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Python Python Collections (Retired) Tuples Combo

Posted by john larson
john larson
16,594 Points

Create a function named combo() that takes two iterables and returns a list of tuples: this code works but..

def combo(iter1, iter2):
    output = []
    for index, value in enumerate(iter1):
# I don't see where [index] is defined for iter2
# I suspect that iterating over iter2 is happening 
# just being inside the for loop
# is that where [index] is coming from also?
# by some association with iter1, being inside the loop? 
        output.append((value, iter2[index]))
    return output
x = combo([1, 2, 3], 'abc')
print(x)
Cheo R
Cheo R
37,150 Points
Cheo R
Cheo R
37,150 Points

Remember that values can be retrieved by getting the index of a list.

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']

def show(lst): 
..   for index, value in enumerate(lst): 
..     print("Index: {}\tValue: {}".format(index, value)) 

 show(list1)
Index: 0    Value: 1
Index: 1    Value: 2
Index: 2    Value: 3

show(list2)
Index: 0    Value: a
Index: 1    Value: b
Index: 2    Value: c

Your code:

for index, value in enumerate(iter1):
    output.append((value, iter2[index]))

Is making a tuple of two values:

  • value found in the first list at position of index, given to you by enumerate
  • value from the second list, at the position of of index, where the index is given to you by enumerate.

Say the first list is longer than the second list:

list3 = [1, 2, 3, 4]

show(c)
Index: 0    Value: 1
Index: 1    Value: 2
Index: 2    Value: 3
Index: 3    Value: 4

What do you think will happen?

combo(list3, list2)
Traceback (most recent call last):
File "python", line 1, in <module>
File "python", line 9, in combo
IndexError: list index out of range

It gives and IndexError because the second list does not have a value at that index.

john larson
john larson
16,594 Points

Cheo, you gave a beautiful in-depth explanation that I will go through in detail. I think you answered my question right off. Are you saying that the index that is referenced here:

  • output.append((value, iter2[index]))
  • is actually the index from iter1
  • and that number is used to collaberate the two iterables?
  • cause if it is, that is very clever, and I wouldn't have thought of it on my own