Want help with debugging a For-loop

Question

I'm trying to run a simulation, which at the end gives a value 'qber'. Now I want to run this simulation multiple times with different values of 'p' each time. And these value of 'p' are actually the elements of a list already defined before. My code is as follows:

x := RandomChoice[{0, π}];
y := RandomChoice[{0, π/2, π, 3 π/2}];
prob = {0.25, 0.05}
f2[{result_, plist_}] := 
   With[{k = RandomReal[]}, 
     Which[
       k < 0.7, {0, plist}, 
       (k > 0.7) && (k < 0.7 + plist[[1]]), 
         {π/10, If[plist[[1]] == prob[[2]], plist, Reverse @ plist]},
       True, {-π/10, If[plist[[1]] == prob[[1]], plist, Reverse @ plist]}]]
 mylist = #[[1]] & /@ NestList[f2, {0, prob}, 9]
 e = Accumulate[mylist]

{0.15, 0.15} 
{0, 0, 0, -(π/10), π/10, 0, -(π/10), 0, 0, -(π/10)}
{0, 0, 0, -(π/10), π/10, 0, -(π/10), 0, 0, -(π/10)}

Then the last list in the above output is used for the values of p, in the following for-loop:

For[i = 1; i <= 10, i++, 
  p = e[[i]]; 
  Module[{}, 
    dat3 = Table[x - y + p, 100];
    dat34 := Mod[dat3, 2 π];
    dat334 := Transpose[{dat3, dat34}];
    ph0 := Select[dat334, #[[1]] == 0 &];
    ph1 := Select[dat334, #[[1]] == π || #[[1]] == -π &];
    ph2 := 
      Select[dat334, 
        #[[1]] == π/2 || #[[1]] == -π/2  || #[[1]] == 3 π/2 || #[[1]] == -3 π/2 &];
    dats := ph0[[All, 2]];
    datt := ph1[[All, 2]];
    datu := ph2[[All, 2]];
    datscos := N[(Cos[(dats)/2])^2];
    dattcos := N[(Cos[(datt)/2])^2];
    datucos := N[(Cos[(datu)/2])^2];
    det1ss := Length[datscos]; 
    det1tt := Length[dattcos];
    det1uu := Length[datucos];
    rr := RandomReal[{0, 1}];
    For[j = 1; det1s = 0, j <= det1ss, j++, 
      If[rr <= datscos[[j]], det1s = det1s + 1];];
    For[k = 1; det1t = 0, k <= det1tt, k++, 
      If[rr <= dattcos[[k]], det1t = det1t + 1];];
    For[l = 1; det1u = 0, l <= det1uu, l++, 
      If[rr <= datucos[[l]], det1u = det1u + 1];];
    det2s := det1ss - det1s;
    det2t := det1tt - det1t;
    det2u := det1uu - det1u;
    aa := {det1s, det2s};
    bb := {det1t, det2t};
    cc := {det1u, det2u};
    det1 := {{1, det1s}, {3, det1t}, {5, det1u}};
    det2 := {{1.5, det2s}, {3.5, det2t}, {5.5, det2u}};
    qber = N[((det2s/(det1s + det2s)) + (det1t/(det1t + det2t)))/2];];
    Print[qber]]

Now when I try to run it, it doesn't give any output. And when I keep the print command outside the for loop(just to check), it says Indeterminate. But I don't think the value of qber can come out to be Indeterminant in anyway. However, I'm unable to find any bug in the code. If you can, then please help me out.

P.S.: I also tried to run the for-loop without using Module, and directly putting all the values inside the for-loop. But still it gives the same error.

Answer 1

I fixed the semicolon/comma thing and put the print statement inside the module but you have some situations where you are dividing by zero causing qber to be indeterminate. This happens when det1s = -det2s or when det1t = -det2t. Adding an if statement to qber prevents dividing by zero.

For[i = 1, i <= 10, i++,
 p = e[[i]];
 Module[{}, dat3 = Table[x - y + p, 100];
  dat34 := Mod[dat3, 2 \[Pi]];
  dat334 := Transpose[{dat3, dat34}];
  ph0 := Select[dat334, #[[1]] == 0 &];
  ph1 := Select[dat334, #[[1]] == \[Pi] || #[[1]] == -\[Pi] &];
  ph2 := Select[
    dat334, #[[1]] == \[Pi]/2 || #[[1]] == -\[Pi]/2 || #[[1]] == 
       3 \[Pi]/2 || #[[1]] == -3 \[Pi]/2 &];
  dats := ph0[[All, 2]];
  datt := ph1[[All, 2]];
  datu := ph2[[All, 2]];
  datscos := N[(Cos[(dats)/2])^2];
  dattcos := N[(Cos[(datt)/2])^2];
  datucos := N[(Cos[(datu)/2])^2];
  det1ss := Length[datscos];
  det1tt := Length[dattcos];
  det1uu := Length[datucos];
  rr := RandomReal[{0, 1}];
  For[j = 1; det1s = 0, j <= det1ss, j++, 
   If[rr <= datscos[[j]], det1s = det1s + 1];];
  For[k = 1; det1t = 0, k <= det1tt, k++, 
   If[rr <= dattcos[[k]], det1t = det1t + 1];];
  For[l = 1; det1u = 0, l <= det1uu, l++, 
   If[rr <= datucos[[l]], det1u = det1u + 1];];
  det2s := det1ss - det1s;
  det2t := det1tt - det1t;
  det2u := det1uu - det1u;
  aa := {det1s, det2s};
  bb := {det1t, det2t};
  cc := {det1u, det2u};
  det1 := {{1, det1s}, {3, det1t}, {5, det1u}};
  det2 := {{1.5, det2s}, {3.5, det2t}, {5.5, det2u}};
 (* If[det1s + det2s != 0 && det1t + det2t != 0,
   qber = N[((det2s/(det1s + det2s)) + (det1t/(det1t + det2t)))/2];
   Print[qber]];*)
  othervals := {det1sval -> det1s, det2sval -> det2s, det1tval -> det1t,
    det2tval -> det2t};
  If[det1s + det2s != 0 && det1t + det2t != 0, 
    qber = N[((det2s/(det1s + det2s)) + (det1t/(det1t + det2t)))/2];
   Print[{qberval -> qber, othervals}], 
   Print[{qberval -> Undefined, othervals}]]
  ]
 ]