# How can I define the majority function?

I came across the majority function while falling into a Wikipedia link rabbit hole (I wish I could remember how I got there...)

The function is defined for $n$ arguments and given by $$\mathrm{maj}(x_1,\ldots,x_n)=\left\lfloor\frac{1}{2}+\frac{\sum\limits_{i=1}^n x_n-\frac{1}{2}}{n}\right\rfloor$$ It's easy enough to define the function for a predefined list. For example, given the list

t = RandomInteger[{0, 1}, 10]
(* {0, 0, 1, 1, 1, 0, 0, 1, 1, 1} *)


I can compute the "majority" with

Floor[1/2 + (Sum[t[[i]], {i, 1, Length[t]}] - 1/2)/Length[t]]
(* 1 *)


Is there a way to define $\mathrm{maj}$ in a way similar to, say, f[x_, y_]?

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If you are interested of the Boolean-valued majority function, you don't need to look further than Majority (which is a special case of BooleanCountingFunction). Consider Majority@@(# == 1 & /@ IntegerDigits[num, 2]). – kirma Mar 2 at 22:03

You can use Total instead of your Sum:

Clear[maj]
maj[list_?VectorQ] := Floor[1/2 + (Total[list] - 1/2)/Length[list]]

maj[{0, 0, 1, 1, 1, 0, 0, 1, 1, 1}]
(* Out: 1*)


If you want to work with truth values, the Majority function is built-in (thanks to @kglr for pointing that out!).

If you'd like, however, you can write your own by applying the definition directly:

majBoole[list_?VectorQ] := Count[list, True] >= Length[list]/2

majBoole[{0, 0, 1, 1, 1, 0, 0, 1, 1, 1} /. {1 -> True, 0 -> False}]
(* Out: True *)

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if the input is a list of truth values you can also use the built-in function Majority (+1) – kglr Mar 2 at 22:06
@kglr Cool, I didn't know that the built-in existed! I should follow my own oft-repeated advice and consult the docs first :-) – MarcoB Mar 2 at 22:08

Since you asked for a form like f[x_, y_], one can define a pure function which takes an arbitrary number of arguments using SlotSequence:

maj = Floor[1/2 + (Total[{##}] - 1/2)/Length[{##}]] &;
maj[1, 0, 0, 1, 1, 1, 1, 0, 0, 1]
(* 1 *)


Alternatively, using BlankSequence:

maj[ins__] := Floor[1/2 + (Total[{ins}] - 1/2)/Length[{ins}]]

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