Chi-Hsiang Wang.

Thanks again. Those are very useful revisions to my code. The functions do a lot more, so I can't use them directly, but I can use the improvements.

And I do normally put Module around the functions to create local variables confined to the function.

I have used the clear at the start, and you are right I should put that in the code as a matter of habit. When I was first learning mathematica the non-local variables and non-resetting of global variables on running the code were my biggest problems. Then I discovered how to fully use "-> " all the time so that I could cut down on setting variables.

The seems to be working wonderfully now. The code is a lot more complicated than the snippets I presented, but the snippets successfully represented the problems I was having.

The great thing about using Mathematica when messing around with numbers and lists is that one can plot and graph the results with such ease, hence I thought I would use this platform.

Thanks again.

Henrik

Thank you so much, the code now seems to work.

I was aware one had to set attributes on the function, but I didn't have the confidence to just try it as I didn't really understand what HoldAll did.

Thanks you so much, I will read up further on this use of HoldAll and its ability to let functions to reference the original variables.

I need to find a decent tutorial on it.

Thank you again. And thanks again to Chi-Hsiang Wang.

I am going to test your patience as alas my problem is not quite solved.

I am doing list manipulation in a function that has a list passed to it from another function. So probably my main problem is one of passing by reference or value or not understanding how to manipulate variables. Apologies again if this in an insanely stupid question

A simplified version of the code

UpperChain = {-10, 1, 2, 3, 4}
LowerChain = {-10, 1, 2, 3, 4}
SpinResult = 7
If[ SpinResult < 19, 
  LowerChain = Win[LowerChain]; UpperChain = Lose[UpperChain];,
  UpperChain = Win[UpperChain]; LowerChain = Lose[LowerChain];
  ];

Win[Chain_] := (
  Bet = Chain[[2]] + Last[Chain];
  (*
  Next line generates an error "is not a variable with a value, so 
its value cannot be changed." I was hoping to end up with Chain={-10,1,2,3,4,5}
  *)
  AppendTo[Chain, Bet];
  (*
  If I use Chain=Append[Chain,
  Bet] this generates an error about the "shape" of the variable 
  *)
  Chain)

Lose[Chain_] := (
  (*
  Delete element 2 and the last element, 
  so that I hope to end up with chain = {-10,2,3}
  *)
  Chain = Delete[Chain, 2];
  Chain = Delete[Length[Chain]];

  Chain)

Hopefully you can see is that the functions "win" and "lose" simply take a parameter of a "chain" list and then attempt to add a member or delete a member.

So basically my question is, what is the best way to pass lists to functions ( or perhaps one might consider them procedures ) where they are manipulated, and the original list is then modified to reflect the result of the function's manipulation, including increasing and decreasing the length of the list. In C++ one would use linked lists and pointers, I was hoping to use the list manipulation code in Mathematica and pass the list as a parameter by reference and I understand with the use of HoldAll one can do something similar, but alas the HoldAll documentation doesn't really explain such an application.

Please replace Append with AppendTo. If using Append, you need

chaintemp = Append[chaintemp, bet];

Hope this is helpful.