Iteration over lists, Python Programming


What if you had a list of integer values, and you need to add  them  up and give the sum?  Here are a number of different types of doing  it.

First,  here  is a type in a style  you  may  have  learned to write  in a Java class (actually, you would have used  for, but Python does not have a for that works  like the one in C and Java).


def addList1(l):

sum  = 0

listLength = len(l)

i =  0

while (i < listLength):

sum  = sum + l[i]

i =  i + 1 return sum


It increments the index i from 0 through the length of the list - 1, and includes the appropriate components of the list into the sum.  This is perfectly right, but pretty verbose and easy to get wrong.

Here is a method of version using  Python's for loop.


def addList2(l):

sum  = 0

for i in range(len(l)):

sum  = sum + l[i]

return sum


A loop of the form


for x in l: something will be executed once for each element in the structure l, with the variable x having each successive element in l on each iteration. So,for  x in range(3): print x will print  0 1 2. Back to addList2,  we look that i will take on variables  from 0 to the length  of the list minus 1, and on every iteration, it will include the appropriate component from l into the addition.  This is more compact and simpler to get right than  the ?rst method of version, but still not the good one  we can do!


This one is even more direct.


def addList3(l):

sum  = 0

for  v in l:

sum  = sum + v return sum


We do not ever really need to work with the indices.  Here, the internal variable v gets on each successive integer  in l, and those values  are goes into sum.


For the truly  lazy, it turns out that  the function we need  is already built  into Python. It is known as sum:


def addList4(l):

return sum(l)


Posted Date: 8/9/2012 7:20:38 AM | Location : United States

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