[Tutor] Converting integers into digit sum (Python 3.3.0)

[spir]

On 12/10/2013 02:31 PM, Rafael Knuth wrote:
> Hej Steven,
>
> thanks for the clarification.
> I have two questions - one about map function and the other about return.
>
>> So, in mathematics we might have a mapping between (let's say) counting
>> numbers 1, 2, 3, 4, ... and the even numbers larger than fifty, 52, 54,
>> 56, ... and so on. The mapping function is 50 + 2*x:
>>
>> x = 1 --> 50 + 2*1 = 52
>> x = 2 --> 50 + 2*2 = 54
>> x = 3 --> 50 + 2*3 = 56
>> x = 4 --> 50 + 2*4 = 58
>>
>> and so on, where we might read the arrow --> as "maps to".
>>
>> So the fundamental idea is that we take a series of elements (in the
>> above case, 1, 2, 3, ...) and a function, apply the function to each
>> element in turn, and get back a series of transformed elements (52, 54,
>> 56, ...) as the result.
>>
>> So in Python, we can do this with map. First we define a function to do
>> the transformation, then pass it to map:
>>
>> def transform(n):
>>      return 50 + 2*n
>>
>> result = map(transform, [1, 2, 3, 4])
>
> #1 Question
>
> In which cases should I use a map function instead of a for loop like
> this for example:
>
> def transform(n, m):
>      for i in range (n, m):
>          print (50 + 2*i)
>
> transform(1,5)
>
>>>>
> 52
> 54
> 56
> 58
>
>> A thought comes to mind... an very important lesson is to learn the
>> difference between return and print, and to prefer return.
>>
>> You have written a function that calculates the digit sum. But it is not
>> *reusable* in other functions, since it cannot do anything but *print*
>> the digit sum. What if you want to store the result in a variable, and
>> print it later? Or print it twice? Or put it in a list? Or add one to
>> it? You're screwed, the function is no use to you at all.
>
> #2 Question
>
> Strangely, I get entirely different results depending on whether I use
> return or print within a function.
> Example:
>
> def transform(n, m):
>      for i in range (n, m):
>          print (50 + 2*i)
>
> transform(1,5)
>
>>>>
> 52
> 54
> 56
> 58
>
> Versus:
>
> def transform(n, m):
>      for i in range(n, m):
>          return (50 + 2*i)
>
> print(transform(1,5))
>
>>>>
> 52
>
> Why do I get entirely different results in each case? &:
> How do I prevent my loop from breaking after the first round when
> using return instead of print?
>
> All the best,
>
> Raf
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