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Python Python Basics (Retired) Putting the "Fun" Back in "Function" Functions

Putting the "Fun" Back in "Function" Understanding my solution

I successfully solved the both parts of the challenge; however, I would love a better explanation of what exactly is going on in #Part 1 of my code. Specifically, why did I need to define total and set it =0?


Part 1

def add_list(lst): total = 0 for item in lst: total += item return total

Part 2

def summarize(lst): return("The sum of {} is {}.".format(lst, add_list(lst)))

3 Answers

Chris Freeman
Chris Freeman
Treehouse Moderator 68,064 Points

Looking the values each time through the loop, the addition line can be thought of as:

# increment old_total to make new_total
new_total = old_total + item

# before start of next loop:
# save the previous value as old_value
old_total = new_total

The loop would unroll like this:

total_1 = total_initial + firs_item
total_2 = total_1 + second_item
total_3 = total_2 + third_item

# which becomes, in the case of [1, 2, 3]
total = 0 + 1
total = 1 + 2
total = 3 + 3

# total now equals 6
Chris Freeman
Chris Freeman
Treehouse Moderator 68,064 Points

In the code:

def add_list(lst):
    total = 0
    for item in lst:
        total += item
    return total

# the line ...

total += item

# is equivalent to

total = total + 1

When this line is evaluated on the first pass through the for loop the right-hand side is evaluated be before the left. Also at this point "total" doesn't yet have a value, so total + 1 can't be done. This will throw an undefined variable error. By setting "total" to 0 before the loop, the initial value is set for the first pass through the loop.

Does that make sense?

That actually does make sense and I completely overlooked the fact that I was looping the code (i.e. "for.."). So each time 'item' loops through it's the same as:

total = 0 +1 
total = 0 +2 
total = 0 + 3

am I correct in thinking this?

Thus, the 2nd half of the code returns

def summarize(lst): 
  return "The sum of {} is {}.".format(lst, add_list(lst))


  return "The sum of [1, 2, 3] is 6"?

[MOD: added ```python formatting. -cf]

Makes perfect sense! Thank you again.