Getting the next item in the List based on current item

Question

I have a List<string>, which contains payrollnumbers as strings, e.g.:

 List<string> payrollnumbers = new List<string>() { "0","1","3" };

I want to get the next value in the list, so:

if currpayrollnumber is 0, I want to get the value 1,
if currpayrollnumber is 1, I want to get the value 3,
etc.

I have the following code, however it does not work like expected:

 List<string> payrollnumbers = new List<string>();
 // fill payrollnumbers ...
 var currpayrollIndex = payrollnumbers.IndexOf(currpayrollnumber);
 var nextPayrollIndex = currpayrollIndex++;
 var payrollnumber = payrollnumbers[nextPayrollIndex];

use `++currpayrollIndex;` instead of `currpayrollIndex++;` btw is using `LinkedList` an option? That would make more sense. You can just use `LinkedListNode.Next` property then. — Sriram Sakthivel, Feb 17 '16 at 12:15
Besides, it seems safer to store the current index, rather than the current value, in case of repeated values — Rhumborl, Feb 17 '16 at 12:16
@TimSchmelter [There is a huge difference](http://stackoverflow.com/questions/3346450/what-is-the-difference-between-i-and-i/3346729#3346729). — Sriram Sakthivel, Feb 17 '16 at 12:19

juharr · Answer 1 · 2016-02-17T12:47:59.217

Because you are using the postfix increment operator (i++) the original value is what gets assigned to nextPayrollIndex before currpayrollIndex is incremented. You could use the prefix increment operator (++i) instead, but I would suggest either making it explicit that you are adding 1 and not increment currpayrollIndex at all.

var nextPayrollIndex = currpayrollIndex + 1;

Or just increment currpayrollIndex and use it instead of nextPayrollIndex.

currpayrollIndex++;
var payrollnumber = payrollnumbers[currpayrollIndex];

I'd also suggest adding checks for when the currpayrollnumber is not in the list (IndexOf will return -1) and when it's the last item (to avoid an ArgumentOutOfRangeException)

score 1 · Answer 2 · answered Feb 17 '16 at 12:28

All you need to do is use ++currpayrollIndex instead of currpayrollIndex++:

var nextPayrollIndex = ++currpayrollIndex;

You can also remove that line entirely and put it in when you access the array:

var payrollnumber = payrollnumbers[++currpayrollIndex];

However it's very unreadable, and might not be noticed by people skimming over the code

score 1 · Answer 3 · edited May 23 '17 at 12:07

You have to change:

 var nextPayrollIndex = currpayrollIndex++;

to

var nextPayrollIndex = ++currpayrollIndex;

Is Your solution currpayrollIndex value is increase by 1 after assign to nextPayrollIndex variable

When You change the moment of increase currpayrollIndex by 1 - You first increase by 1 currpayrollIndex value and next assign to nextPayrollIndex variable

It is good explanation here:

What is the difference between i++ and ++i?

Also You may want to change You code to this:

var currpayrollIndex = payrollnumbers.IndexOf(currpayrollnumber);
var payrollnumber = payrollnumbers[++currpayrollIndex ];

it will by more readable

Getting the next item in the List based on current item

3 Answers3