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 Loop
for val in sequence: Body of for
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 Loop
Example: Python 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 print("The sum is",sum)
The sum is 48
The range() function
We can generate a sequence of numbers using
range(10) will generate numbers from 0 to 9 (10 numbers). We can also define the
step size as
step size defaults to 1 if not provided. This function does not store all the values in memory, it would be inefficient. So it remembers the
step sizeand generates the next number on the go. To force this function to output all the items, we can use the function
The following example will clarify this.
>>> range(10) range(0, 10) >>> list(range(10)) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> list(range(2,8)) [2, 3, 4, 5, 6, 7] >>> list(range(2,20,3)) [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 though a sequence using indexing. Here is an example.
# Program to iterate # through a list # using indexing # List of genre genre = ['pop','rock','jazz'] # iterate over the list using index for i in range(len(genre)): print("I like",genre[i])
I like pop I like rock I like jazz
for loop with else
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.
break statement can be used to stop a
for loop. In such case, the
else part is ignored. Hence, a
else part runs if no break occurs.
Here is an example to illustrate this.
# Program to show # the control flow # when using else block # in a for loop # a list of digit list_of_digits = [0,1,2,3,4,5,6] # take input from user input_digit = int(input("Enter a digit: ")) # search the input digit in our list for i in list_of_digits: if input_digit == i: print("Digit is in the list") break else: print("Digit not found in list")
Enter a digit: 3 Digit is in the list
Enter a digit: 9 Digit not found in list
Here, we have a list of digits from 0 to 6. We ask the user to enter a digit and check if the digit is in our list or not. If the digit is present,
for loop breaks prematurely. So, the
else part does not run. But if the items in our list exhausts (digit not found in our list), the program enters the