I need some help , can anybody explain the Custom sorting code below ; how exactly the code blocks are working?

Question

please read the code first; i am trying to get familiar with custom sorting so I need help understanding how 'eksge' is sorted into 'geeks' and what's happening inside def sortbypattern function, how 'g' is made to appear first then followed by 'e' ,'e' ,'k' ,'s'; can anyone explain me the algorithm and a dry run

def sortbyPattern(pat, str):
  
    priority = list(pat)
  
    # Create a dictionary to store priority of each character
    myDict = { priority[i] : i for i in range(len(priority))}
  
    str = list(str)
  
    # Pass lambda function as key in sort function
    str.sort( key = lambda ele : myDict[ele])
  
    # Reverse the string using reverse()
    str.reverse()
  
    new_str = ''.join(str)
    return new_str
  
  
if __name__=='__main__':
    pat = "asbcklfdmegnot"
    str =  "eksge"
    new_str = sortbyPattern(pat, str)
    print(new_str)

Output : geeks

Answer 1

I don't know what the use of such a function may be, but it's rather simple:

• pat , then priority (which by the way is a totally useless step), and finally myDict specify the expected ordering of the chars
• sort sorts your string comparing each char based on the priority from myDict instead of a standard alphabetical ordering

Answer 2

The sortbyPattern function takes pattern and the string to be sorted as parameters. The pattern is working as a priority list here. The less the index number the lower the priority.

Inside the function, the priority is a list of the characters in the pattern . The pattern given is asbcklfdmegnot . If you print priority , you'll get:

['a', 's', 'b', 'c', 'k', 'l', 'f', 'd', 'm', 'e', 'g', 'n', 'o', 't']

So, as the lower the index, the lower the priority, we can make a dict having priority score similar to their corresponding index. That is what done in the line myDict = { priority[i] : i for i in range(len(priority))} . The dict looks like this:

{'a': 0, 's': 1, 'b': 2, 'c': 3, 'k': 4, 'l': 5, 'f': 6, 'd': 7, 'm': 8, 'e': 9, 'g': 10, 'n': 11, 'o': 12, 't': 13}

Now, str = list(str) is called to make a list of characters inside the input str variable, eksge . Then str.sort( key = lambda ele : myDict[ele]) is applied. This basically sorted the str based on the value of each character in str in the dict myDict . So the str is sorted in ascending order based on the value of each character it got from myDict . In this situation, we got in str :

['s', 'k', 'e', 'e', 'g']

Finally, str.reverse() made the str reversed, hence str in desired order.

Point to be noted, if you had done a reverse=True in the sort parameter, you wouldn't need to call that str.reverse()