Inner while loop doesn't get executed

Question

Found that the control never reaches the inner while loop in this snippet for deleting a duplicate number

numbers=[1,6,6,7]
k=len(numbers)
i=0
j=0
while i in range(k-1):
    while j in range(i+1,k):
        if numbers[i] == numbers[j]:
            numbers.remove(numbers[j])
            k-=1
            j-=1
        j += 1
    i += 1
print(numbers)

Answer 1

Your code does not make j start at i+1 . Instead it starts at zero and never changes. The inner loop never runs because 0 is outside of the range you are testing.

Try this simple change:

i=0
while i < k+1:
    j=i+1
    while j < k:
        if numbers[i] == numbers[j]:
            ...

The main change is moving the initialization of j inside the first while loop, so it updates each time you go through it, and never starts out less than or equal to i .

The other change I made is much less important. Rather than using i in range(...) and j in range(...) for the while loop conditions, I just did an inequality test. This is exactly the same as what the range membership test does under the covers, but avoids unnecessary testing for things that can't happen (like j being too small, now). It also makes the loop look a lot less like a for loop, which uses for i in range(...) syntax a lot (with a different meaning).

Another issue you may run into later, with some sets with multiple sets of duplicates is that your code to remove the j th element probably doesn't do what you intend. The call numbers.remove(numbers[j]) removes the first value equal to numbers[j] from the list, which is going to be the one at index i rather than the one at index j . To delete a list item by index, you want to use del numbers[j] .

Answer 2

You could remove duplicates from a list in Python by using the dict.fromkeys() .

numbers=[1,6,6,7]
final_numbers = list(dict.fromkeys(numbers))
print(final_numbers)

In this example, we use the dict.fromkeys() method to create a dictionary from numbers variable. We then use list() to convert our data from a dictionary back to a list. Then, on the final line, we print out our revised list.

Another option is to use set . Sets are used to store collections of unique items in Python. Unlike lists , sets cannot store duplicate values. We can convert our list to a set to remove duplicated items.

numbers=[1,6,6,7]
final_numbers = list(set(numbers))
print(final_numbers)

Answer 3

It doesn't reach because j and i starts at 0 value and in the inner while loop the condition is j in range(i+1, k) which means range(1, 4) and 0 in range(1, 4) would be False . Anyways, you should avoid using j and i as counters and use a for loop instead.

But the solution is easier and doesn't need to traverse the list, if you wanna remove the duplicate values, you can do as below:

numbers = [1, 6, 6, 7]
print(list(set(numbers)))

The result is: [1, 6, 7]