Insertion Sort With No Built In Functions

Question

I'm trying to create an insertion sort program in python with no built in functions. The only problem is that my program will only sort the first item in a list. What am I doing wrong?

My sort

       def insertionsort(list1):
         for index in range(len(list1)):
           value = list1[index]
           position = index
           while position > 0 and list1[position-1] > value:
              list1[position]=list1[position-1]
              position = position-1
           list1[position] = value
           return(list1)

Example Output

List = [3,1,5,2]

Output = [1,3,5,2]

Insertion sort examples can be found in almost any Python text book (e.g., in Liang). It's a shame to ask such questions on SO. — DYZ, Jan 29 '17 at 19:12
If I recall correctly this question was asked not an hour ago... — Willem Van Onsem, Jan 29 '17 at 19:16

score 1 · Answer 1 · edited Jan 29 '17 at 19:28

1

Fix your indentation like: (The return should be outside the loop)

def insertionsort(list1):
  for index in range(len(list1)):
    value = list1[index]
    position = index
    while position > 0 and list1[position-1] > value:
      list1[position]=list1[position-1]
      position = position-1
    list1[position] = value
  return(list1)


aList = [3, 1, 5, 2]

aList = insertionsort (aList)
print (aList)

And indeed, you can find this anywhere:

https://interactivepython.org/runestone/static/pythonds/SortSearch/TheInsertionSort.html

edited Jan 29 '17 at 19:28

user2390182

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answered Jan 29 '17 at 19:17

Jacques de Hooge

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Also, you can just remove the `return` as this does an in-place sort. – user2390182 Jan 29 '17 at 19:30
Right. But then it will be exactly like in the link (apart from the var names)... – Jacques de Hooge Jan 29 '17 at 19:31
No shame in that ;) you cited the source. – user2390182 Jan 29 '17 at 19:32

Pranav Gupta · Answer 2 · 2017-01-29T19:50:59.503

Firstly in insertion sort we assume that our first element is sorted. Hence we start iterating in the list from the first element. Secondly, You are using range function over here. range(4) will include numbers starting from 0-3 and will exclude 4. So the correction your code requires is that you need to use range function in below manner: for index in range(1,len(list1)) instead of for index in range(len(list1)) And correct the indent of your return statement.Return statement should be present only when the for loop has done its task as the function code execution will stop once a return is encountered. The sort function sums up as below.

def insertionsort(list1):
     for index in range(1,len(list1)):
       value = list1[index]
       position = index
       while position > 0 and list1[position-1] > value:
          list1[position]=list1[position-1]
          position = position-1
       list1[position] = value
     return(list1)

Another suggestion is to try a dry run of your code before asking for help.It avoids small problems that you can solve on your own and create a better concept for yourself.

Cheers..!!

Kedar Kodgire · Answer 3 · 2017-01-29T19:22:14.060

0

Try out this piece of code :

def insertionSort(alist):
  for index in range(1,len(alist)):

    currentvalue = alist[index]
    position = index

    while position>0 and alist[position-1]>currentvalue:
        alist[position]=alist[position-1]
        position = position-1

    alist[position]=currentvalue


alist = [3,1,5,2]
insertionSort(alist)
print(alist)

I hope it helps.

edited Jan 29 '17 at 19:22

answered Jan 29 '17 at 19:19

Kedar Kodgire

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Insertion Sort With No Built In Functions

3 Answers3