Comparing list elements

Question

I make a simple code:

arr =  [4, 9, 5, 3, 2, 10]
kpmp = [] # Empty list to store the comparison

for i in arr:
    if i>0:
        x = 0
        j = i
        while j>=0:
            j-=1
            if arr[i] > arr[j]:
                x+=1
    kpmp.append(x)
print(kpmp)

But, I got this error:

Traceback (most recent call last):
  File "python", line 11, in <module>
IndexError: list index out of range

Which is this line if arr[i] > arr[j]:

The expected output I'd like to print is a list [0, 0, 1, 3, 4, 0]
Explanation:
arr = [4, 9, 5, 3, 2, 10]
1. 4 has nothing to compare so the result is 0
2. 9 > 4 the result is 0
3. 4 < 5 < 9 the result is 1
4. 3 < 4 < 5 < 9 the result is 3
5. 2 < 3 < 4 < 5 < 9 the result is 4
6. Well, 10 is the greatest in the list so the result is 0

I got stuck here, kinda basic knowledge with Python, though.
Thanks for your help!

Answer 1

You are currently using elements of list as indices, which will cause an out of range error if they are greater than the length of your list.

Since arr contains 9 , this will throw an out of range error. Also, you need to define x before your if statement, or else you will get a variable not found error.

Here is a different solution to your problem using a list comprehension and slicing :

x = [4, 9, 5, 3, 2, 10]
final = [sum(k > j for k in x[:i]) for i, j in enumerate(x)]

# [0, 0, 1, 3, 4, 0]

Explanation:

sum(k > j for k in x[:i])

This counts all previous values that are greater than the current value. x[:i] gets all elements before the current element in your list.

Answer 2

It looks like you are trying to use i and j as indices, but they will instead be the elements of the list itself. If you want use indices you have a couple of options:

for i in range(len(arr)):

or

for i, _ in enumerate(arr):

The mistake is evident if you try to print the i in your original code

arr =  [4, 9, 5, 3, 2, 10]
for i in arr:
    print(i)

This will print all elements of the list, each on their own line

4
9
5
3
2
10

Answer 3

For fixing the above code:

• First you will need to iterate through range of length of list.
• And other change is moving statement of decreasing of j after comparison rather than before.

You can try as below:

arr =  [4, 9, 5, 3, 2, 10]
kpmp = [] # Empty list to store the comparison
# iterate through length of arr
for i in range(len(arr)):
    if i>0:
        x = 0
        j = i
        while j>=0:
            if arr[i] < arr[j]:
                x+=1
            j-=1 # decrease j after comparison rather than before

    kpmp.append(x)
print(kpmp)

Result:

[0, 0, 1, 3, 4, 0]

Using list comprehension:

You can also try using list comprehension as below:

kmpp = [sum(arr[i] < arr[j] for j in range(i)) for i in range(len(arr))]

Answer 4

There is a mistake in your code

for i in arr:
     print(i)         # will print  4 9 5 3 2 10

This will put every element in arr.

So arr[9] will throw an error as the array size is 6.