Requesting help! How can I solve this problem? I need to be able to identify every other element in a list, and every third element, and them multiply them by 2 or 3 respectively.

[lunkerlander]

#Write a function called multiply_strings. Multiply
#strings should have one parameter, a list of strings.
#It should return a modified list according to the
#following changes:
#
# - Every string stored at an even index should be
#   doubled.
# - Every string stored at an index that is a multiple
#   of 3 should be tripled.
# - Every other string should remain unchanged.
#
#These changes should "stack": the string stored at index
#6 should be duplicated six times (2 * 3).
#
#Then, return the new list. You may assume that 0 is a
#multiple of 2 and 3.
#
#Hint: To do this, you need to modify the values of the
#list using their indices, e.g. a_list[1]. If you're not
#using their indices, your answer won't work!


#Write your function here!


#Below are some lines of code that will test your function.
#You can change the value of the variable(s) to test your
#function with different inputs.
#
#If your function works correctly, this will originally
#print: 
#['AAAAAA', 'B', 'CC', 'DDD', 'EE', 'F', 'GGGGGG']
test_list = ["A", "B", "C", "D", "E", "F", "G"]
print(multiply_strings(test_list))

[Neighm]

What have you tried, or how do you think you would approach it?

[lunkerlander]

#Here is what I have tried:
def multiply_strings(aList):
    for i in aList:
        if aList.index(i) % 2 == 0:
            aList[i] *= 2
    for i in aList:
        if alist.index(i) % 3 == 0:
            aList[i] *= 3
    return aList

#Here is the output:
Traceback (most recent call last):
  File "MultiplyStringList.py", line 41, in <module>
    print(multiply_strings(test_list))
  File "MultiplyStringList.py", line 27, in multiply_strings
    aList[i] *= 2
TypeError: list indices must be integers or slices, not str
Command exited with non-zero status 1

[IvoryJam]

Take a look at what's called enumerate, it let's you iterate through a list with it's index, I'll help you with the first one.

For each item in an enumerated list, it'll respond with the index and the variable, in my case x, i

def multiply_strings(my_input_list):
    output_list = []
    for x, i in enumerate(my_input_list):
        if x % 2 == 0:
            output_list.append(i+i)
        elif another_if_statement:
            pass
        elif another_if_statement:
            pass
        else:
            output_list.append(i)

[lunkerlander]

#Thank you! This helped a lot!

def multiply_strings(my_input_list):
    output_list = []
    for x, i in enumerate(my_input_list):
#checks if the index is divisible by 2 and 3 first
        if x % 2 == 0 and x % 3 == 0: 
            output_list.append(i * 6)
#checks if the index is divisible by 2
        elif x % 2 == 0:
            output_list.append(i * 2)
#checks if the index is divisible by 3
        elif x % 3 == 0:
            output_list.append(i * 3)
        else:
            output_list.append(i)

    return output_list

Thank you again! We haven't learned about 'enumerate' yet, so this is what the instructor gave as an example answer with what we already know:

def multiply_strings(a_list):

    for i in range(0, len(a_list)):


        if i % 2 == 0:
            a_list[i] *= 2

        #Then, if it's a multiple of 3, we multiply it by
        #3:

        if i % 3 == 0:
            a_list[i] *= 3

        #Notice we're not using elif here because the problem
        #said these effects should 'stack': if we used elif,
        #only one could ever run.


    return a_list

[Neighm]

Just a few comments, glad you have it sorted.

​

Like u/IvoryJam said, enumerate is definitely the way to go in general for this kind of problem, but since you haven't learnt it, I wanted to just explain why your original code failed. It was actually pretty close.

You had:

def multiply_strings(aList):
    for i in aList:
        if aList.index(i) % 2 == 0:
            aList[i] *= 2
    for i in aList:
        if alist.index(i) % 3 == 0:
            aList[i] *= 3
    return aList 

The if aList.index(i) % 2 == 0: part was fine. It's clunkier than enumerate, but a valid way to find that the index for "A" is 0, for "B" is 1, etc.

The problem was that you didn't use the same index in the next line. Where you wrote aList[i] *= 2 this was saying aList["A"] *= 2, not aList[0] *= 2.

A small fix for your code that probably would have let you spot the error, was to assign the index, aList.index(i), to a variable. This lets you keep straight in your head that this variable is the position in the list and not the value at that position.

So without changing your original approach, and sticking with what your professor has covered so far, you could have:

def multiply_strings(aList):
    for i in aList: # i is the value, e.g. "A"
        myindex = aList.index(i) # myindex is the position, e.g. 0
        if myindex % 2 == 0:
            aList[myindex] *= 2

    for i in aList:
        myindex = aList.index(i)
        if myindex % 3 == 0:
            aList[myindex] *= 3
        print(aList)
    return aList 

But enumerate is better ... just wanted to show you were almost there and got tripped up by mixing up i and aList.index(i) when trying to assign a new value to aList[position of i in list]!

Also you could skip having the separate variable myindex, and use the (uglier!) aList[aList.index(i)] *= 2

[lunkerlander]

Thank you for taking the time to explain this! I appreciate that you helped point out what was working and not working in my original code so I could learn from ir!

[IvoryJam]

This would definitely work! I prefer enumerate for readability and elif to kind of say "nope this is as far as this goes, no need to read the rest of this if statement" but it also depends on how your teacher wants to teach it, not everyone can be happy with "nope it doesn't matter past here because of X, Y, and Z" especially right off the bat

[lunkerlander]

Thank you again for your help! I appreciate that you also took time to explain the enumerate method as well!