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? Or repeating some part of the program until something happens?
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 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.
number = 23
guess = int(raw_input('Enter an integer: '))
if guess == number:
print('Congratulations, you guessed it!')
elif guess < number:
print('No, it is a little higher than that')
else:
# this is the `else` clause, you must have guessed > number to reach here
print('No, it is a little lower than that')
print('Done')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!
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 raw_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 the [enter] key, the raw_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, this is used to indicate to Python that an indented 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.
Note for C/C++ Programmers
There is no
switchstatement in Python. You can use anif..elif..elsestatement to do the same thing (and in some cases, use a dictionary to do it quickly)
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.')
running = False # this causes the while loop to stop
elif guess < number:
print('No, it is a little higher than that.')
else:
print('No, it is a little lower than that.')
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.
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.
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, like integers or strings.
Example (save as for.py):
for i in range(5):
print(i)
print('The for loop is over')Output:
$ python for.py
0
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 range with a number that will be counted up to, starting at 0. For example, range(5), will start at 0 and count by 1 up to (but not including 5), giving [0, 1, 2, 3, 4].
Note that range() generates a list of numbers all at once for us to use. Lists are explained in the data structures chapter.
The for loop then iterates over this range - for i in range(5) is equivalent to for i in [0, 1, 2, 3, 4]. The i variable is assigned to each value in the sequence, one at a time, and then the block of statements in the body of the loop is executed. In this case, we just print the value in the block of statements.
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 object! We will explore this idea in detail in later chapters.
Note for C/C++/Java/C# Programmers
The Python
forloop is radically different from the C/C++forloop. C# programmers will note that theforloop in Python is similar to theforeachloop in C#. Java programmers will note that the same is similar tofor (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 justfor i in range(5). As you can see, theforloop is simpler, more expressive and less error prone in Python.
While inside a loop body, you can use break to immediately exit the loop, and continue to skip over the current iteration to the next iteration. Official docs. A Byte of Python's section on break and continue.
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.