Python 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 language, 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 break Statement
With the break statement we can stop the loop before it has looped through all the items:
Example
Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"] for x in fruits: print(x) if x == "banana": break |
Example
Exit the loop when x is "banana", but this time the break comes before the print:
fruits = ["apple", "banana", "cherry"] for x in fruits: if x == "banana": break print(x) |
The continue Statement
With the continue statement we can stop the current iteration of the loop, and continue with the next:
Example
Do not print banana:
fruits = ["apple", "banana", "cherry"] for x in fruits: if x == "banana": continue 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) |
Note that range(6) is not the values of 0 to 6, but the values 0 to 5.
The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):
Example
Using the start parameter:
for x in range(2, 6): print(x) |
The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):
Example
Increment the sequence with 3 (default is 1):
for x in range(2, 30, 3): 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!") |
Nested Loops
A nested loop is a loop inside a loop.
The "inner loop" will be executed one time for each iteration of the "outer loop":
Example
Print each adjective for every fruit:
adj = ["red", "big", "tasty"] fruits = ["apple", "banana", "cherry"] for x in adj: for y in fruits: print(x, y) |
Recursion
Python also accepts function recursion, which means a defined function can call itself.
Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.
The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.
In this example, tri_recursion() is a function that we have defined to call itself ("recurse"). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).
To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.
Example
Recursion Example
def tri_recursion(k): if(k>0): result = k+tri_recursion(k-1) print(result) else: result = 0 return result print("\n\nRecursion Example Results") tri_recursion(6) |
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