Python Helsinki: 3

Introduction to Programming: Part 3

Loops

Some notes

Writing conditions

Any Boolean expression or combination thereof is a valid condition in a loop. For example, the following program prints out every third number, but only as long as the number is less than 100 and not divisible by 5:

number = int(input("Please type in a number: "))

while number < 100 and number % 5 != 0:
    print(number)
    number += 3
Two examples of the program's execution with different inputs:

Sample output

Please type in a number: 28
28
31
34
37

Sample output

Please type in a number: 96
96
99

Debugging

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Building strings

The += operator allows us to write this a little more compactly, this also applies to f-strings, which may come in handy if values stored in variables are needed as parts of the resulting string. For example this would work:

course = "Introduction to Programming"
grade = 4

verdict = "You have received "
verdict += f"the grade {grade} "
verdict += f"from the course {course}"

print(verdict)

Sample output

You have received the grade 4 from the course Introduction to Programming

String

The * operator can also be used with a string, when the other operand is an integer.

The string operand is then repeated the number of times specified by the integer. For example this would work:

word = "banana"
print(word*3)

Sample output

bananabananabanana

As strings are essentially sequences of characters, any single character in a string can also be retrieved. The operator [] finds the character with the index specified within the brackets.

The index refers to a position in the string, counting up from zero. The first character in the string has index 0, the second character has index 1, and so forth.

For example, this program

input_string = input("Please type in a string: ")
print(input_string[0])
print(input_string[1])
print(input_string[3])
would print out this:

Sample output

Please type in a string: monkey
m
o
k

IndexError: string index out of range

If you tried the above examples for yourself, you may already have come across the error message IndexError: string index out of range.

This error appears if you try to access an index which is not present in the string.

input_string = input("Please type in a string: ")
print("The tenth character: " + input_string[9])
Sample output
Please type in a string: introduction to programming
The tenth character: i

Sample output

Please type in a string: python

Traceback (most recent call last):
File "", line 1, in 
IndexError: string index out of range

Substrings and slices

A substring of a string is a sequence of characters that forms a part of the string.

For example, the string example contains the substrings exam, amp and ple, among others.

In Python programming, the process of selecting substrings is usually called slicing, and a substring is often referred to as a slice of the string. The two terms can often be used interchangeably.

If you know the beginning and end indexes of the slice you wish to extract, you can do so with the notation [a:b]. This means the slice begins at the index a and ends at the last character before index b - that is, including the first, but excluding the last.

You can think of the indexes as separator lines drawn on the left side of the indexed character, as illustrated in the image below:

Let’s have a closer look at some sliced strings:

input_string = "presumptious"

print(input_string[0:3])
print(input_string[4:10])

# if the beginning index is left out, it defaults to 0
print(input_string[:3])

# if the end index is left out, it defaults to the length of the string
print(input_string[4:])

Sample output

pre
umptio
pre
umptious

Half open intervals

In Python string processing the interval [a:b] is half open, which in this case means that the character at the beginning index a is included in the interval, but the character at the end index b is left out.

Why is that?

There is no profound reason for this feature. Rather it is a convention inherited from other programming languages.

Half open intervals may feel unintuitive, but in practice they do have some advantages. For example, you can easily calculate the length of a slice with b-a. On the other hand, you must always remember that the character at the end index b will not be included in the slice.

Searching for substrings

The in operator can tell us if a string contains a particular substring.

The Boolean expression a in b is true, if b contains the substring a.

The operator in returns a Boolean value, so it will only tell us if a substring exists in a string, but it will not be useful in finding out where exactly it is.

Instead, the Python string method find can be used for this purpose.

It takes the substring searched for as an argument, and returns either the first index where it is found, or -1 if the substring is not found within the string.

The image below illustrates how it is used:

The continue command

Another way to change the way a loop is executed is the continue command. It causes the execution of the loop to jump straight to the beginning of the loop, where the condition of the loop is. Then the execution continues normally with checking the condition:

More helper variables with loops

Example

sentence = input('Please type in a sentence: ')

words = sentence.split(' ')
for word in words:
    print(word[0])

Sample output

Please type in a sentence: Love Python a lot
L
P
a
l

Else without if

In Python, a while loop can have an else block without an if statement. This structure is unique to Python and can be useful in certain scenarios. Here’s an example:

count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1
else:
    print("Loop completed successfully")

In this example, the while loop executes as long as the condition count < 5 is true.

The loop body increments the count and prints its value.

Once the condition becomes false (when count reaches 5), the else block is executed.

Note

The else block in a while loop runs if the loop completes normally, without encountering a break statement.

It’s particularly useful for scenarios where you want to perform an action after a loop finishes its normal execution, such as in search algorithms or when you need to confirm that a loop has completed without interruption

The function definition

Before a function can be used, it must be defined. Any function definition begins with the keyword def, short for define. Then comes the name of the function, followed by parentheses and a colon character. This is called the header of the function. After this, indented just like while and if blocks, comes the body of the function.

For example, the following code defines the function message:

def message():
    print("This is my very own function!")