5.1 — Controlling print(), Nested Loops, Lists

Controlling the flow of execution with return statement

As we have seen, return statement allows us to return a value from a function back to the code that calls the function.

But at the same time return statement also ends execution of the function.

When return statement is executed, no further code in the function gets executed.

1def display(message):
2    print("*** " + message + " ***")
3    return
4    print("This will never be displayed")
5    
6    
7display("hello")

Output

*** hello ***

return can be very useful when placed inside an if statement if we want to exit from the function under certain conditions.

Using return statement, we can simplify the prime number example:

1# We saw this before
2def is_prime(num):
3    prime = True
4
5    if num < 2:
6        prime = False
7    else:
8        for i in range(2, num):
9            if num % i == 0:
10                prime = False
11                break
12
13    return prime

1# Simplified version
2def is_prime(num):
3    if num < 2:
4        return False
5
6    for i in range(2, num):
7        if num % i == 0:
8            return False
9
10    return True

Important: we must make sure that all branches/cases in the function return the correct values.

In previous example, if we forget the last return statement in the simplified is_prime function, return None will happen implicitly, which would be incorrect.

Be careful — incorrect indentation changes logic (1)

1def remove_spaces(s):
2    result = "" 
3    for i in range(len(s)):
4        letter = s[i]
5        if letter != " ":
6            result = result + letter
7
8    return result

1def remove_spaces(s):
2    result = "" 
3    for i in range(len(s)):
4        letter = s[i]
5    if letter != " ":
6        result = result + letter
7
8    return result

Be careful — incorrect indentation changes logic (2)

1def remove_spaces(s):
2    result = "" 
3    for i in range(len(s)):
4        letter = s[i]
5        if letter != " ":
6            result = result + letter
7
8    return result

1def remove_spaces(s):
2    result = "" 
3    for i in range(len(s)):
4        letter = s[i]
5        if letter != " ":
6            result = result + letter
7
8        return result  # what will this do?

ASCII code and special characters

Recall ASCII table from a previous lecture:

Python has built-in functions to convert ASCII code (decimal) to/from a single character.

1print(ord("a"))  # 97, the ASCII code for letter "a"
2print(ord("A"))  # 65
3print(ord("$"))  # 36
4
5print(ord("hi"))  # doesn't work for more than one character
6# TypeError: ord() expected a character, but string of length 2 found
7
8print(chr(70))  # F, the character for ASCII code 70
9
10print(chr(103))  # g

Write a program that shifts each letter in a string to the left by 3 steps according to ASCII table.
i.e. A → >, B → ?, C → @, D → A, E → B, etc.

1word = "Python"
2
3result = ""
4for i in range(len(word)):
5    code = ord(word[i])  # Get ASCII code for the letter
6    code = code - 3  # Shift code by 3
7    # Get letter for the code and add it to result:
8    result = result + chr(code)  
9    
10print(result)  # Mvqelk

Text is stored in a file as a sequence of character codes.

cat dog

116

100

111

103

Multiple lines in text:

cat
dog

116

100

111

103

The newline character, which represents “enter” or “return” key, is also stored when text contains multiple lines.

Escape characters

There are special characters, which we may not directly include in a string, e.g.:

newline character: This is the character representing “enter” or “return” key.
tab character: This is the character representing “tab” key.

Such special characters can be used in a string using escape characters.

1# Trying to enter a newline character directly fails:
2message = "Hello
3world"

Output

    message = "Hello
                   ^
SyntaxError: EOL while scanning string literal

To include a newline character in a string we can use the escape character \n in the string:

1message = "hello\nworld"
2print(message)

Output

hello
world

\n is stored as a single character even though it looks like two.

print(ord("\n"))  # 10

Another escape character is \t which represents the tab character.
It is useful as a separator when displaying values:

1# print uses space as separator by default
2print("Khalid", 85)
3print("Reza", 90)
4
5# Using tab as separator
6print("Khalid", 85, sep="\t")
7print("Reza", 90, sep="\t")

Output

Khalid 85
Reza 90
Khalid	85
Reza	90

Controlling print() function

In previous example, we used a keyword argument sep= to tell print which separator to use between values.

Unlike the usual arguments, keyword arguments are given in the form name=value; in the example sep is the name of argument and "\t" is the value.

1# separator can be any string
2print("Alice", 90, 3.14, sep=",")
3# Alice,90,3.14

1print("Alice", 90, 3.14, sep="|")
2# Alice|90|3.14
3
4# even longer than one character
5print("Alice", 90, 3.14, sep="-----")
6# Alice-----90-----3.14
7
8print("Alice", 90, 3.14, sep="")  # No separator!
9# Alice903.14

By default, print() function displays a newline character \n at end of line.

1print("Good", "morning")
2print()  # no arguments, just prints "\n"
3print(123, 3.14)

Output

Good morning

123 3.14

We can change the end character using another keyword argument to print() function, end=.

1print("A sequence of numbers:")
2print(1, end=",")
3print(4, end=",")
4print(9, end=",")

Output

A sequence of numbers:
1,4,9,

end= is useful in a loop:

1N = 10
2for i in range(N):
3    print(i*i, end=", ")  # comma and a space

Output

0, 1, 4, 9, 16, 25, 36, 49, 64, 81,

Change the above example to not print the last comma. E.g.,

Output

0, 1, 4, 9, 16, 25, 36, 49, 64, 81

1N = 10
2for i in range(N):
3    if i == N - 1:
4        print(i * i)
5    else:
6        print(i * i, end=", ")

Multiline strings

Using \n, we can create a single string that contains all of the following lines:

1shopping_list = "Shopping list\n- Milk\n- Eggs\n- Apples\n"
2print(shopping_list)

Output

Shopping list
- Milk
- Eggs
- Apples

Python provides a better create multiline strings using triple quotes: ''' or """.

1>>> shopping_list = """Shopping list
2- Milk
3- Eggs
4- Apples
5"""
6>>> shopping_list
7'Shopping list\n- Milk\n- Eggs\n- Apples\n'

Nested Loops

We can have a for/while loop inside other for/while loops.

This is useful when we have two sequences and we need all combinations/pairs of items from the sequences.

Write a program that prints all pairs of numbers that sum to $7$ when two six-sided dice are rolled.

1# outer loop for first die d1:
2for d1 in range(1, 7):  
3    # inner loop for second die d2
4    for d2 in range(1, 7):
5        if d1 + d2 == 7:
6            print(d1, d2)

Output

Write a program that takes two string—one with consonants and other with vowels—and combines each consonant with every vowel to print a syllable.

1consonants = "bdfghjklmn"
2vowels = "aeiou"

Output

ba be bi bo bu 
da de di do du 
fa fe fi fo fu 
ga ge gi go gu 
ha he hi ho hu 
ja je ji jo ju 
ka ke ki ko ku 
la le li lo lu 
ma me mi mo mu 
na ne ni no nu

1consonants = "bdfghjklmn"
2vowels = "aeiou"
3
4for i in range(len(consonants)):
5    for j in range(len(vowels)):
6        syllable = consonants[i] + vowels[j]
7        print(syllable, end=" ")
8    print()  # to start printing from next line

Lists

A list is like a container that holds a sequence of objects.
Objects contained in a list are called elements or items.
Lists are ordered! The order in which the items are stored in the list matters.
Each item is associated with an index (index $0$ : first item, index $1$ : second item, etc.)

Creating a list

A list is created using square brackets, with each item separated by a comma.

1prime_numbers = [2, 3, 5, 7, 11, 13]
2print(prime_numbers)
3# [2, 3, 5, 7, 11, 13]
4
5print(type(prime_numbers))
6# <class 'list'>

A list can contain items of any type. For example we can have a list of strings:

1days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
2
3# Number of items in the list
4print(len(days))  # 7
5
6empty_list = []
7print(len(empty_list))  # 0

A list can contain any number of items, from zero to as many as the computer’s memory allows.

A list can contains objects of different types.

1# list with mixed types
2numbers = [1, 'two', 3.75]

Items of a list don’t need to be unique.

1# list with duplicate values
2numbers = [5, "five", 5]

Why use a list?

1# Suppose we want to store grades for multiple students
2
3grades1 = 80
4grades2 = 100
5grades3 = 65
6# ...
7# How many variables?!!
8
9# Use just one variable name "grades"
10grades = [80, 100, 65]

Indexing a list

We can access an item inside a list using indexing (square brackets), just as we did for strings.

1days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
2first_day = days[0]
3second_day = days[1]
4last_day = days[6]
5print(first_day, second_day, last_day)  # Mon Tue Sun
6
7# No item at index 7
8print(days[7])  
9# IndexError: list index out of range

1days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
2
3# Negative indexing can be used as well
4print(days[-1], days[-2])  # Sun Sat
5
6numbers = [1, 'two', 3.75]
7print(numbers[0] + numbers[2])  # 4.75
8
9print(numbers[0] + numbers[1])
10# TypeError: unsupported operand type(s) for +: 'int' and 'str'

Modifying the content of a list

We can modify the content of a list after it has been created.

We can change a single item using its index and the assignment operator.

1days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
2days[0] = "Sun"
3print(days)
4# ['Sun', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
5
6days[7] = "No such day"
7# IndexError: list assignment index out of range

for loops are very useful for looping through all the items in a list.

1# compute an average of grades
2grades = [85, 78.5, 98, 75, 100]
3total = grades[0] + grades[1] + grades[2] + grades[3] + grades[4] 
4avg = total / 5
5print(avg)  # prints 87.3

For instance, we could use a for loop to compute the average of grades.

1grades = [85, 78.5, 98, 75, 100]
2
3total = 0
4for i in range(5):
5   total += grades[i]
6
7avg = total / 5
8print(avg)  # 87.3

Generalized version:

1grades = [85, 78.5, 98, 75, 100]
2
3total = 0
4N = len(grades) 
5for i in range(N):
6   total += grades[i]
7
8avg = total / N
9print(avg)  # 87.3

Try the problem “Dot product” on Ed Lessons.

Try the problem “DNA Cut-site” on Ed Lessons.

Try the problem “Palindrome” on Ed Lessons (from last week).

Try the problem “Smallest and largest divisors” on Ed Lessons (from last week)..

Controlling the flow of execution with .css-70ydf1{background-color:#d6cbc5;color:black;border-radius:3px;padding:0.1em 0.3em;font-family:Hack;font-size:0.75em;white-space:pre;}return statement

Be careful — incorrect indentation changes logic (1)

Be careful — incorrect indentation changes logic (2)

ASCII code and special characters

Escape characters

Controlling print() function

Multiline strings

Nested Loops

Lists

Creating a list

Why use a list?

Indexing a list

Modifying the content of a list

Controlling the flow of execution with return statement