user3124306 user3124306 - 10 months ago 62
C# Question

Rearranging a string of numbers from highest to lowest C#

What I'm trying to do is to create a function that will rearrange a string of numbers like "1234" to "4321". I'm certain that there are many much more efficient ways to do this than my method but I just want to see what went wrong with what I did because I'm a beginner at programming and can use the knowledge to get better.

My thought process for the code was to:

  • find the largest number in the inputted string

  • add the largest number into a list

  • remove the largest number from the inputted string

  • find the largest number again from the (now shorter) string

So I made a function that found the largest number in a string and it worked fine:

static int LargestNumber(string num)
int largestnumber = 0;
char[] numbers = num.ToCharArray();
foreach (var number in numbers)
int prevNumber = (int) char.GetNumericValue(number);
if (prevNumber >= largestnumber)
largestnumber = prevNumber;

return largestnumber;

Now the rearranging function is what I am having problems with:

static List<int> Rearrange(string num)
List<int> rearranged = new List<int>(); // to store rearranged numbers
foreach (var number in num) //for every number in the number string
string prevnumber = number.ToString(); // the previous number in the loop
if (prevnumber == LargestNumber(num).ToString()) // if the previous number is the larges number in the inputted string (num)
rearranged.Add(Convert.ToInt32(prevnumber)); // put the previous number into the list

// removing the previous number (largest) from the inputted string and update the inputted string (which should be now smaller)
StringBuilder sb = new StringBuilder(num);
sb.Remove(num.IndexOf(number), 1);
num = sb.ToString();
return rearranged; // return the final rearranged list of numbers

When I run this code (fixed for concatenation):

var rearranged = Rearrange("3250");
string concat = String.Join(" ", rearranged.ToArray());


All I get is:


I'm not sure what I'm missing or what I'm doing wrong - the code doesn't seem to be going back after removing '5' which i s the highest number then removing the next highest number/

Answer Source

Your issue is your if statement within your loop.

if (prevnumber == LargestNumber(num).ToString()

You only ever add to your List rearranged if the value of prevnumber is the largest value, which is false for every number but 5, so the only value that ever gets added to the list is 5.

That's the answer to why it's only returning 5, but I don't think that will make your method work properly necessarily. You're doing a very dangerous thing by changing the value of the collection you are iterating over (the characters in num) from within the loop itself. Other answers have been written for you containing a method that rearranges the numbers as you've described.