Randomizing Elements in a List (C#)
July 12th, 2007 | View Comments
For a project I am currently working on, I needed the ability to randomly sort a list of strings. Before I describe this further, the following is the C# code I used:
- private Random ranNum;
- private List<string> GetRandomStringList(List<string> inputList)
- {
- ranNum = new Random();
- int randomNumber;
- for (int i = 0; i < inputList.Count; i++)
- {
- int randomNumber;
- randomNumber = ranNum.Next(inputList.Count);
- string currentString = inputList[i];
- string randomString = inputList[randomNumber];
- inputList[i] = randomString;
- inputList[randomNumber] = currentString;
- }
- return inputList;
- }
The approach I use in the above code is fairly simple. I iterate through the list, and at each list item, I store two pieces of information – the current list item designated by the i index (currentString) and a random list item (randomString) picked randomly through randomNumber:
With the two pieces of information, I simply swap their locations in the list. I set the value at position i to that of randomString, and I set the value at position randomNumber to be currentString:
Now, the values in our list are out of order, and since we had no control over where randomNumber was going to be, you can say that the list is randomly out of order. This store and swap operation continues until our index value i has reached the last element of the list. Since no operations are duplicated beyond the length of our list, this operation runs in linear time.
EDIT: Be sure to read OJ’s comments to this post where he outlines ways in improving the code.
Cheers!
Kirupa 
July 12th, 2007 at 9:35 pm
Hi Kirupa,
I just have a few points from a development perspective:
The ‘ranNum’ variable should be local to the GetRandomStringList(), since that’s where you’re constructing it anyway. Either that or construct the ‘ranNum’ variable in the object’s constructor rather than recreate it each time the function is called.
Let’s assume I used your current code in the following manner:
List x = SomeBigLongListOfStuff();
List y = GetRandomStringList( x );
List z = GetRandomStringList( x );
I’d expect x, y and z to all be different – but they’re all exactly the same! They reference the same list, and hence have the same content.
The function name GetRandomStringList() doesn’t imply that the list you pass in (‘inputList’) is going to be modified, and hence is a bit misleading. IMHO, I think you should either change the function signature to ‘private void RandomiseStringList( List list )’ which implies that ‘list’ will be modified, or you should create a whole new list inside your GetRandomStringList() function and leave the input parameter alone. This improves readability of your code, and makes this much more intuitive.
Creating the ‘currentString’ variable isn’t necessary. You could do the exact same thing without adding another variable to your code:
for( int i = 0; i
Cheers
OJ
July 12th, 2007 at 9:35 pm
PS. Any chance you could modify my comment so that the code appears… please?
July 12th, 2007 at 10:11 pm
Good points OJ, and I definitely agree with you about how my current version doesn’t help inform the developer whether the input will get mutated or not. Do you know of a good book/resource for learning proper coding styles and techniques?
What is the full code you want me to post? I am guessing it is:
for (int i = 0; i < inputList.Count; i++)
{
randomNumber = ranNum.Next(inputList.Count);
string randomString = inputList[randomNumber];
inputList[i] = randomString;
inputList[randomNumber] = inputList[i];
}
If it isn’t, you can e-mail it to me at kirupa.at.kirupa.com, and I’ll add it up
Cheers!
Kirupa
July 13th, 2007 at 5:22 pm
Hi Kirupa,
Yup, that was it
Thanks very much. The first example had a few references to “List”, which should have been “List<string>”, but other than that it’s all good.
As far as books go, I think that any prospective coder will get a great deal out of reading Steve Mcconnell’s Code Complete 2. I’ve read this one a few times, and I get something new out of it teach time! (That makes me sound like I don’t pay attention when I’m reading
). Anyway, I think you’ll enjoy it, there’s some fantastic stuff in there.
Cheers mate
OJ
July 13th, 2007 at 11:19 pm
Thanks for the plug mate! I’m in the process of writing up a blog post that was inspired by our discussion.
Cheers
OJ
July 19th, 2007 at 5:33 pm
[...] This blog post was inspired by a brief chat I had recently with Kirupa of kirupa.com. I subscribe to his blog’s RSS feed as he comes out with some really good stuff. His recent post which showed a way of shuffling a List of strings (using C#) inspired a bit of thought on the topic of code readability, how and when it’s learned (if at all) and why there is so little of it around. [...]
July 21st, 2007 at 7:13 am
Hi,
Unless i’m mistaken OJ’s code doesn’t actually work.
Shouldn’t the two assignments to inputList be the other way round? Otherwise you move the random string to position i and then put the random string back where it started?
E.g.
inputList[0] = “String1″
inputList[1] = “String2″
i=0, randomNumber=1
randomString = inputList[1] = “String2″
inputList[0] = randomString = “String2″
inputList[randomNumber] = inputList[0] = “String2″
inputList[0] = “String2″
inputList[1] = “String2″
July 21st, 2007 at 7:18 am
Gav – you are right. I didn’t test OJ’s code, but it looks like swapping them like you mention works correctly:
inputList[randomNumber] = inputList[i];
inputList[i] = randomString;
Cheers!
Kirupa
July 26th, 2007 at 1:17 am
[...] Writing Intuitive CodeA nice article extending the comments made in my Randomizing Elements in a List post on how code readability and maintainability can be improved. [...]
June 10th, 2009 at 8:17 pm
[...] I subscribe to his blog’s RSS feed as he comes out with some really good stuff. His recent post which showed a way of shuffling a List of strings (using C#) inspired a bit of thought on the topic [...]
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