don't have Ainput defined like a function it gives me in both points 0 results instead of 1/4,0. Do you have any suggestions how could I define the function or how could I find the y-coordinate? Thank you.

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    $\begingroup$ Could use KroneckerDelta: In[166]:= f[x_] := 1/4*KroneckerDelta[x - 2/5] Map[f, {2/5, 3/5}] Out[167]= {1/4, 0} $\endgroup$ Dec 15, 2018 at 16:00

1 Answer 1

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#[[1, 2]] Boole[x == #[[1, 1]]] &@Ainput /. {{x -> 2/5}, {x -> 3/5}}

{1/4, 0}

Also

Function[x, #[[1, 2]] Boole[x == #[[1, 1]]] &@Ainput] /@ {2/5, 3/5}

{1/4, 0}

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  • $\begingroup$ The OP might be interested in the difference between Function[x, #[[1, 2]] Boole[x == #[[1, 1]]] &@Ainput] /@ {2/5, 2/5 + 10.^-15} and Function[x, #[[1, 2]] Boole[x - #[[1, 1]] == 0] &@Ainput] /@ {2/5, 2/5 + 10.^-15} $\endgroup$
    – Michael E2
    Dec 16, 2018 at 1:49

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