When I list the integers in a list, an extra one gets added at the end [Python]-CodePudding

numberlist = [1, 4, 7, 5, 6, 2, 4]

for i in numberlist:
    pos = numberlist.index(i)
    next = pos   1
    ordering = [i, numberlist[next]]
    print(ordering)

This is the code I'm having problems with. When I run it, it's supposed to print multiple lists containing 2 items: The value of i and the number right after it. It does that, however an extra number is added for the last iteration and there is no consistency of what number it may be, in this case the output is:

[1, 4]
[4, 7]
[7, 5]
[5, 6]
[6, 2]
[2, 4]
[4, 7]

A 7 is added at the end, and while I thought it may have been just a repetition of a number that is already in the list, if I add another value to the list,

numberlist = [1, 4, 7, 5, 6, 2, 4, 6]

The output becomes:

[1, 4]
[4, 7]
[7, 5]
[5, 6]
[6, 2]
[2, 4]
[4, 7]
[6, 2]

A 2 is added, but 2 isn't present in the list to begin with, and I also noticed that the 6 isn't taken into consideration, it prints the 7 again. However, if instead of a 6, I added a 3, the code would just give me an error, as it should.

  File "c:/Users/santi/Desktop/code/order/#test zone.py", line 8, in <module>
    ordering = [i, numberlist[next]]
IndexError: list index out of range

What's happening here?

CodePudding user response：

numberlist = [1, 4, 7, 5, 6, 2, 4]

Based on your example and your for loop, there are 7 elements in the list.

Once i is 6 which is the last position on the list, it will still carry out all the codes in the for loop. What I could suggest is to use a if condition to break the for loop when it reaches the last element in the list.

numberlist = [1, 4, 7, 5, 6, 2, 4]

for position, number in enumerate(numberlist):
    # Break for loop once it is the last position
    # Need to -1 from len() function as index position is from 0 to 6
    if position == len(numberlist) - 1:
        break
    # proceed to next position with  = 1
    position  = 1
    ordering = [number, numberlist[position]]
    print(ordering)

CodePudding user response：

This code is quite interesting in the way it has exposed some issues that beginners will run into. The first one is to choose more meaningful variable names (see comments) and second one is to learn how to debug a small program.

The earlier comments and post really did not address these issues, even though they do point out the symptoms.

Revise the OP code with more meaningful variable names to demo the intent and avoid pitfalls:

If the L list doesn't contain duplicate number (4, this case), the problem will NOT be able to surface itself.

L = [1, 4, 7, 5, 6, 2, 4]
#       *              *   

for x in L:                          # use x to mean the extracted number.  i is for index usually
 
    next_pos = L.index(x)   1
    
    pair = [x, L[next_pos] ] # take current one and next num.
    
    print(pair)              # when it reached the end of L, 4 in the case, it will find the first 4 (pos. 1) and that's why you have pair (4, 7)

The right way to print the pair of adjacent numbers should be:

L = [1, 4, 7, 5, 6, 2, 4]

for a, b in zip(L, L[1:]):
    print(a, b)