## Example 1¶

Iterating through a list

In [1]:
# We'll learn how to automate this sort of list in the next lecture
list1 = [1,2,3,4,5,6,7,8,9,10]

In [2]:
for num in list1:
print(num)

1
2
3
4
5
6
7
8
9
10


Great! Hopefully this makes sense. Now let's add an if statement to check for even numbers. We'll first introduce a new concept here--the modulo.

### Modulo¶

The modulo allows us to get the remainder in a division and uses the % symbol. For example:

In [3]:
17 % 5

Out[3]:
2

This makes sense since 17 divided by 5 is 3 remainder 2. Let's see a few more quick examples:

In [4]:
# 3 Remainder 1
10 % 3

Out[4]:
1
In [5]:
# 2 Remainder 4
18 % 7

Out[5]:
4
In [6]:
# 2 no remainder
4 % 2

Out[6]:
0

Notice that if a number is fully divisible with no remainder, the result of the modulo call is 0. We can use this to test for even numbers, since if a number modulo 2 is equal to 0, that means it is an even number!

Back to the for loops!

## Example 2¶

Let's print only the even numbers from that list!

In [7]:
for num in list1:
if num % 2 == 0:
print(num)

2
4
6
8
10


We could have also put an else statement in there:

In [8]:
for num in list1:
if num % 2 == 0:
print(num)
else:
print('Odd number')

Odd number
2
Odd number
4
Odd number
6
Odd number
8
Odd number
10


## Example 3¶

Another common idea during a for loop is keeping some sort of running tally during multiple loops. For example, let's create a for loop that sums up the list:

In [9]:
# Start sum at zero
list_sum = 0

for num in list1:
list_sum = list_sum + num

print(list_sum)

55


Great! Read over the above cell and make sure you understand fully what is going on. Also we could have implemented a += to perform the addition towards the sum. For example:

In [10]:
# Start sum at zero
list_sum = 0

for num in list1:
list_sum += num

print(list_sum)

55


## Example 4¶

We've used for loops with lists, how about with strings? Remember strings are a sequence so when we iterate through them we will be accessing each item in that string.

In [11]:
for letter in 'This is a string.':
print(letter)

T
h
i
s

i
s

a

s
t
r
i
n
g
.


## Example 5¶

Let's now look at how a for loop can be used with a tuple:

In [12]:
tup = (1,2,3,4,5)

for t in tup:
print(t)

1
2
3
4
5


## Example 6¶

Tuples have a special quality when it comes to for loops. If you are iterating through a sequence that contains tuples, the item can actually be the tuple itself, this is an example of tuple unpacking. During the for loop we will be unpacking the tuple inside of a sequence and we can access the individual items inside that tuple!

In [13]:
list2 = [(2,4),(6,8),(10,12)]

In [14]:
for tup in list2:
print(tup)

(2, 4)
(6, 8)
(10, 12)

In [15]:
# Now with unpacking!
for (t1,t2) in list2:
print(t1)

2
6
10


Cool! With tuples in a sequence we can access the items inside of them through unpacking! The reason this is important is because many objects will deliver their iterables through tuples. Let's start exploring iterating through Dictionaries to explore this further!

## Example 7¶

In [16]:
d = {'k1':1,'k2':2,'k3':3}

In [17]:
for item in d:
print(item)

k1
k2
k3


Notice how this produces only the keys. So how can we get the values? Or both the keys and the values?

We're going to introduce three new Dictionary methods: .keys(), .values() and .items()

In Python each of these methods return a dictionary view object. It supports operations like membership test and iteration, but its contents are not independent of the original dictionary – it is only a view. Let's see it in action:

In [18]:
# Create a dictionary view object
d.items()

Out[18]:
dict_items([('k1', 1), ('k2', 2), ('k3', 3)])

Since the .items() method supports iteration, we can perform dictionary unpacking to separate keys and values just as we did in the previous examples.

In [19]:
# Dictionary unpacking
for k,v in d.items():
print(k)
print(v)

k1
1
k2
2
k3
3


If you want to obtain a true list of keys, values, or key/value tuples, you can cast the view as a list:

In [20]:
list(d.keys())

Out[20]:
['k1', 'k2', 'k3']

Remember that dictionaries are unordered, and that keys and values come back in arbitrary order. You can obtain a sorted list using sorted():

In [21]:
sorted(d.values())

Out[21]:
[1, 2, 3]

## Example 7¶

### The range() function¶

• We can generate a sequence of numbers using range() function. range(10) will generate numbers from 0 to 9 (10 numbers).
• We can also define the start, stop and step size as range(start, stop,step_size). step_size defaults to 1 if not provided.
• The range object is "lazy" in a sense because it doesn't generate every number that it "contains" when we create it. However, it is not an iterator since it supports in, len and getitem operations.
In [3]:
for x in range(1, 3):
for y in range(1, 11):
print('%d * %d = %d' % (x, y, x*y))

1 * 1 = 1
1 * 2 = 2
1 * 3 = 3
1 * 4 = 4
1 * 5 = 5
1 * 6 = 6
1 * 7 = 7
1 * 8 = 8
1 * 9 = 9
1 * 10 = 10
2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
2 * 4 = 8
2 * 5 = 10
2 * 6 = 12
2 * 7 = 14
2 * 8 = 16
2 * 9 = 18
2 * 10 = 20


## Conclusion¶

We've learned how to use for loops to iterate through tuples, lists, strings, and dictionaries. It will be an important tool for us, so make sure you know it well and understood the above examples.

More resources

https://wiki.python.org/moin/ForLoop