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I have data file with the following structure:

{{100, 201, value1}, {100, 301, value2}, {200, 301, value3}}

How to construct a function f[i_,j_] so that f[100,201]=value1, f[100,301]=value2, f[200,301]=value3. For all other arguments which are not presented in a file one have to obtain zero, i.e. f[100,104]=0?

P.S. This problem arises in connection with atomic physics. It is necessary to construct a function which calls the transitions rates between states i and j tabulated in a file. i=100 means 1S state of hydrogen for example, j=201 = 2p state and etc.

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f[_, _]=0;

(f[#, #2] = #3) & @@@ {{100, 201, value1}, {100, 301, value2}, {200, 301, value3}};


f[100,201]
f[100,104]
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This is essentially Lookup with a default.

data = {{100, 201, value1}, {100, 301, value2}, {200, 301, value3}}
listOfRules = Most[#] -> Last[#] & /@ data
f = Function[{x, y}, Lookup[listOfRules, Key@{x, y}, 0]]
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