### Multplexer 4x1 program

I am utterly lost, I am making a program based on a 4x1 multiplexer and output the truth table. But I am not sure how to go about it,this is the multiplexer https://imgur.com/FS9kco2, from what I understand, a and b have to be certain combination to get A-D and send those inputs to Q. So I think I have to do a nand on a,b get that result and do a nand on result,b and so on. And based on those results I allow A-D to get send to Q? But this is all I have so far, not sure where to go from here.
 ``12345678910111213141516171819`` `````` bool A, B, C, D, SELa, SELb; bool result1,result2,result3,result4; //return 1 for a 2 for b 3 for c 4 for d cout << "Multiplexer" << endl; cout << "--------------" << endl; SELa = false; SELb = false; result1 = (!(SELa && SELb)); result2 = (!(result1 && SELb)); result3 = (!(result2 && SELb)); result4 = (!(result3 && SELb)); ``````
> bool A, B, C, D, SELa, SELb;
Ok so far

> bool result1,result2,result3,result4;
Try just
bool Q;

> //return 1 for a 2 for b 3 for c 4 for d
Why are you returning integers?
If you just need a number, you only need SELa and SELb.

 ``12345678`` ``````bool multiplex(bool A, bool B, bool C, bool D, bool sela, bool selb) { bool Q; // now use sela and selb to decide which of A to D should // be assigned to Q. return Q; }``````

 A-D to get send to Q?

Looks like A, B, C, D, and Q have same type, but do we actually care what it is?
 ``1234567`` ``````template U multiplex(U A, U B, U C, U D, bool sela, bool selb) { U Q; // now use sela and selb to decide which of A to D should // be assigned to Q. return Q; }``````

There is no need for the Q in that function either, since each case can return approriate value.
As a C++20 starter, consider:

 ``12345678910111213141516171819202122232425262728293031323334`` ``````#include #include template requires (sizeof ...(Args) > 1) && (std::convertible_to && ...) bool And(Args... args) { return (... && args); } template bool Nand(Args... args) { return !And(args...); } bool evaluate(bool selb, bool sela, bool D, bool C, bool B, bool A) { const auto notb { Nand(selb, selb) }; const auto nota { Nand(sela, sela) }; return Nand(Nand(A, notb, nota), Nand(B, notb, sela), Nand(C, nota, selb), Nand(D, selb, sela)); } int main() { std::cout << "b a D C B A Q\n"; for (bool b : {0, 1}) for (bool a : {0, 1}) for (bool D : {0, 1}) for (bool C : {0, 1}) for (bool B : {0, 1}) for (bool A : {0, 1}) if (auto Q { evaluate(b, a, D, C, B, A) }; Q) std::cout << b << ' ' << a << ' ' << D << ' ' << C << ' ' << B << ' ' << A << " " << Q << '\n'; }``````

which shows the truth table for all true Q:

 ``` b a D C B A Q 0 0 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 1 0 1 1 0 0 0 1 1 1 1 0 0 1 0 0 1 1 0 0 1 0 1 1 1 0 0 1 1 0 1 1 0 0 1 1 1 1 1 0 1 0 0 1 0 1 0 1 0 0 1 1 1 0 1 0 1 1 0 1 0 1 0 1 1 1 1 0 1 1 0 1 0 1 0 1 1 0 1 1 1 0 1 1 1 1 0 1 0 1 1 1 1 1 1 1 0 0 1 0 0 1 1 0 0 1 0 1 1 1 0 0 1 1 0 1 1 0 0 1 1 1 1 1 0 1 1 0 0 1 1 0 1 1 0 1 1 1 0 1 1 1 0 1 1 0 1 1 1 1 1 1 1 1 0 0 0 1 1 1 1 0 0 1 1 1 1 1 0 1 0 1 1 1 1 0 1 1 1 1 1 1 1 0 0 1 1 1 1 1 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 ```

a very beginner friendly take on it:
 ``1234567891011`` ``````int mux(int a, int b, int c, int d, bool sel1, bool sel2) { unsigned char sel2num = sel2*2+sel1; //converts to int of binary 00, 01, 10, 11 switch sel2num { case 0: return a; case 1: return b; case 2: return c; case 3: return d; } }``````

your case to sel mapping may be some other way, but all you have to do is figure out which goes where and order them as you need.
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