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.

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:

$ python if.py
Enter an integer : 50
No, it is a little lower than that
Done

$ python if.py
Enter an integer : 22
No, it is a little higher than that
Done

$ python if.py
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.

Note for C/C++ Programmers

There is no switch statement in Python. You can use an if..elif..else statement to do the same thing (and in some cases, use a dictionary to do it quickly)

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:

$ python while.py
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.

Note for C/C++ Programmers

Remember that you can have an else clause for the while loop.

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:

$ python for.py
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.

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.

Note for C/C++/Java/C# Programmers

The Python for loop is radically different from the C/C++ for loop. C# programmers will note that the for loop in Python is similar to the foreach loop in C#. Java programmers will note that the same is similar to for (int i : IntArray) in Java 1.5.

In C/C++, if you want to write for (int i = 0; i < 5; i++), then in Python you write just for i in range(0,5). As you can see, the for loop is simpler, more expressive and less error prone in Python.

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:

$ python break.py
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

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:

$ python continue.py
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.

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.

Next, we will see how to create and use functions.