Modifying parameters: Set::setps in the part assignment is not a symbol [duplicate]

Question

Ok, this will probably be a silly question, but I can't get my head around it. I'm making a simple module that, given a list and two integers, will swap the position of the elements in the list.(probably there's a function that does it, but I'm interested in doing it myself) This is what I have so far:

Exchange[list_, i_, j_] := Module[{temp},
  temp = list[[i]];
  list[[i]] = list[[j]];
  list[[j]] = temp;
  Return[list];
  ]

Then I do: list1 = {1, 2, 3, 4} And finally:

Exchange[list1, 1, 2]

But it returns this error: Set::setps: "{1,2,3,4} in the part assignment is not a symbol." And the list is unchanged. I looked at the help but I don't understand how I'm supposed to make a valid assignment. I know that what is wrong are the lines 2 and 3 of the module... Any help is greatly appreciated

Answer 1

The problem here is that in Mathematica, parameters of a function are not local variables. So trying to modify a parameter of a function inside it's body will lead to an error. The reason is that function arguments are evaluated, when the function is called so that it is actually the result of this evaluation that's textually substituted for the function parameters within the body. To fix this (without using advanced techniques), assign that parameter to a local variable and work with that variable inside the body of the function. Here's a simple fix to your function:

exchange[list_, i_, j_] := Module[{temp}, 
  temp = list; 
  temp[[i]] = list[[j]]; 
  temp[[j]] = list[[i]];
  temp
]

Now:

exchange[list1, 1, 2]

{2, 1, 3, 4}

Note that I've removed Return in your definition as it's not needed here. Also note that the original list remains intact and is not changed.

Answer 2

If you want to change the original list "directly" (inplace) you can do it with HoldFirst:

SetAttributes[Exchange, HoldFirst];

Exchange[list_, a_, b_] := list[[{a, b}]] = list[[{b, a}]]

list1 = {1, 2, 3, 4};

Exchange[list1, 3, 1];

list1

{3, 1, 2, 4}

Multiple swaps:

list1 = {1, 2, 3, 4};

Exchange[list1, ##] & @@@ {{4, 1}, {2, 3}} // Flatten;

list1

{4, 3, 2, 1}