Loops in Python

Loops

A loop is a programming construct that enables repetitive processing of a sequence of statements. Python provides two types of loops to its users: the ‘for loop’ and the ‘while loop’. The ‘for’ and ‘while’ loops are iteration statements that allow a block of code (the body of the loop) to be repeated a number of times.

The For Loop

Python implements an iterator-based ‘for loop’. It is a type of ‘for loop’ that iterates over a list of items through an explicit or implicit iterator.

The loop is introduced by the keyword ‘for’ which is followed by a random variable name which will contain the values supplied by the object.

This is the syntax of Python’s ‘for loop’:

for variable in list:

statements

else:

statements

Here is an example of a ‘for loop’ in Python:

pizza = [“New York Style Pizza”, “Pan Pizza”, “Thin n Crispy Pizza”, “Stuffed Crust Pizza”]

for choice in pizza:

if choice == “Pan Pizza”:

print(“Please pay $16. Thank you!”)

print(“Delicious, cheesy ” + choice)

else:

print(“Cheesy pan pizza is my all-time favorite!”)

print(“Finally, I’m full!”)

Run this and you’ll get the following output on Python Shell:

Delicious, cheesy New York Style Pizza

Please pay $16. Thank you!

Delicious, cheesy Pan Pizza

Delicious, cheesy Thin n Crispy Pizza

Delicious, cheesy Stuffed Crust Pizza

Cheesy pan pizza is my all-time favorite!

Finally, I’m full!

Using a break statement

A Python break statement ends the present loop and instructs the interpreter to starts executing the next statement after the loop. It can be used in both ‘for’ and ‘while’ loops. Besides leading the program to the statement after the loop, a break statement also prevents the execution of the ‘else’ statement.

To illustrate, a break statement may be placed right after the print function of the ‘if statement’:

pizza = [“New York Style Pizza”, “Pan Pizza”, “Thin n Crispy Pizza”, “Stuffed Crust Pizza”]

for choice in pizza:

if choice == “Pan Pizza”:

print(“Please pay $16. Thank you!”)

break

print(“Delicious, cheezy ” + choice)

else:

print(“Cheezy pan pizza is my all-time favorite!”)

print(“Finally, I’m full!”)

The Python Shell will now show:

Delicious, cheezy New York Style Pizza

Please pay $16. Thank you!

Finally, I’m full!

Using Continue Statement

The continue statement brings back program control to the start of the loop. You can use it for both ‘for’ and ‘while’ loops.

To illustrate, the continue statement may be placed right after the print function of the ‘for loop’ to replace the break statement:

pizza = [“New York Style Pizza”, “Pan Pizza”, “Thin n Crispy Pizza”, “Stuffed Crust Pizza”]

for choice in pizza:

if choice == “Pan Pizza”:

print(“Please pay $16. Thank you!”)

continue

print(“Delicious, cheesy ” + choice)

else:

print(“Cheesy pan pizza is my all-time favorite!”)

print(“Finally, I’m full!”)

The output will be:

Delicious, cheesy New York Style Pizza

Please pay $16. Thank you!

Delicious, cheesy Thin n Crispy Pizza

Delicious, cheesy Stuffed Crust Pizza

Cheesy pan pizza is my all-time favorite!

Finally, I’m full!

Using the range() Function with the for Loop

The range() function can be combined with the ‘for loop’ to supply the numbers required by the loop. In the following example, the range(1, x+1) provided the numbers 1 to 50 needed by the ‘for loop’ to add the sum of 1 until 50:

x = 50

total = 0

for number in range(1, x+1):

total = total + number

print(“Sum of 1 until %d: %d” % (x, total))

The Python Shell will display:

l.py

Sum of 1 until 50: 1275

The While Loop

A Python ‘while loop’ repeatedly carries out a target statement while the condition is true.

The loop iterates as long as the defined condition is true. When it ceases to be true and becomes false, control passes to the first line after the loop.

The ‘while loop’ has the following syntax:

while condition

statement

statement

Here is a simple ‘while loop’:

counter = 0

while (counter < 10):

print(‘The count is:’ , counter)

counter = counter + 1

print(“Done!”)

If you run the code, you should see this output:

l.py

The count is: 0

The count is: 1

The count is: 2

The count is: 3

The count is: 4

The count is: 5

The count is: 6

The count is: 7

The count is: 8

The count is: 9

Done!

Using Pass Statement

The pass statement tells the Python interpreter to ‘do nothing’. The interpreter simply continues with the program’s execution whenever the pass statement is encountered. This attribute makes it a good placeholder whenever Python syntactically requires a line but the program itself does not require action. It can be very useful when you’re creating a program and you need to focus on specific areas of your code, but you still want to reserve some loops or test run the incomplete code.

Here is how you would use a pass statement to fill gaps within a code:

def function_name(x):

pass