Python For Loops

In Python, the 'for' loop is used to iterate over a sequence or any iterable object such as a list, tuple, string, or range.

It repeatedly executes a block of code for each element in the sequence until the iteration of the sequence is completed.

When the iterator is exhausted, the loop terminates.

If there is 'else' clause, it will be executed, when the iterator is exhausted.

for: The for loops begins with the keyword 'for'.

item_variable: 'for' keyword is followed by a variable named 'item_variable' that represents the element/item of the sequence being iterated over. You can choose a variable name. 'in' is another keyword that shows relation between variable and sequence.

sequence: 'sequence' is the iterable object (e.g., a list, tuple, string, range, or any other iterable) over which the loop iterates. You need to replace it with your iterable object.

The sequence is followed by a colon (:) which is part of the syntax.

In the following example, 'for' loop is used to iterate over a list of fruit names:

Output of the above example

Apple
Banana
Orange
Watermelon
                                

Explanation of example:

fruit_list = ["Apple", "Banana", "Orange", "Watermelon"] is a list of string of fruit names.

for fruit in fruit_list: defines the 'for' loop where item is the variable and 'fruit_list' is the list sequence.

print(fruit) is the code block under for loop which prints each element of fruit list.

In the following example, 'for' loop is used to iterate over a tuple of numbers:

Output of the above example

1
2
3
4
                                

Explanation of example:

num_tuple = (1, 2, 3, 4) is a tuple of numbers.

for number in num_tuple: defines the 'for' loop where 'number' is the variable and 'num_tuple' is the tuple sequence.

print(number) is the code block under for loop which prints each element of 'num_tuple' tuple.

In the following example, 'for' loop is used to iterate over a set of numbers:

Output of the above example

1
2
3
4
                                

Explanation of example:

num_set = {1, 2, 3, 4} is a set of numbers.

for numberItem in num_set: defines the 'for' loop where 'numberItem' is the variable and 'num_set' is the set sequence.

print(numberItem) is the code block under for loop which prints each element of 'num_set' set.

In the following example, 'for' loop is used to iterate over a string:

Output of the above example

P
y
t
h
o
n
                                

Explanation of example:

var_String = "Python": This line declares a string variable named 'var_String' and assigns value "Python" to the variable.

for letter in var_String: defines the 'for' loop where 'letter' is the variable and 'var_String' is the string sequence.

print(letter) is the code block under 'for' loop which prints each element of 'var_String' string.

In the following example, 'for' loop is used to iterate over a range function:

Output of the above example

0
1
2
3
4
5
                                

Explanation of example:

for num in range(6): defines the 'for' loop where 'num' is the variable and 'range(6)' is the range() function. The range(6) function generates a sequence of 6 numbers starting from 0 up to 5.

print(num) is the code block under 'for' loop which prints each value of num generated by the range(6) function.

The 'for' loop cab be used to iterate through the keys and values of the dictionary In the following example, 'for' loop is used to iterate over a dictionary:

Output of the above example

0
1
2
3
4
5
                                

Explanation of examples:

Dictionary key example:

personDict = {"name": "John", "age": 25, "country": "UK"}: This line defines a dictionary named 'personDict'.

for key in personDict: defines a 'for' loop where 'key' is the variable that refers to each key of the 'personDict' dictionary.

print(key) is the code block under the 'for' loop that prints each key of the 'personDict' dictionary.

Dictionary value example:

for value in personDict.values(): defines another 'for' loop where 'key' is the variable that refers to each value of the 'personDict' dictionary. 'personDict.values()' defines the values of the dictionary.

print(value) is the code block under the 'for' loop that prints each value of the 'personDict' dictionary.

Dictionary key & value example:

for key, value in personDict.items(): defines yet another 'for' loop where 'key' and 'value' are the variables that refer to each key and value of items of the 'personDict' dictionary. 'personDict.items())' defines the items of the dictionary.

print(key, ":", value) is the code block under the 'for' loop that prints each key and value of the 'personDict' dictionary.

In Python, 'for' loop takes an 'else' block.

The code inside the 'else' block runs once when the iteration of the sequence is completed.

Output of the above example

Apple
Banana
Orange
For loop finished & else block runs.
                                

Explanation of example:

fruit_list = ["Apple", "Banana", "Orange"]: This line defines a list.

for fruit in fruit_list: defines a 'for' loop where 'fruit' is the variable representing the item of 'fruit_list' list.

print(fruit) is the code block under the 'for' loop that prints each item of the 'fruit_list' list.

else: The 'else' block runs once the iteration of the 'for' loop is completed.

print("For loop finished & else block runs.") It prints the message when the 'else' block runs.

In Python, 'for' loop can have nested loops (loop within a loop).

The nested for loops are used to iterate over multiple sequences as demonstrated in the following example:

Output of the above example

Green
Red
Apple
Banana
                                

Explanation of example:

fruitcolor_list = [["Green", "Red"],["Apple", "Banana"]]: This line defines a list with two elements of lists - ["Green", "Red"] and ["Apple", "Banana"].

for itemlist in fruitcolor_list: defines a 'for' loop where 'itemlist' is the variable representing the list item of 'fruitcolor_list' list.

for item in itemlist: defines a nested 'for' loop where 'item' is the variable representing the each element of each 'itemlist' list.

print(item) is the code block under the nested 'for' loop which prints each value of the 'itemlist' list.

In Python, 'break' is used to terminate (stop) the 'for' early at some point.

'continue' is used to skip the current iteration and moves to the next one.

'break' and 'continue' are used with for loop as demonstrated in the following examples:

Output of the above example

0
1
2
The number was 3 and the loop exited.
                                

Explanation of example:

for num in range(11): defines a 'for' loop where 'num' is the variable representing the number of range() function.

if num == 3: checks if the num value is equal to 3.

print("The number was {} and the loop exited.".format(num)) prints the message if the num value is equal to 3.

break exits the 'for' loop when the 'num' is equal to 3.

print(num) prints the num value as long as the num value is less than 3.

Output of the above example

1
3
5
7
9
                                

Explanation of example:

for number in range(10): defines a 'for' loop where 'number' is the variable representing the number of range() function.

if number % 2 == 0: checks if the number is an even number.

continue skips the current iteration if the number is even and moves to the next one.

print(number) prints the number value if the number is not even.

You can find even numbers by skipping the odd numbers in the same way.

The enumerate() function adds a counter (an index) to an iterable and returns it as an enumerate object, which can then be used directly in 'for' loop. The counter can be used as the index of each element of a sequence like list as demonstrated in the following example:

Output of the above example

0 John
1 Jenny
2 Milton
                                

Explanation of example:

personList = ['John', 'Jenny', 'Milton']: This line defines a list with person names.

for index, person in enumerate(personList):: Here enumerate() function takes 'personLis' as parameter. 'index' takes the current index and 'person' takes the value of current element of 'personList'.

print(index, person) is the code block under 'for' loop which prints the current index and person name of the 'personList'.

List comprehension provides a concise way to create a list using 'for' loop as demonstrated in the following example:

Output of the above example

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
                                

Explanation of example:

squares = [x**2 for x in range(10)]: This line defines a list comprehension where the expression 'x**2' is used to create square of each number of the 'range(10)' (from 0 to 9). Expression is placed before the 'for' loop. Finally, it creates a list with the squares of each number of the range.

print(squares) prints a new list of squared numbers.

Practice the above Python code in the following Python Editor by creating more 'for' loop with different sequences. Create more Python for loop in the following Python code editor and click Execute button to check the output. Output will be displayed in Python Code Output section.

Python Editor Execute

print("Example of for loop with list:") #Example of list fruit_list = ["Apple", "Banana", "Orange", "Watermelon"] # a list of fruit names for fruit in fruit_list: print(fruit) print("\nExample of for loop with tuple:") #Example of tuple num_tuple = (1, 2, 3, 4) # a tuple of numbers for number in num_tuple: print(number) print("\nExample of for loop with set:") #Example of set num_set = {1, 2, 3, 4} # a set of numbers for numberItem in num_set: print(numberItem) print("\nExample of for loop with else block:") #Example of else block fruit_list = ["Apple", "Banana", "Orange"] for fruit in fruit_list: print(fruit) else: print("For loop finished & else block runs.") print("\nExample of for loop with nest loop:") #Example of nested loop fruitcolor_list = [["Green", "Red"],["Apple", "Banana"]] for itemlist in fruitcolor_list: for item in itemlist: #nested loop print(item)

Python Code Output