04_loops_and_conditionals.md

Loops and Conditionals

For Loops

A for loop is used for iterating over a sequence. The sequence can be a list, a tuple, a dictionary, a set, or a string.

cities = ['San Francisco', 'Los Angeles', 'New York', 'Atlanta']

for city in cities:
    print(city)

To iterate over a dictionary, you can call the items() method on it which returns a tuple of key and value for each item.

data = {'city': 'San Francisco', 'population': 881549, 'coordinates': (-122.4194, 37.7749) }

for x, y in data.items():
    print(x, y)

The built-in range() function allows you to create sequence of numbers that you can iterate over

for x in range(5):
    print(x)

The range function can also take a start and an end number

for x in range(1, 10, 2):
    print(x)

Conditionals

Python supports logical conditions such as equals, not equals, greater than etc. These conditions can be used in several ways, most commonly in if statements and loops.

An if statement is written by using the if keyword.

Note: A very common error that programmers make is to use = to evaluate a equals to condition. The = in Python means assignment, not equals to. Always ensure that you use the == for an equals to condition.

for city in cities:
    if city == 'Atlanta':
        print(city)

You can use else keywords along with if to match elements that do not meet the condition

for city in cities:
    if city == 'Atlanta':
        print(city)
    else:
        print('This is not Atlanta')

Python relies on indentation (whitespace at the beginning of a line) to define scope in the for loop and if statements. So make sure your code is properly indented.

You can evaluate a series of conditions using the elif keyword.

Multiple criteria can be combined using the and and or keywords.

cities_population = {
    'San Francisco': 881549,
    'Los Angeles': 3792621,
    'New York': 8175133,
    'Atlanta':498044
}

for city, population in cities_population.items():
    if population < 1000000:
        print('{} is a small city'.format(city))
    elif population > 1000000 and population < 5000000:
        print('{} is a big city'.format(city))
    else:
        print('{} is a mega city'.format(city))

Control Statements

A for-loop iterates over each item in the sequence. Sometimes is desirable to stop the execution, or skip certain parts of the for-loops. Python has special statements, break, continue and pass.

A break statement will stop the loop and exit out of it

for city in cities:
    print(city)
    if city == 'Los Angeles':
        print('I found Los Angeles')
        break

A continue statement will skip the remaining part of the loop and go to the next iteration

for city in cities:
    if city == 'Los Angeles':
        continue
    print(city)

A pass statement doesn't do anything. It is useful when some code is required to complete the syntax, but you do not want any code to execute. It is typically used as a placeholder when a function is not complete.

for city in cities:
    if city == 'Los Angeles':
        pass
    else:
        print(city)

Exercise

The Fizz Buzz challenge.

Write a program that prints the numbers from 1 to 100 and for multiples of 3 print Fizz instead of the number and for the multiples of 5 print Buzz. If it is divisible by both, print FizzBuzz.

So the output should be something like below

1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz, 11, Fizz, 13, 14, FizzBuzz, ...

Breaking down the problem further, we need to create for-loop with following conditions

• If the number is a multiple of both 3 and 5 (i.e. 15), print FizzBuzz
• If the number is multiple of 3, print Fizz
• If the number is multiple of 5, print Buzz
• Otherwise print the number

Hint: See the code cell below. Use the modulus operator % to check if a number is divisible by another. 10 % 5 equals 0, meaning it is divisible by 5.

for x in range(1, 10):
    if x%2 == 0:
        print('{} is divisible by 2'.format(x))
    else:
        print('{} is not divisible by 2'.format(x))

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