Python While Loops
Python Loops
Python has two primitive loop commands:
- while loops
- for loops
The while Loop
With the while loop we can execute a set of statements as long as a condition is true.
Example
Print i as long as i is less than 6:
i = 1
while i < 6:
print(i)
i += 1
remember to increment i, or else the loop will continue forever.
The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i , which we set to 1.
The break Statement
With the break statement we can stop the loop even if the while condition is true:
Example
Exit the loop when i is 3:
i = 1
while i < 6:
print(i)
if i == 3:
break
i += 1
The continue Statement
With the continue statement we can stop the current iteration, and continue with the next:
Example
Continue to the next iteration if i is 3:
i = 0
while i < 6:
i += 1
if i == 3:
continue
print(i)
The else Statement
With the else statement we can run a block of code once when the condition no longer is true:
Example
Print a message once the condition is false:
i = 1
while i < 6:
print(i)
i += 1
else:
print("i is no longer less than 6")
Python For Loops
A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).
This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.
With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
Example
Print each fruit in a fruit list:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
print(x)
The for loop does not require an indexing variable to set beforehand.
Looping Through a String
Even strings are iterable objects, they contain a sequence of characters:
Example
Loop through the letters in the word "banana":
for x in "banana":
print(x)
The range() Function
To loop through a set of code a specified number of times, we can use the range() function,
The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.
Example
Using the range() function:
for x in range(6):
print(x)
Else in For Loop
The else keyword in a for loop specifies a block of code to be executed when the loop is finished:
Example
Print all numbers from 0 to 5, and print a message when the loop has ended:
for x in range(6):
print(x)
else:
print("Finally finished!")