What is for loop in Python?
The for loop in Python is used to iterate over a sequence [list, tuple, string] or other iterable objects. Iterating over a sequence is called traversal.
Syntax of for Loop
for val in sequence:
loop body
Here, val
is the variable that takes the value of the item inside the sequence on each iteration.
Loop continues until we reach the last item in the sequence. The body of for loop is separated from the rest of the code using indentation.
Flowchart of for Loop
Example: Python for Loop
# Program to find the sum of all numbers stored in a list
# List of numbers
numbers = [6, 5, 3, 8, 4, 2, 5, 4, 11]
# variable to store the sum
sum = 0
# iterate over the list
for val in numbers:
sum = sum+val
print["The sum is", sum]
When you run the program, the output will be:
The sum is 48
The range[] function
We can generate a sequence of numbers using range[]
function. range[10]
will generate numbers from 0 to 9 [10 numbers].
We can also define the start, stop and step size as range[start, stop,step_size]
. step_size defaults to 1 if not provided.
The range
object is "lazy" in a sense because it doesn't
generate every number that it "contains" when we create it. However, it is not an iterator since it supports in
, len
and __getitem__
operations.
This function does not store all the values in memory; it would be inefficient. So it remembers the start, stop, step size and generates the next number on the go.
To force this function to output all the items, we can use the function list[]
.
The following example will clarify this.
print[range[10]]
print[list[range[10]]]
print[list[range[2, 8]]]
print[list[range[2, 20, 3]]]
Output
range[0, 10] [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [2, 3, 4, 5, 6, 7] [2, 5, 8, 11, 14, 17]
We can use the range[]
function in for
loops to iterate through a sequence of numbers. It can be combined with the len[]
function to iterate through a sequence using indexing. Here is an example.
# Program to iterate through a list using indexing
genre = ['pop', 'rock', 'jazz']
# iterate over the list using index
for i in range[len[genre]]:
print["I like", genre[i]]
Output
I like pop I like rock I like jazz
for loop with else
A for
loop can have an optional else
block as well. The else
part is executed if the items in the sequence used in for loop exhausts.
The break keyword can be used to stop a for loop. In such cases, the else part is ignored.
Hence, a for loop's else part runs if no break occurs.
Here is an example to illustrate this.
digits = [0, 1, 5]
for i in digits:
print[i]
else:
print["No items left."]
When you run the program, the output will be:
0 1 5 No items left.
Here, the for loop prints items of the list until the loop exhausts. When the for loop exhausts, it executes the
block of code in the else
and prints No items left.
This for...else
statement can be used with the break
keyword to run the else
block only when the break
keyword was not executed. Let's take an example:
# program to display student's marks from record
student_name = 'Soyuj'
marks = {'James': 90, 'Jules': 55, 'Arthur': 77}
for student in marks:
if student == student_name:
print[marks[student]]
break
else:
print['No entry with that name found.']
Output
No entry with that name found.
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]
Try it Yourself »
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]
Try it Yourself »
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
Try it Yourself »
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]
Try it Yourself »
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]
Try it Yourself »
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.
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]:
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]
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!"]
Try it Yourself »
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]
Try it Yourself »
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.