Loops

Materials:

• Code-along Jupyter notebook

• Filled-in Jupyter notebook

Often, programs need to do the same task several times repeatedly. You may need to run a task just a handful of times, or maybe thousands of times.

For example, say you want to print out all of the numbers 0 - 5. You could write print() 5 times:

Python

print(0)
print(1)
print(2)
print(3)
print(4)
print(5)

However, if you want to expand this even a few numbers further, it gets very tedious very quickly.

To save us from having to have the same code duplicated over and over, we have loops. They are incredibly powerful tools for examining large amounts of information. Here we will be looking at while loops and for loops.

while loops

while loops are somewhat similar to if statements as both depend on conditions to do actions. You can think of a while loop as an if statement that keeps on repeating as long as the condition stays true.

Here is an example of a basic while loop:

Python

i = 0 # initialize index

while i < 5: # runs 5 times

   print(i)
   i += 1 # increment i

Output

0
1
2
3
4

Let’s break down the code.

• i = 0: We initialize i as 0 before the while loop

• while i < 5:: This begins the while loop and specifies the condition being tested. Essentially it means as long as i is less than 5, keep doing the code below.

• print(i): Print current value of i

• i += 1: We increase the value of i at the end of each loop

• Once i += 1 runs, the while loop checks the current value of i

• If i is less than 5, the code in the while loop runs again. Otherwise, the loop is over.

We have to be careful not to create an infinite loop. For instance, if you removed the line with i += 1, i would never reach 5, and the loop would keep printing 0. If you do this, you can always halt execution of the cell.

Below is more complicated while loop. We have a list of names, and an index i keeping track of where we are in the list. We want to print out names until we reach 'Jimmy', so we keep a boolean variable called notJimmy set to True initially. We go through the list one name at a time and check to see if the name is 'Jimmy'. If the name is 'Jimmy', we change notJimmy to be False; otherwise, we don’t do anything. Regardless, we continue to the end of the loop, printing the name and the value of notJimmy and incrementing i by one. If not Jimmy is true, the loop keeps going; otherwise, it is finished.

Python

friends = ['Jim', 'Bob', 'Jimbob', 'Jimmy', 'James'] # list of friend names

notJimmy = True # initialize condition as True
i = 0 # initialize index i as 0

while notJimmy: # while notJimmy is True

   friend = friends[i] # select one of the friend names

   if friend == 'Jimmy': # if the friend name is Jimmy
      notJimmy = False # change notJimmy to be false

   print(friend, ',', notJimmy) # print each name and also the val of not Jimmy
   i += 1 # increment i

Output

Jim , True
Bob , True
Jimbob , True
Jimmy , False

Question 1: while loops:

Create a variable x with the value of 8. Divide x by 2 and re-assign this value to x. Continue to do this until x is less than 0.00001. Print out how many divisions this takes.

Python

### your code here:

Solution

Python

x = 8
i += 1

while x >= 0.00001:

   x /= 2
   i += 1

print(i)

Output

20

for loops

for loops are one of the most powerful tools that base Python has to offer. for loops take iterables (lists, dictionaries, sets, tuples, even strings) and perform the same actions to each item contained within them.

In the code below, each number in a list gets added to 20, and then the sum is printed. We call this iterating over the items in the list. Note the keywords for and in.

Python

num_list = [0, 1, 2, 3, 4, 5] # list of numbers

for n in num_list: # one at a time, make each of those numbers n

   print(n + 20) # print that number + 20

Output

20
21
22
23
24
25

Let’s break down this code:

• num_list = [0, 1, 2, 3, 4, 5]: Makes a list of integers 0-5.

• for n in num_list:: Take the first item in num_list and assign its value to n.

• print(n + 20): Add n and 20 and print the sum.

• We then go back to the start of the loop, take the next item, assign it to n, and start all over again.

For ordered iterables, like lists, tuples, and strings, for loops iterate over these groups in order.

Just like normal variable names, the variable name we use after for is arbitrary, though short and descriptive is best.

Python

for triangle in num_list: # 0 is not a triangle

   print(triangle)

Output

0
1
2
3
4
5

If you want to quickly create a range of numbers to iterate over, the range() function generates numbers from 0 to the int you provide (but not including it).

Python

for i in range(4): # includes 0, but not 4

   print(i)

Output

0
1
2
3

We can start to use for loops to do tasks with strings, as well.

Python

my_breakfast = ['eggs', 'cereal', 'oatmeal', 'toast']

for food in my_breakfast: # for each string in the list of string

   sentence = 'I like to eat ' + food + '.'
   print(sentence)

Output

I like to eat eggs.
I like to eat cereal.
I like to eat oatmeal.
I like to eat toast.

We can use the enumerate() function to iterate over items in a list and get their indexes at the same time.

When use use enumerate(), we need to provide two variables names separated by a comma. The first represents the current index, and the second is the item at that index.

Python

for i, food in enumerate(my_breakfast):
   print('index:', i)
   print('food:', food)

Output

index: 0
food: eggs
index: 1
food: cereal
index: 2
food: oatmeal
index: 3
food: toast

This is a very useful approach for iterating over multiple lists of the same length at once.

Python

my_lunch = ['sandwich', 'chips', 'fruit', 'juice']
my_dinner = ['pasta', 'salad', 'bread', 'dessert']

for i, breakfast in enumerate(my_breakfast):
   lunch = my_lunch[i]
   dinner = my_dinner[i]

   print("my food today:", breakfast, lunch, dinner, i)

Output

my food today: eggs sandwich pasta 0
my food today: cereal chips salad 1
my food today: oatmeal fruit bread 2
my food today: toast juice dessert 3

Question 2: Looping with indexes

Below are four lists: x1, x2, y1, and y2.

Using a single for loop, subtract the values of x1 and x2 at each index, and take the square of the difference. Do the same for y1 and y2. Add the two squares together. Store all 4 squares in a list in the same order.

Python

x1 = [6.3, 7.1, 3.7, 3.2, 0.1]
x2 = [-5.7, -17.5, -3.2, -19.3,-18.2]
y1 = [34.6, 28.4, 60.0, 68.1, 83.9]
y2 = [188.7,  75.9, 100.1, 61.1, 180.2]

# your code here:

Solution

vals = list()

for i, x_1 in x1:

   x_2 = x2[i]
   y_1 = y1[i]
   y_2 = y2[i]

   val = (x_1 - x_2)**2 + (y_1 - y_2)**2
   vals.append(val)

Adding in conditionals

for loops can become quite powerful when you include conditionals that change behavior based on the item in the current iteration.

Python

for food in my_breakfast:

   if food == 'eggs': # if food is currently 'eggs'

      sentence = 'I do not like to eat ' + food + '.'

   else: # for all other values of food

      sentence = 'I like to eat ' + food + '.'

   print(sentence)

Output

I do not like to eat eggs.
I like to eat cereal.
I like to eat oatmeal.
I like to eat toast.

We can even use full if-elif-else statements.

Python

for food in my_breakfast:

   if len(food) < 5: # if the length of string is less than 5

      sentence = 'I do not like to eat ' + food + '.'

   elif len(food) < 6: # if the length is 5

      sentence = 'I sometimes like to eat ' + food + '.'

   else: # if the length is greater than 5

      sentence = 'I like to eat ' + food + '.'

   print(sentence)

Output

I do not like to eat eggs.
I like to eat cereal.
I like to eat oatmeal.
I sometimes like to eat toast.

Question 3: More loops

Iterate over all integers from 0 to 1000 and print all multiples of 41 (numbers that can be divided by 41 with no remainder). How many multiples are there?

Python

### put your code below:

Solution

Python

i = 0 # counts the number of multiples

for n in range(1000):

   if n % 41 == 0: #if the remainder is 0

      print(n)
      i += 1

print('number of multiples:', i)

Output

0
41
82
123
164
205
246
287
328
369
410
451
492
533
574
615
656
697
738
779
820
861
902
943
984
number of multiples: 25

Nested for loops

Just like you can use if statements in a for loop, you can also put for loops inside of other for loops. This is great if you want to use all combinations of two lists, for instance.

Python

hats = ['bowler', 'fedora', 'beret']
shirts = ['plaid', 'striped', 'polka dot']

print('Outfit combinations:')

for shirt in shirts:

   for hat in hats:

      print(shirt, "shirt with a", hat)

Output

Outfit combinations:
plaid shirt with a bowler
plaid shirt with a fedora
plaid shirt with a beret
striped shirt with a bowler
striped shirt with a fedora
striped shirt with a beret
polka dot shirt with a bowler
polka dot shirt with a fedora
polka dot shirt with a beret

Be careful, however. If you use very long collections of items and nest more than 2 loops, the runtime can become very slow.

Comprehensions

If the outcome of your for loop is to produce a list, dictionary, set, or tuple, and you are using minimal code in your loop, then comprehensions may be perfect for you.

Python

[ food + ' time' for food in my_breakfast ] # creates a list

Output

['eggs time', 'cereal time', 'oatmeal time', 'toast time']

Python

{ food:len(food) for food in my_breakfast } # creates a dictionary

Output

{'eggs': 4, 'cereal': 6, 'oatmeal': 7, 'toast': 5}

Resources

• Software Carpentry

• W3 School - List Comprehensions