A question in combo task. ( I solved it, but I want to know why my first attempt did not work)

Question

You can see my question in the code as comments, but to summarize:

why did the first attempt code not loop until num was bigger than the length of values in x or y?
What is the difference that made second block code to work but not work for the first block of code

stringcases.py

# combo([1, 2, 3], 'abc')
# Output:
# [(1, 'a'), (2, 'b'), (3, 'c')]

# This was my first attempt to answer this question
def combo(x, y):
    combo_list = []
    num = 0
    for values in x , y:  
        results = (x[num], y[num])
        combo_list.append(results)
        num += 1
    return combo_list

# When I tired to run my first attempt code on eclispe
#  comb([1, 2, 3, 4, 5], "abcde")
# I am not sure why, but for some reason, the output is 
# [(1, 'a'), (2, 'b')]

# combo_list was not appending the remaining value in x or y.
# since I included num += 1 at the end of for loop
# I thought that all the values in x and y will be included
# until the index num value is bigger than total values in x or y


# After struggling for an hour I solved this task with this code
def combo(x, y):
    combo_list = []
    num = 0
    for values in x:  # I took out y
        results = (x[num], y[num])
        combo_list.append(results)
        num += 1
    return combo_list
# when I ran this, the combo_list included all the values
# unlike the first attempt, it included all the values of x and y

# Consequently, my question is
# 1. why did the first attemp code not loop until 
#    num was bigger than len of values in x or y?
# 2. What is the difference that made second block code to work
#    but not for the first block of code

Answer 1 · 2017-07-11T08:30:48Z

July 11, 2017 8:30am

Hi there!

It's happening because you're creating a tuple of only two elements, so python goes through to the first element in the tuple (x) and gets the list [1,2,3,4...] and that becomes value. Then the next loop it goes through the second element in the tuple and value becomes the string "abcd..." After that the tuple is over and the for loop ends.

To demonstrate:

>>>for name in "Bob", "Jane":
       print(name)
'Bob'
'Jane'

Note how it doesn't print 'B' - 'o' - 'b' - 'J' - 'a' - 'n' - 'e' ?

It works when you remove y, because now it's not a multidimensional tuple anymore, but a flat list you're iterating through and so the for loop goes through each item in the list x. Because y and x are always the same length in this example, it works fine whichever you loop through - you could do

for i in range(len(x)):

And use i rather than keeping track of num :) In the real world, you'd probably want to do some check that both inputs are the same length and only loop the length of the shorter if not to avoid an IndexError, but that's not needed here.

Hope it helps :)

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taejooncho

taejooncho

A question in combo task. ( I solved it, but I want to know why my first attempt did not work)

1 Answer

Jon Mirow

Jon Mirow

taejooncho

taejooncho