forloop.adoc

For Loop

The for loop is more complex, but it’s also the most commonly used loop.

In Python, the for loop is very powerful. It has a number of different forms, including one which is remarkably simple.

Maybe we have a list of donuts. And we want to print out each one.

list_of_donuts = ["chocolate", "glazed", "jelly"]

for donut in list_of_donuts:
    print(donut)

# you'll see as output
chocolate
glazed
jelly

This is called a for-each loop. It steps thru the list, and For Each donut in list_of_donuts, it does whatever the code in its block specifies - here just printing out the donut. It has the ability to iterate over the items of any sequence, such as a list or a string. A sequence here is a list, or a string, or a couple of other data structures we haven’t yet talked about.

Just like slicing a string, a list (or any sequence of data) can be thought of a series of data point where the index start at zero and goes to len(thing)-1. Just like a string "hello" which goes from 0..4, our donut list goes from 0..2 - in this example you see the range() function for the first time. And in python code, you see a lot of range().

list_of_donuts = ["chocolate", "glazed", "jelly"]

for index in range(len(list_of_donuts)):
    print(list_of_donuts[index])

Here the index was set to 0 and then 1, and then 2, and the index was used to retrieve from the list the donut name at the index. list_of_donuts[1] is "glazed". This example’s out is the same as the for-each loop above.

Let’s use range() is much more common usage.

Here’s one where we go from 0 to 4.

for j in range(5):
    # loop body code
    print(j)

Notice how in this case, the loop prints out 0..4!

Now say we wanted to print out the sum of all the numbers from 1 to 10.

# performing sum of 1 to 10
sum = 0
for i in range(1, 11):
    sum = sum + i
print("Sum of first 10 natural numbers is ", sum)

Wait! Notice how we have to ask range to one beyond the last number we want added to the sum! That’s because the range starts at 0 and includes every whole number up to, but not including, the number that you have provided as the stop number.

There are three versions to the range()'s mechanism.

• range(stop) when you provide one number it acts as the stop value.

• range(start, stop) when you provide two numbers, the first is start, the second is stop.

• range(start, stop, step) when you provide three, they are taken to mean start, stop and step.

So if you have:

• range(5) gives 0, 1, 2, 3, 4

• range(2, 5) gives 2, 3, 4

• range(1, 8, 2) gives 1, 3, 5, 7

Let’s show you a glimpse of the break statement.

for p in range(1,6):
    if p == 4:
        break

print("Loop " + str(p) + " times")

Jumps out of the loop when p is equal to 4.

What about if we print from 10 to 1 with a for loop and a while loop (hint: we need to decrement).

for p in range(10, 0, -1):
    print(p)

Look, we had to range(10, 0, -1) to get the one to print. Otherwise if we had had used range(10,1,-1), we would have only printed 10 to 2.

The range() function gets used a lot in Python, and is the envy of many programmers in other languages who have use the for loop that was handed down from the C programming language. So use it widely, and don’t gloat — too much :-)