List and Functions

In Python there are a number of built-in functions that can be used on lists that allow you to quickly look through a list without writing your own loops:

>>> numbers = [1, 52, 64, 72, 32]
>>> print len(numbers)
5
>>> print max(numbers)
72
>>> print min(numbers)
1
>>> print sum(numbers)
221
>>> print float(sum(numbers))/len(numbers)
44.2

The sum() function can be applied only when elements of the list are numbers otherwise you’ll get TypeError.

>>> guitars
['Gibson', 'Fender', 'Jackson']
>>> sum(guitars)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unsupported operand type(s) for +: 'int' and 'str'

The other functions that we uses such as max(), len() work with lists of strings and other types that can be comparable.

Let’s rewrite an earlier program that computed the average of a list of numbers entered by the user using a list.

First the program to compute an average without a list:

total = 0
count = 0
while True:
     userinput = raw_input('Enter a number: ')
     if userinput == 'done' : break
     value = float(userinput)
     total = total + value
     count = count + 1
average = total / count 
print ‘Averge: ‘, average
Enter a number: 5
Enter a number: 6
Enter a number: 7
Enter a number: 3
Enter a number: 4
Enter a number: 5
Enter a number: 63
Enter a number: 4
Enter a number: 5
Enter a number: 6
Enter a number: 7
Enter a number: done
Average: 10.4545454545

In this program we have to variables: count and sum. Their values before the start of the while loop are equal to zero. Using while loop each time user enters a number the value is added to the variable total and each entry is counted using count variable. While loop can go forever because the condition is always True so we simply added the if statement because we need to be able to stop the program at some point. If user types a string done than the while loop is over and execution of the program continues from calculating average value and printing out the average value as result.

The program could be simplified using built in functions to calculate sum and count at the end using lists of course.

num = list()
while True:
    userinput = raw_input('Enter a number: ')
    if userinput == 'done': break
    value = float(userinput)
    num.append(value)
average = sum(num) / len(num)
print 'Averge: ', average
Enter a number: 5
Enter a number: 3
Enter a number: 4
Enter a number: 2
Enter a number: 6
Enter a number: 67
Enter a number: 232
Enter a number: 123125
Enter a number: 2312563
Enter a number: 21312523645
Enter a number: 123123123542
Enter a number: 23123123
Enter a number: 213123
Enter a number: 2
Enter a number: done

First we’ve created a list called num and it’s an empty list. In while loop each time the user enters a number the number is added to a list using append built in function. At the end of a program, we simply compute the sum of the numbers in the list and divide it by the count of the numbers in the list to come up with the average value.