Control Flow
In the programs we have seen till now, there has always been a series of statements faithfully executed by Python in exact top-down order. What if you wanted to change the flow of how it works? For example, you want the program to take some decisions and do different things depending on different situations, such as printing 'Good Morning' or 'Good Evening' depending on the time of the day?
As you might have guessed, this is achieved using control flow statements. There are three control flow statements in Python - 'if', 'for' and 'while'.
Which of these is NOT a control flow statement in Python?
The 'if' statement
The 'if' statement is used to check a condition: if the condition is true, we run a block of statements (called the if-block), else we process another block of statements (called the else-block). The else clause is optional.
Example (save as 'if.py'):
number = 23
guess = int(input('Enter an integer : '))
if guess == number:
# New block starts here
print('Congratulations, you guessed it.')
print('(but you do not win any prizes!)')
# New block ends here
elif guess < number:
# Another block
print('No, it is a little higher than that')
# You can do whatever you want in a block ...
else:
print('No, it is a little lower than that')
# you must have guessed > number to reach here
print('Done')
# This last statement is always executed,
# after the if statement is executed.Output: ''' Enter an integer : 23
Congratulations, you guessed it.
(but you do not win any prizes!)
Done '''
How It Works
In this program, we take guesses from the user and check if it is the number that we have. We set the variable 'number' to any integer we want, say '23'. Then, we take the user's guess using the 'input()' function. Functions are just reusable pieces of programs. We'll read more about them in the next chapter.
We supply a string to the built-in 'input' function which prints it to the screen and waits for input from the user. Once we enter something and press kbd:[enter] key, the 'input()' function returns what we entered, as a string. We then convert this string to an integer using 'int' and then store it in the variable 'guess'. Actually, the 'int' is a class but all you need to know right now is that you can use it to convert a string to an integer (assuming the string contains a valid integer in the text).
Next, we compare the guess of the user with the number we have chosen. If they are equal, we print a success message. Notice that we use indentation levels to tell Python which statements belong to which block. This is why indentation is so important in Python. I hope you are sticking to the "consistent indentation" rule. Are you?
Notice how the 'if' statement contains a colon at the end - we are indicating to Python that a block of statements follows.
Then, we check if the guess is less than the number, and if so, we inform the user that they must guess a little higher than that. What we have used here is the 'elif' clause which actually combines two related 'if else-if else' statements into one combined 'if-elif-else' statement. This makes the program easier and reduces the amount of indentation required.
The 'elif' and 'else' statements must also have a colon at the end of the logical line followed by their corresponding block of statements (with proper indentation, of course)
You can have another 'if' statement inside the if-block of an 'if' statement and so on - this is called a nested 'if' statement.
Remember that the 'elif' and 'else' parts are optional. A minimal valid 'if' statement is:
if True:
print('Yes, it is true')After Python has finished executing the complete 'if' statement along with the associated 'elif' and 'else' clauses, it moves on to the next statement in the block containing the 'if' statement. In this case, it is the main block (where execution of the program starts), and the next statement is the 'print('Done')' statement. After this, Python sees the ends of the program and simply finishes up.
Even though this is a very simple program, I have been pointing out a lot of things that you should notice. All these are pretty straightforward (and surprisingly simple for those of you from C/C++ backgrounds). You will need to become aware of all these things initially, but after some practice you will become comfortable with them, and it will all feel 'natural' to you.
Which part of an if statement is optional?
The while Statement
The 'while' statement allows you to repeatedly execute a block of statements as long as a condition is true. A 'while' statement is an example of what is called a looping statement. A 'while' statement can have an optional 'else' clause.
Example (save as 'while.py'):
number = 23
running = True
while running:
guess = int(input('Enter an integer : '))
if guess == number:
print('Congratulations, you guessed it.')
# this causes the while loop to stop
running = False
elif guess < number:
print('No, it is a little higher than that.')
else:
print('No, it is a little lower than that.')
else:
print('The while loop is over.')
# Do anything else you want to do here
print('Done')Output: ''' Enter an integer : 50
No, it is a little lower than that.
Enter an integer : 22
No, it is a little higher than that.
Enter an integer : 23
Congratulations, you guessed it.
The while loop is over.
Done '''
How It Works
In this program, we are still playing the guessing game, but the advantage is that the user is allowed to keep guessing until he guesses correctly - there is no need to repeatedly run the program for each guess, as we have done in the previous section. This aptly demonstrates the use of the 'while' statement.
We move the 'input' and 'if' statements to inside the 'while' loop and set the variable 'running' to 'True' before the while loop. First, we check if the variable 'running' is 'True' and then proceed to execute the corresponding while-block. After this block is executed, the condition is again checked which in this case is the 'running' variable. If it is true, we execute the while-block again, else we continue to execute the optional else-block and then continue to the next statement.
The 'else' block is executed when the 'while' loop condition becomes 'False' - this may even be the first time that the condition is checked. If there is an 'else' clause for a 'while' loop, it is always executed unless you break out of the loop with a 'break' statement.
The 'True' and 'False' are called Boolean types and you can consider them to be equivalent to the value '1' and '0' respectively.
What is the output of this while loop?
count = 0
while count < 3:
count += 1
print(count)The 'for' loop
The 'for..in' statement is another looping statement which iterates over a sequence of objects i.e. go through each item in a sequence. We will see more about sequences in detail in later chapters. What you need to know right now is that a sequence is just an ordered collection of items.
Example (save as 'for.py'):
for i in range(1, 5):
print(i)
else:
print('The for loop is over')Output: ''' 1
2
3
4 The for loop is over '''
How It Works
In this program, we are printing a sequence of numbers. We generate this sequence of numbers using the built-in 'range' function.
What we do here is supply it two numbers and 'range' returns a sequence of numbers starting from the first number and up to the second number. For example, 'range(1,5)' gives the sequence '[1, 2, 3, 4]'. By default, 'range' takes a step count of 1. If we supply a third number to 'range', then that becomes the step count. For example, 'range(1,5,2)' gives '[1,3]'. Remember that the range extends up to the second number i.e. it does not include the second number.
Note that 'range()' generates only one number at a time, if you want the full list of numbers, call 'list()' on the 'range()', for example, 'list(range(5))' will result in '[0, 1, 2, 3, 4]'. Lists are explained in the data structures chapter.
Complete this code to print numbers from 1 to 5:
print(i)
The 'for' loop then iterates over this range - 'for i in range(1,5)' is equivalent to 'for i in [1, 2, 3, 4]' which is like assigning each number (or object) in the sequence to i, one at a time, and then executing the block of statements for each value of 'i'. In this case, we just print the value in the block of statements.
Remember that the 'else' part is optional. When included, it is always executed once after the 'for' loop is over unless a break statement is encountered.
Remember that the 'for..in' loop works for any sequence. Here, we have a list of numbers generated by the built-in 'range' function, but in general we can use any kind of sequence of any kind of objects! We will explore this idea in detail in later chapters.
The break Statement
The 'break' statement is used to break out of a loop statement i.e. stop the execution of a looping statement, even if the loop condition has not become 'False' or the sequence of items has not been completely iterated over.
An important note is that if you break out of a 'for' or 'while' loop, any corresponding loop 'else' block is not executed.
Example (save as 'break.py'):
while True:
s = input('Enter something : ')
if s == 'quit':
break
print('Length of the string is', len(s))
print('Done')Output: ''' Enter something : Programming is fun Length of the string is 18 Enter something : When the work is done Length of the string is 21 Enter something : if you wanna make your work also fun: Length of the string is 37 Enter something : use Python! Length of the string is 11 Enter something : quit Done '''
What happens when you use 'break' in a loop with an else clause?
How It Works
In this program, we repeatedly take the user's input and print the length of each input each time. We are providing a special condition to stop the program by checking if the user input is ''quit''. We stop the program by breaking out of the loop and reach the end of the program.
The length of the input string can be found out using the built-in 'len' function.
Remember that the 'break' statement can be used with the 'for' loop as well.
Swaroop's Poetic Python
The input I have used here is a mini poem I have written:
''' Programming is fun When the work is done if you wanna make your work also fun: use Python! '''
The 'continue' Statement
The 'continue' statement is used to tell Python to skip the rest of the statements in the current loop block and to continue to the next iteration of the loop.
Example (save as 'continue.py'):
while True:
s = input('Enter something : ')
if s == 'quit':
break
if len(s) < 3:
print('Too small')
continue
print('Input is of sufficient length')
# Do other kinds of processing here...Output: ''' Enter something : a Too small Enter something : 12 Too small Enter something : abc Input is of sufficient length Enter something : quit '''
What's the difference between break and continue?
What does the continue statement do?
Example (save as 'continue.py'):
while True:
s = input('Enter something : ')
if s == 'quit':
break
if len(s) < 3:
print('Too small')
continue
print('Input is of sufficient length')
# Do other kinds of processing here...Output: ''' Enter something : a Too small Enter something : 12 Too small Enter something : abc Input is of sufficient length Enter something : quit '''
How It Works
In this program, we accept input from the user, but we process the input string only if it is at least 3 characters long. So, we use the built-in 'len' function to get the length and if the length is less than 3, we skip the rest of the statements in the block by using the 'continue' statement. Otherwise, the rest of the statements in the loop are executed, doing any kind of processing we want to do here.
What happens in this code for the input "hi"?
while True:
text = input("Enter text: ")
if len(text) < 3:
continue
print("Processing:", text)Note that the 'continue' statement works with the 'for' loop as well.
Summary
We have seen how to use the three control flow statements - 'if', 'while' and 'for' along with their associated 'break' and 'continue' statements. These are some of the most commonly used parts of Python and hence, becoming comfortable with them is essential.
Final Review:
Match the statements with their purpose:
Next, we will see how to create and use functions.