Python 3 Course Tutorial - Control Structures

Booleans

Another type in Python is the Boolean type. There are two Boolean values: True and False.
They can be created by comparing values, for instance by using the equal operator ==.

>>> my_boolean = True
>>> my_boolean
True

>>> 2 == 3
False
>>> "hello" == "hello"
True

Be careful not to confuse assignment (oneequals sign) with comparison (twoequals signs).

Comparison

Another comparison operator, the not equal operator (!=), evaluates to True if the items being compared aren't equal, and False if they are.

>>> 1 != 1
False
>>> "eleven" != "seven"
True
>>> 2 != 10
True

Python also has operators that determine whether one number (float or integer) is greater than or smaller than another. These operators are > and < respectively.

>>> 7 > 5
True
>>> 10 < 10
False

The greater than or equal to, and smaller than or equal to operators are >= and <=.
They are the same as the strict greater than and smaller than operators, except that they return True when comparing equal numbers.

>>> 7 <= 8
True
>>> 9 >= 9.0
True

Greater than and smaller than operators can also be used to compare strings lexicographically (the alphabetical order of words is based on the alphabetical order of their component letters).

if Statements

You can use if statements to run code if a certain condition holds.
If an expression evaluates to True, some statements are carried out. Otherwise, they aren't carried out.
An if statement looks like this:

if expression:
   statements

Python uses indentation (white space at the beginning of a line) to delimit blocks of code. Other languages, such as C, use curly braces to accomplish this, but in Python indentation is mandatory; programs won't work without it. As you can see, the statements in the if should be indented.

Here is an example if statement:

if 10 > 5:
   print("10 greater than 5")

print("Program ended")

The expression determines whether 10 is greater than five. Since it is, the indented statement runs, and "10 greater than 5" is output. Then, the unindented statement, which is not part of the if statement, is run, and "Program ended" is displayed.

Result:

>>>
10 greater than 5
Program ended
>>>

Notice the colon at the end of the expression in the if statement.

As the program contains multiple lines of code, you should create it as a separate file and run it.

To perform more complex checks, if statements can be nested, one inside the other.
This means that the inner if statement is the statement part of the outer one. This is one way to see whether multiple conditions are satisfied.

For example:

num = 12
if num > 5:
   print("Bigger than 5")
   if num <=47:
      print("Between 5 and 47")

Result:

>>>
Bigger than 5
Between 5 and 47
>>>

else Statements

An else statement follows an if statement, and contains code that is called when the if statement evaluates to False.
As with if statements, the code inside the block should be indented.

x = 4
if x == 5:
   print("Yes")
else:
   print("No")

Result:

>>> 
No
>>>

You can chain if and else statements to determine which option in a series of possibilities is true.
For example:

num = 7
if num == 5:
  print("Number is 5")
else: 
  if num == 11:
    print("Number is 11")
  else:
    if num == 7:
      print("Number is 7")
    else: 
      print("Number isn't 5, 11 or 7")

Result:

>>>
Number is 7
>>> 

elif Statements

The elif (short for else if) statement is a shortcut to use when chaining if and else statements.
A series of if elif statements can have a final else block, which is called if none of the if or elifexpressions is True.
For example:

num = 7
if num == 5:
   print("Number is 5")
elif num == 11:
   print("Number is 11")
elif num == 7:
   print("Number is 7")
else:
   print("Number isn't 5, 11 or 7")

Result:

>>>
Number is 7
>>>

In other programming languages, equivalents to the elif statement have varying names, including else if, elseif or elsif.

Boolean Logic

Boolean logic is used to make more complicated conditions for if statements that rely on more than one condition.
Python's Boolean operators are and, or, and not.
The and operator takes two arguments, and evaluates as True if, and only if, both of its arguments are True. Otherwise, it evaluates to False.

>>> 1 == 1 and 2 == 2
True
>>> 1 == 1 and 2 == 3
False
>>> 1 != 1 and 2 == 2
False
>>> 2 < 1 and 3 >  6
False

Python uses words for its Boolean operators, whereas most other languages use symbols such as &&, || and !.

Boolean Or

The or operator also takes two arguments. It evaluates to True if either (or both) of its arguments are True, and False if both arguments are False.

>>> 1 == 1 or 2 == 2
True
>>> 1 == 1 or 2 == 3
True
>>> 1 != 1 or 2 == 2
True
>>> 2 < 1 or 3 >  6
False

Boolean Not

Unlike other operators we've seen so far, not only takes one argument, and inverts it.
The result of not True is False, and not False goes to True.

>>> not 1 == 1
False
>>> not 1 > 7
True

You can chain multiple conditional statements in an if statement using the Booleanoperators.

Operator Precedence

Operator precedence is a very important concept in programming. It is an extension of the mathematical idea of order of operations (multiplication being performed before addition, etc.) to include other operators, such as those in Boolean logic.

The below code shows that == has a higher precedence than or:

>>> False == False or True
True
>>> False == (False or True)
False
>>> (False == False) or True
True

Python's order of operations is the same as that of normal mathematics: parentheses first, then exponentiation, then multiplication/division, and then addition/subtraction.

The following table lists all of Python's operators, from highest precedence to lowest.

Operators in the same box have the same precedence.

while Loops

An if statement is run once if its condition evaluates to True, and never if it evaluates to False.
A while statement is similar, except that it can be run more than once. The statements inside it are repeatedly executed, as long as the condition holds. Once it evaluates to False, the next section of code is executed.
Below is a while loop containing a variable that counts up from 1 to 5, at which point the loop terminates.

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

print("Finished!")

Result:

>>>
1
2
3
4
5
Finished!
>>>

The code in the body of a while loop is executed repeatedly. This is called iteration.

The infinite loop is a special kind of while loop; it never stops running. Its condition always remains True.
An example of an infinite loop:

while 1==1:
  print("In the loop") 

This program would indefinitely print "In the loop". 

You can stop the program's execution by using the Ctrl-C shortcut or by closing the program.

break

To end a while loop prematurely, the break statement can be used.
When encountered inside a loop, the break statement causes the loop to finish immediately.

i = 0
while 1==1:
  print(i)
  i = i + 1
  if i >= 5:
    print("Breaking")
    break

print("Finished")

Result:

>>>
0
1
2
3
4
Breaking
Finished
>>>

Using the break statement outside of a loop causes an error.

continue

Another statement that can be used within loops is continue.
Unlike break, continue jumps back to the top of the loop, rather than stopping it.

i = 0
while True:
   i = i +1
   if i == 2:
      print("Skipping 2")
      continue
   if i == 5:
      print("Breaking")
      break
   print(i)

print("Finished")

Result:

>>>
1
Skipping 2
3
4
Breaking
Finished
>>>

Basically, the continue statement stops the current iteration and continues with the next one.

Using the continue statement outside of a loop causes an error.

Lists

Lists are another type of object in Python. They are used to store an indexed list of items.
A list is created using square brackets with commas separating items.
The certain item in the list can be accessed by using its index in square brackets.
For example:

words = ["Hello", "world", "!"]
print(words[0])
print(words[1])
print(words[2])

Result:

>>>
Hello
world
!
>>>

The first list item's index is 0, rather than 1, as might be expected.

An empty list is created with an empty pair of square brackets.

empty_list = []
print(empty_list)

Result:

>>>
[]
>>>

Most of the time, a comma won't follow the last item in a list. However, it is perfectly valid to place one there, and it is encouraged in some cases.

Typically, a list will contain items of a single item type, but it is also possible to include several different types.
Lists can also be nested within other lists.

number = 3
things = ["string", 0, [1, 2, number], 4.56]
print(things[1])
print(things[2])
print(things[2][2])

Result:

>>>
0
[1, 2, 3]
3
>>>

Lists of lists are often used to represent 2D grids, as Python lacks the multidimensional arrays that would be used for this in other languages.

Indexing out of the bounds of possible list values causes an IndexError.
Some types, such as strings, can be indexed like lists. Indexing strings behaves as though you are indexing a list containing each character in the string.
For other types, such as integers, indexing them isn't possible, and it causes a TypeError.

str = "Hello world!"
print(str[6])

Result:

>>>
w
>>>

List Operations

The item at a certain index in a list can be reassigned.
For example:

nums = [7, 7, 7, 7, 7]
nums[2] = 5
print(nums)

Result:

>>>
[7, 7, 5, 7, 7]
>>>

Lists can be added and multiplied in the same way as strings.
For example:

nums = [1, 2, 3]
print(nums + [4, 5, 6])
print(nums * 3)

Result:

>>>
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 1, 2, 3, 1, 2, 3]
>>>

Lists and strings are similar in many ways - strings can be thought of as lists of characters that can't be changed.

To check if an item is in a list, the in operator can be used. It returns True if the item occurs one or more times in the list, and False if it doesn't.

words = ["spam", "egg", "spam", "sausage"]
print("spam" in words)
print("egg" in words)
print("tomato" in words)

Result:

>>>
True
True
False
>>>

The in operator is also used to determine whether or not a string is a substring of another string.

To check if an item is not in a list, you can use the not operator in one of the following ways:

nums = [1, 2, 3]
print(not 4 in nums)
print(4 not in nums)
print(not 3 in nums)
print(3 not in nums)

Result:

>>>
True
True
False
False
>>>

List Functions

Another way of altering lists is using the append method. This adds an item to the end of an existing list.

nums = [1, 2, 3]
nums.append(4)
print(nums)

Result:

>>>
[1, 2, 3, 4]
>>>

The dot before append is there because it is a method of the list class. Methods will be explained in a later lesson.

To get the number of items in a list, you can use the len function.

nums = [1, 3, 5, 2, 4]
print(len(nums))

Result:

>>>
5
>>>

Unlike append, len is a normal function, rather than a method. This means it is written before the list it is being called on, without a dot.

The insert method is similar to append, except that it allows you to insert a new item at any position in the list, as opposed to just at the end.

words = ["Python", "fun"]
index = 1
words.insert(index, "is")
print(words)

Result:

>>>
['Python', 'is', 'fun']
>>>

The index method finds the first occurrence of a list item and returns its index.
If the item isn't in the list, it raises a ValueError.

letters = ['p', 'q', 'r', 's', 'p', 'u']
print(letters.index('r'))
print(letters.index('p'))
print(letters.index('z'))

Result:

>>>
2
0
ValueError: 'z' is not in list
>>> 

There are a few more useful functions and methods for lists.
max(list): Returns the list item with the maximum value
min(list): Returns the list item with minimum value
list.count(obj): Returns a count of how many times an item occurs in a list
list.remove(obj): Removes an object from a list
list.reverse(): Reverses objects in a list

Range

The range function creates a sequential list of numbers.
The code below generates a list containing all of the integers, up to 10.

numbers = list(range(10))
print(numbers)

Result:

>>>
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>

The call to list is necessary because range by itself creates a range object, and this must be converted to a list if you want to use it as one.

If range is called with one argument, it produces an object with values from 0 to that argument.
If it is called with two arguments, it produces values from the first to the second.
For example:

numbers = list(range(3, 8))
print(numbers)

print(range(20) == range(0, 20))

Result:

>>>
[3, 4, 5, 6, 7]

True
>>>

range can have a third argument, which determines the interval of the sequence produced. This third argument must be an integer.

numbers = list(range(5, 20, 2))
print(numbers)

Result:

>>>
[5, 7, 9, 11, 13, 15, 17, 19]
>>>

Loops

Sometimes, you need to perform code on each item in a list. This is called iteration, and it can be accomplished with a while loop and a counter variable.
For example:

words = ["hello", "world", "spam", "eggs"]
counter = 0
max_index = len(words) - 1

while counter <= max_index:
   word = words[counter]
   print(word + "!")
   counter = counter + 1

Result:

>>>
hello!
world!
spam!
eggs!
>>>

The example above iterates through all items in the list, accesses them using their indices, and prints them with exclamation marks.

Iterating through a list using a while loop requires quite a lot of code, so Python provides the forloop as a shortcut that accomplishes the same thing.
The same code from the previous example can be written with a for loop, as follows:

words = ["hello", "world", "spam", "eggs"]
for word in words:
  print(word + "!")

Result:

>>>
hello!
world!
spam!
eggs!
>>>

The for loop in Python is like the foreach loop in other languages.

The for loop is commonly used to repeat some code a certain number of times. This is done by combining for loops with range objects.

for i in range(5):
  print("hello!")

Result:

>>>
hello!
hello!
hello!
hello!
hello!
>>>

You don't need to call list on the range object when it is used in a for loop, because it isn't being indexed, so a list isn't required.