Non-Programmer's Tutorial for Python 2.6/For Loops

And here is the new typing exercise for this chapter:

onetoten = range(1, 11)
for count in onetoten:
    print count

and the ever-present output:

1
2
3
4
5
6
7
8
9
10

The output looks awfully familiar but the program code looks different. The first line uses the range function. The range function uses two arguments like this range(start, finish). start is the first number that is produced. finish is one larger than the last number. Note that this program could have been done in a shorter way:

for count in range(1, 11):
    print count

Here are some examples to show what happens with the range command:

>>> range(1, 10)
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(-32, -20)
[-32, -31, -30, -29, -28, -27, -26, -25, -24, -23, -22, -21]
>>> range(5,21)
[5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
>>> range(5)
[0, 1, 2, 3, 4]
>>> range(21, 5)
[]

The next line for count in onetoten: uses the for control structure. A for control structure looks like for variable in list:. list is gone through starting with the first element of the list and going to the last. As for goes through each element in a list it puts each into variable. That allows variable to be used in each successive time the for loop is run through. Here is another example (you don't have to type this) to demonstrate:

demolist = ['life', 42, 'the universe', 6, 'and', 9, 'everything']
for item in demolist:
    print "The Current item is:",
    print item

The output is:

The Current item is: life
The Current item is: 42
The Current item is: the universe
The Current item is: 6
The Current item is: and
The Current item is: 9
The Current item is: everything

Notice how the for loop goes through and sets item to each element in the list. Notice how if you don't want print to go to the next line add a comma at the end of the statement (i.e. if you want to print something else on that line). So, what is for good for? The first use is to go through all the elements of a list and do something with each of them. Here's a quick way to add up all the elements:

list = [2, 4, 6, 8]
sum = 0
for num in list:
    sum = sum + num

print "The sum is:", sum

with the output simply being:

The sum is: 20

Or you could write a program to find out if there are any duplicates in a list like this program does:

list = [4, 5, 7, 8, 9, 1, 0, 7, 10]
list.sort()
prev = list[0]
del list[0]
for item in list:
    if prev == item:
        print "Duplicate of", prev, "found"
    prev = item

and for good measure:

Duplicate of 7 Found

Okay, so how does it work? Here is a special debugging version to help you understand (you don't need to type this in):

l = [4, 5, 7, 8, 9, 1, 0, 7, 10]
print "l = [4, 5, 7, 8, 9, 1, 0, 7, 10]", "\t\tl:", l
l.sort()
print "l.sort()", "\t\tl:", l
prev = l[0]
print "prev = l[0]", "\t\tprev:", prev
del l[0]
print "del l[0]", "\t\tl:", l
for item in l:
    if prev == item:
        print "Duplicate of", prev, "found"
    print "if prev == item:", "\tprev:", prev, "\titem:", item
    prev = item
    print "prev = item", "\t\tprev:", prev, "\titem:", item

with the output being:

l = [4, 5, 7, 8, 9, 1, 0, 7, 10]        l: [4, 5, 7, 8, 9, 1, 0, 7, 10]
l.sort()                l: [0, 1, 4, 5, 7, 7, 8, 9, 10]
prev = l[0]             prev: 0
del l[0]                l: [1, 4, 5, 7, 7, 8, 9, 10]
if prev == item:        prev: 0         item: 1
prev = item             prev: 1         item: 1
if prev == item:        prev: 1         item: 4
prev = item             prev: 4         item: 4
if prev == item:        prev: 4         item: 5
prev = item             prev: 5         item: 5
if prev == item:        prev: 5         item: 7
prev = item             prev: 7         item: 7
Duplicate of 7 found
if prev == item:        prev: 7         item: 7
prev = item             prev: 7         item: 7
if prev == item:        prev: 7         item: 8
prev = item             prev: 8         item: 8
if prev == item:        prev: 8         item: 9
prev = item             prev: 9         item: 9
if prev == item:        prev: 9         item: 10
prev = item             prev: 10        item: 10

The reason I put so many print statements in the code was so that you can see what is happening in each line. (By the way, if you can't figure out why a program is not working, try putting in lots of print statements so you can see what is happening.) First the program starts with a boring old list. Next the program sorts the list. This is so that any duplicates get put next to each other. The program then initializes a prev(ious) variable. Next the first element of the list is deleted so that the first item is not incorrectly thought to be a duplicate. Next a for loop is gone into. Each item of the list is checked to see if it is the same as the previous. If it is a duplicate was found. The value of prev is then changed so that the next time the for loop is run through prev is the previous item to the current. Sure enough, the 7 is found to be a duplicate. (Notice how \t is used to print a tab.)

The other way to use for loops is to do something a certain number of times. Here is some code to print out the first 9 numbers of the Fibonacci series:

a = 1
b = 1
for c in range(1, 10):
    print a,
    n = a + b
    a = b
    b = n

with the surprising output:

1 1 2 3 5 8 13 21 34

Everything that can be done with for loops can also be done with while loops but for loops give an easy way to go through all the elements in a list or to do something a certain number of times.