Python for Loop: Iterating Through Sequences in Python

The for loop in Python has the ability to iterate over the items of any sequence, such as a list or a string.

Syntax

If a sequence contains an expression list, it is evaluated first. Then, the first item (at 0th index) in the sequence is assigned to the iterating variable iterating_var.

Next, the statements block is executed. Each item in the list is assigned to iterating_var, and the statement(s) block is executed until the entire sequence is exhausted.

The following flow diagram illustrates the working of for loop −

Since the loop is executed for each member element in a sequence, there is no need for explicit verification of Boolean expression controlling the loop (as in while loop).

The sequence objects such as list, tuple or string are called iterables, as the for loop iterates through the collection. Any iterator object can be iterated by the for loop.

The view objects returned by items(), keys() and values() methods of dictionary are also iterables, hence we can run a for loop with them.

Python’s built-in range() function returns an iterator object that streams a sequence of numbers. We can run a for loop with range.

Using "for" with a String

A string is a sequence of Unicode letters, each having a positional index. The following example compares each character and displays if it is not a vowel (‘a’, ‘e’, ‘I’, ‘o’ or ‘u’)

Example

Output

On executing, this code will produce the following output −

Btfl s bttr thn gly.
Explct s bttr thn mplct.
Smpl s bttr thn cmplx.
Cmplx s bttr thn cmplctd.

Using "for" with a Tuple

Python’s tuple object is also an indexed sequence, and hence we can traverse its items with a for loop.

Example

In the following example, the for loop traverses a tuple containing integers and returns the total of all numbers.

Output

On executing, this code will produce the following output −

Total = 539

Using "for" with a List

Python’s list object is also an indexed sequence, and hence we can traverse its items with a for loop.

Example

In the following example, the for loop traverses a list containing integers and prints only those which are divisible by 2.

Output

On executing, this code will produce the following output −

34
54
78
44
80

Using "for" with a Range Object

Python’s buil-in range() function returns a range object. Python’s range object is an iterator which generates an integer with each iteration. The object contains integrrs from start to stop, separated by step parameter.

Syntax

The range() function has the following syntax −

Parameters

• Start − Starting value of the range. Optional. Default is 0

• Stop − The range goes upto stop-1

• Step − Integers in the range increment by the step value. Option, default is 1.

Return Value

The range() function returns a range object. It can be parsed to a list sequence.

Example

Output

On executing, this code will produce the following output −

[0, 1, 2, 3, 4]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[1, 3, 5, 7, 9]

Example

Once we obtain the range, we can use the for loop with it.

Output

On executing, this code will produce the following output −

0 1 2 3 4
10 11 12 13 14 15 16 17 18 19
1 3 5 7 9

Example: Factorial of a Number

Factorial is a product of all numbers from 1 to that number say n. It can also be defined as product of 1, 2, up to n.

We use the range() function to get the sequence of numbers from 1 to n-1 and perform cumumulative multplication to get the factorial value.

Output

On executing, this code will produce the following output −

factorial of 5 is 120

In the above program, change the value of N to obtain factorial value of different numbers.

Using "for" Loop with Sequence Index

To iterate over a sequence, we can obtain the list of indices using the range() function

We can then form a for loop as follows:

On executing, this code will produce the following output −

index: 0 number: 34
index: 1 number: 54
index: 2 number: 67
index: 3 number: 21
index: 4 number: 78

Using "for" with Dictionaries

Unlike a list, tuple or a string, dictionary data type in Python is not a sequence, as the items do not have a positional index. However, traversing a dictionary is still possible with different techniques.

Running a simple for loop over the dictionary object traverses the keys used in it.

On executing, this code will produce the following output −

10
20
30
40

Once we are able to get the key, its associated value can be easily accessed either by using square brackets operator or with the get() method. Take a look at the following example −

It will produce the following output −

10 : Ten
20 : Twenty
30 : Thirty
40 : Forty

The items(), keys() and values() methods of dict class return the view objects dict_items, dict_keys and dict_values respectively. These objects are iterators, and hence we can run a for loop over them.

The dict_items object is a list of key-value tuples over which a for loop can be run as follows −

It will produce the following output −

(10, 'Ten')
(20, 'Twenty')
(30, 'Thirty')
(40, 'Forty')

Here, "x" is the tuple element from the dict_items iterator. We can further unpack this tuple in two different variables. Check the following code −

It will produce the following output −

10 : Ten
20 : Twenty
30 : Thirty
40 : Forty

Similarly, the collection of keys in dict_keys object can be iterated over. Take a look at the following example −

It will produce the same output −

10 : Ten
20 : Twenty
30 : Thirty
40 : Forty


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