Trouble with local variables in C#

Question

I have a couple of variables that need to be assigned inside a for loop. Apparently, when the loop exits, C# ignores whatever happened in there, and the variables are returned to their original status. Specifically, I need them to be the last and next-to-last elements of a List. Here's the code:

int temp1, temp2;
for (int i = 0; i < toReturn.Count; i++) {
     if (i == toReturn.Count - 2) { // Next-to-last element
         temp1 = toReturn[i];
     } else if (i == toReturn.Count - 1) { // Last element
         temp2 = toReturn[i];
     }
}
// At this point, temp1 and temp2 are treated as uninitialized

Note: Nevermind the bad variable names, they're really temporary variables. Anything more complex would confuse things.

Now, there are two ways (that I know of) to solve this: one is figuring out how to make the variables live after the loop exits, the other is to do something like in Python, where you can do temp = my_list[-1] to get the last element of a list. Is any of these possible in C#?

Edit: When I try to compile, I get a "use of unassigned local variable 'temp1'" error. This code isn't even run, it's just sitting inside of a method that never gets called. If this helps, I'm trying to use the variables inside another loop.

Answer 1

If toReturn.Count is 0, the loop never runs and temp1 and temp2 are never initialized.

Answer 2

Why not just do...

int temp1 = 0;
int temp2 = 0;
    if (toReturn.Count > 1)
        temp1 = toReturn[toReturn.Count - 2];
    if (toReturn.Count > 0)
        temp2 = toReturn[toReturn.Count - 1];

Answer 3

What's this do?

if (toReturn.Count > 1) {
    temp1 = toReturn[toReturn.Count - 2]
    temp2 = toReturn[toReturn.Count - 1]
}

Answer 4

try to give temp1 and temp2 an initial value i.e 0 or whatever is appropriate because they might never be initialized

Answer 5

int temp1 = 0; // Or some other value. Perhaps -1 is appropriate.
int temp2 = 0; 

for (int i = 0; i < toReturn.Count; i++) {
     if (i == toReturn.Count - 2) { // Next-to-last element
         temp1 = toReturn[i];
     } else if (i == toReturn.Count - 1) { // Last element
         temp2 = toReturn[i];
     }
}

The compiler requires that temp1 and temp2 are definitely assigned before trying to read their values. The compiler doesn't know your for-loop will assign the variables. It doesn't know if the for loop ever runs at all. It also doesn't know if your if-conditions will ever be true.

The above code makes sure that temp1 and temp2 have been assigned to something. If you want to be sure whether temp1 and temp2 have been assigned in the loop, consider keeping track of this:

int temp1 = 0;
int temp2 = 0;
bool temp1Assigned = false;
bool temp2Assigned = false;

for (int i = 0; i < toReturn.Count; i++) {
     if (i == toReturn.Count - 2) { // Next-to-last element
         temp1 = toReturn[i];
         temp1Assigned = true;
     } else if (i == toReturn.Count - 1) { // Last element
         temp2 = toReturn[i];
         temp2Assigned = true;
     }
}

Answer 6

If you want a default value:

int count = toReturn.Count;
int temp1 = count > 1 ? toReturn[count - 2] : 0;
int temp2 = count  > 0 ? toReturn[count - 1] : 0;

If you don't care about default value and have previous checks for count in place:

int count = toReturn.Count;
int temp1 = toReturn[count - 2];
int temp2 = toReturn[count - 1];