Loops¶

Consider the code below: It prints the numbers 1 through 10 using what we've learned so far.
This notebook is about how to do the same task less tediously. Loops are a way to repeatedly execute some code, in a simple and succinct way.

Indeed, we can use a for loop to do the same in only two lines:

#pseudo code
for number in number_list:
    print number

Loops are part of flow control. The code inside the loop is (usually) executed several times, whereas lines such as above are only executed one time. The program also needs to know when the loop is over and we return to 'linear' flow. Like in if blocks, this is made clear with indentation.

for loops¶

In Python, for loops are written like this:

for variable in sequence:
    this code is executed inside the loop
    and this code too

now we are not in the loop anymore

Read: for each *variable* in *sequence, execute the expression* (that is code you want to repeat for each variable)

• variable is a variable and can be called whatever you want.

• sequence is a sequence we iterate over. It is some kind of collection of items, for instance: a str of characters, a range, a list etc.

• variable references the current position of our *iterator* within the iterable variable. It will iterate over (run through) every item of the collection and then go away when it has visited all items.

• The body of the loop is indented.

• When we write a command on the same indentation level as the for statement the loop is over. This will be executed after the loop.

for loops with ranges¶

Instead of writing out a list with numerical value we can create it using a range:

range() function¶

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 by default, and stops at a specified number (which is not included in the range).

Based on what we learned so far you might think that it creates a list, but it does not. In fact, range does not do anything by itself, but can be used inside a for loop to create the sequence to loop over.

The syntax is: range(start, stop, step)

The step parameter tells the function how many steps to skip and which direction to count (+ for up and - for down).

Examples:

• range(8) gives you integers from 0 through 7.

• range(2, 9) will give you integers from 2 to 8.

• range(10, 20, 2) will give you even numbers from 10 to 18. Remember, the upper limit of the range is excluded!

• range(9, 0, -1) will start from 9 and give you integers down to 1.

Another useful function for for loops is enumerate. It will create a list of tuples where the second element is the current item and the first element is its position:

Exercise 1¶

~ 20 minutes

a. Use a for loop to iterate over range(4). Which numbers does it produce?

b. Now write a for loop using range that produces the numbers 1 to 4.

c. What numbers does the following range generate inside a for loop? Write out the loop to check.

range(12,0,-3)

d. Loop through numbers 1-20:

• If the number is 4 or 13, print "x is unlucky"
• Otherwise:
  • If the number is even, print "x is even"
  • If the number is odd, print "x is odd"

check Conditions.ipynb

e. In the code below we're counting from 0 as python usually does. Can you fix so that it starts writing from 1?

# get both items and their position
for index, country in enumerate(countries):
    print("My number" + str(index) + " favorite country is: " + country)

while loops¶

We can also iterate over a sequence using a while loop, which has a different format:

while condition:
    expression

while loops continue to execute while a certain condition is True, and will end when it becomes False.

user_response = "Something..."
while user_response != "please":
    user_response = input("Ah ah ah, you didn't say the magic word: ")

while loops require more careful setup than for loops, since you have to specify the termination conditions manually.

Be careful! If the condition doesn't become False at some point, your loop will continue *forever*!

Exercise 2¶

~15 minutes

a. What does the following loop do?

i = 1
    while i < 5:
        i + i
        print(i)

Hint: is the value of i changing?

b. What does the following loop do?

i = 0
    while i <= 5:
        i = i + 1
        print(i)

c. Fix the infinite loop below so that it doesn't run endlessly anymore:

# this code runs forever...
    x = 0
    while x != 11:
        x += 2
        print(x)

Python loop control¶

Controlled exit, skipping a block of code, or ignoring external factors that might influence your code, can be achieved with the Python statements: break, continue, and pass.

*break* statement¶

The keyword break gives us the ability to exit out of a loop whenever we want, and can be used in both while and for loops.

Example:

for letter in 'Python':
    if letter == 'h':
        break
    print('Current Letter:', letter)

The break statement needs to be within the block of code under your loop statement, ususally after a conditional if statement.

*continue* statement¶

The continue statement in Python gives you the option to skip over the part of a loop where a condition is met, but to go on to complete the rest of the loop. That is, it disrupts the iteration of the loop that fulfills the condition and returns the control to the beginning of the loop. It works with both while and for loops.

Example:

for letter in 'Python':
    if letter == 'h':
        continue
    print('Current Letter :', letter)

The difference in using continue rather than break is that the loop will continue despite the disruption when the condition is met.

*pass* statement¶

The pass statement is used when a statement is required syntactically but you do not want any command or code to execute. It's a null operation.

Example:

for letter in 'Python':
    if letter == 'h':
        pass
    print('Current Letter :', letter)

Exercise¶

~ 15 minutes

Write a loop that:

• iterates over each character in the string "I live in CPH, and I like it here.";
• for each character checks if it is a space;
• if it is a space, then just continue with the loop;
• if the character is not a space, do the following:
• check if it is a comma , ;
• if the character is a comma ,, break the loop;
• if the character is not a comma, print it.