(58) Python For Loops

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.

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)

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) Try it Yourself »

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!") Try it Yourself » Note: The else block will NOT be executed if the loop is stopped by a break statement. Example Break the loop when x is 3, and see what happens with the else block: for x in range(6): if x == 3: break print(x) else: print("Finally finished!") #If the loop breaks, the else block is not executed.

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) Try it Yourself » 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)

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 pass Statement

for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error. Example for x in [0, 1, 2]: pass # having an empty for loop like this, would raise an error without the pass statement