In Python, ‘for loops’ are the fundamental control flow statements that are generally used to execute the particular condition in the block of code repeatedly for a fixed number of iterations. In contrast to other traditional programming languages where ‘for loops’ typically require condition checks and increment operations manually, Python simplifies this whole process by depending on the sequence iteration.
Now in this tutorial, you are going to learn everything about Python for loops including its syntax, working examples, and best practices. We will also help you to explore the more advanced concepts like nested for loops, loop interruptions, filtering data using for loops, and many more. Let’s learn and master ‘for loops’ to enhance your Python programming skills.
What are For Loops in Python?
For Loops in Python are primarily used to iterate through sequences such as lists, strings, tuples, dictionaries, sets, and even custom collections. Every iteration simply processes one element at a time which makes it efficient for various tasks like data traversal, complex calculations, and creating patterns.
The for loops in Python continue till all the elements in the sequence are processed. Unlike while loops, for loops handle the sequence automatically and do not require manual condition checks which makes it a suitable and preferred choice for many programming requirements.
When to Use For Loops
- Sequential Iterations: When you need to iterate over a sequence (like lists, strings, or ranges) in a linear manner.
- Fixed Number of Iterations: When the number of repetitions is pre-known or can be defined based on the size of a sequence.
- Data Processing: When you have to filter, transform, or process data in a data structure such as lists, dictionaries, or files.
- Pattern Generation: For generating formatted patterns in console outputs, i.e., number or star pyramids.
- Nested Iterations: In the case of multi-dimensional data structures like lists of lists or matrices.
How to Use For Loops in Python
For loops in Python are used for sequential iterations for a certain number of times, that is, the length of the sequence. Iterations on the sequences in Python are called traversals.
Syntax of for loops in Python
Let us see the Python Syntax of For Loop with examples:
for a in sequence:
body of for loop
Working of For Loops in Python
The following flowchart explains the working of for loops in Python:
- Start of Loop: First, the for loop starts with selecting the first item(with zeroth index) from the sequence like lists, strings, range, etc.
- Condition Check:
- The for loop then simply checks whether the last element has been reached.
- If it had been reached, meaning yes, then, the loop will terminate, and the control flow moves to the statement after the loop.
- If not, then the body of the loop will get executed.
- Body Execution:
- The condition present inside the code block inside the for loop executes for the current item in the sequence.
- This may typically involve operations like calculations, printing, or modifying data.
- Move to the Next Element: The for loop then moves to the next item in the sequence automatically.
- Repeat Until the End: Steps 2 to 4 will be repeated until all elements in the sequence have been processed.
- Exit Loop: Once the sequence reaches its end, the control then exits the loop and continues with the code after it.
Code Example for better understanding:
Output:
Now In this example, the loop iterates through each fruit, processes it by printing, and exits after all elements are traversed, as shown in the flowchart.
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How to Use For Loops on Strings in Python
In Python, strings are sequences of characters, and for loops can be used to iterate over each character in a string. This is helpful when you need to manipulate or analyze each character individually.
Example:
Output:
Common Use Cases:
- Counting the number of specific characters in a string.
- Checking if certain characters exist in a string.
- Modifying or reversing strings.
How to Use range() Function In Python For Loops
We can specify a particular range using an inbuilt Python function, named range(), to iterate the loop a specified number of times through that range.
Example:
Output:
Note: The range here is not from 1 to 10 but from 0 to 9 (10 numbers).
- Specifying start and stop points in the range() function:
Example:
Output:
Note: Here, in this example, the starting point is 3 and the ending point is 7. So, the list will be generated from 3 to 6 (4 numbers).
- Using Python for loops with the range() function:
Example:
We can simply use Python For loop with the range() function as shown in the example below.
Output:
By default, the increment in the range() function when used with loops is set to 1; however, we can change or specify a particular increment by including a third parameter in the range() function, as illustrated in the following example:
Output:
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Loop Interruptions in Python For Loops
1. Break Statement
Just as in while loops, for loops can also be prematurely terminated using the break statement. The break statement will immediately terminate the execution of the loop and transfer the control of the program to the end of the loop.
Example:
Output:
2. Continue Statement
Just as with while loops, the continue statement can also be used in Python for loops to terminate the ongoing iteration and transfer the control to the beginning of the loop to continue the next iteration.
Output:
Note that the fifth iteration was interrupted as we have used the continue statement when the value of i is 5. Therefore, the control was passed to the beginning of the loop, and the program started executing the sixth iteration and did not print the value of i when it was 5.
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Else in Python For Loops
For loop in Python can have an optional else block. The else block will be executed only when all iterations are completed. When break is used in for loop to terminate the loop before all the iterations are completed, the else block is ignored.
Example:
Output:
Nested For Loop in Python
As the name suggests, nested loops are the loops that are nested inside an existing loop, that is, nested loops are the body of another loop.
Example:
Output:
In the above example, we have used nested loops to print a number pyramid pattern. The outer loop is used to handle the number of rows in the pattern, and the inner loop or the nested loop is used to handle the number of columns in the pattern. Note that we have also used the third parameter in the range() function and have set the increment to 2, that is why the number in every column is incremented by 2.
For Loops in Python Data Structures
In Python, there are many data structures such as lists, tuples, sets, and dictionaries that generally provide the versatility to store and organize different kinds of data. For Loops in Python are very important concept in order to perform iterations over these data structures that typically make it very easy to process, filter, and modify their content efficiently. Here we have explained how you can use them for loops in Python Data Structures.
1. Lists
Lists are ordered, mutable collections where you can hold elements of any data type. For loops can be applied for traversals and manipulating each list element.
Example:
Output:
Use Cases:
- Applying functions to each list element
- Filtering elements based on conditions
- Creating new lists using list comprehensions
Nested Loops on Lists:
Output:
2. Tuples
Tuples are fixed, ordered groups and can be used for fixed data. For loops can also be applied in the same manner you’d apply them for lists.
Example:
Output:
Use Cases:
- Iterating over multiple fixed values
- Handling return values from functions
Advantages Over Lists:
Tuples are also faster and require less space, and thus appropriate when data is not supposed to be modified.
3. Sets
Sets are groups of distinctive elements where the order is not specified. For loops can nevertheless allow access through each element even when the order is not specified.
Example:
Output:
Use Cases:
- Removing duplicates from data handling
- Performing mathematical set operations like union and intersection
Important Note: Since sets are not arranged, successive iterations can provide different orders.
4. Dictionaries
Dictionaries store key-value pairs and are one of the most commonly used data structures in Python. For loops can be used to iterate over keys, values, or both.
- Iterating Over Keys
Output:
- Iterating Over Values
Output:
- Iterating Over Key-Value Pairs
Output:
Use Cases:
- Extracting information from structured data
- Updating or restructuring the key-value pairs
- Data aggregation activities
Handling Nested Dictionaries:
Output:
Enumerate Function in Python For Loops
The enumerate() function adds the counter for the iterable and returns the value and the index for each pass through the loop. This is useful when you need the capability for the ability to know the loop position.
Syntax:
Output:
- iterable: Any iterable (like lists, strings, or tuples)
- start: The starting index (default is 0)
Example:
Output:
Benefits of Enumerate:
- Avoid manually counting counter variables
- Cleaner and easier to read
Reverse for loops in Python
We can invert the for loop in Python in two ways:
- One is by utilizing the reversed() function.
Output:
- The second method is using range(n,-1,-1)
Output:
How to Use For Loops to Flatten Nested Lists (2D Lists)
Nested lists, also known as two-dimensional lists, are lists of lists. They can be used for the description of grids, tables, or matrices. Flattening the nested list is the conversion of the nested list into a one-dimensional list form. This can be achieved through nested for loops where the outer for loop iterates over each sublist and the inner for loop iterates over each element in each sublist.
Example:
Output:
How to Use the Zip Function with For Loops
The zip() function is applied for parallel iterations over multiple sequences (lists, tuples, etc.). This pairs elements from each list together (or n-tuples when multiple lists are provided). This is helpful for combining associated data from two lists.
Example:
Output:
Looping Backward with a Negative Step in Range
In Python, range() also accepts the start, stop, and step parameters. For looping backward, the step is given a negative value. This is useful for those operations where you require counting backward or need the reversal of the sequences.
Example:
Output:
How to Filter Data Using For Loops
Data filtering is the act of selecting elements for some given criteria. One simple technique for data filtering using only the base libraries is the for loop. This is beneficial for data cleansing and data verification processes.
Example:
Output:
How to Create Patterns Using For Loops
Pattern generation is one good technique for practicing and understanding nested loops. In this case, the outer loop is for the rows, and the inner loop is for the columns, printing the ‘*’ for the triangle pattern.
Example:
Output:
Example Questions on Python For Loops
1. Counting Even and Odd Numbers in a List
A for loop can be used to traverse a list and check if each element is even or odd. The modulo operator (%) helps determine whether a number is divisible by 2. If the remainder is zero, the number is even; otherwise, it’s odd.
Example:
Output:
2. Finding the Maximum Number in a List
To find the highest value from the list by using the list elements alone, the variable value is assigned the value of the list’s first element. In the list loop, each value is matched against the highest value, and the variable is assigned when the value is larger.
Example:
Output:
3. Reversing Each Word in a String List
This task involves looping through the list of strings and reversing each word. To reverse the letters of a word, the slice operation is performed using the syntax [::-1].
Example:
Output:
4. Sum of Digits in a Number
This problem involves splitting the given figure into individual digits, converting them into integers, and summing them up. For looping through the given figure in the form of the string, the for loop is being used.
Example:
Output:
5. Printing a Multiplication Table
To display the multiplication table for the specified number, a for loop iterates through the range and multiplies the specified number by each value from the range.
Example:
Output:
6. Checking for Prime Numbers
A prime number is a number greater than 1 that has no divisors other than 1 and itself. To check if a number is prime, a for loop iterates from 2 to the square root of the number, checking if it’s divisible by any number.
Example:
Output:
Difference between For and while loop in Python
| Parameter |
For Loop |
While Loop |
| Format |
It allows initialization, condition checking, and iteration statements that are written on the top of the loop. |
It only allows initialization and condition checking at the top of the loop. |
| Declaration |
for(initialization; condition; iteration){ //body of ‘for’ loop} |
while ( condition) { statements; //body of loop} |
| Condition |
It iterates infinite time if no condition is given. |
It shows an error if no condition is given. |
| Initialization |
If initialization is once done in the “for” loop, it never is repeated. |
If initialization is done while condition checking, then it is required each time when the loop iterates itself. |
| Iteration Statement |
As the iteration statement is written at the top, it will execute only after all the statements are executed. |
The iteration statement can be placed anywhere in the syntax of the loop. |
Conclusion
With this, we have come to the end of this Python For Loops Tutorial. Mastering for loops is the essence of writing clean and efficient code. Having knowledge about the syntax, everyday applications, and recent features like the range() function, nested loops, and loop control statements will make you easily handle various coding scenarios.
Start implementing for loops for your projects immediately and unlock their potential. If you need to explore deeper into the domain of Python programming, check out our exhaustive tutorials and interview questions for Python for enhanced coding skills and job opportunities.
FAQs on Python For Loops
1. What is the purpose of a for loop in Python?
A for loop is used for looping through the sequences like lists, strings, and ranges. This is helpful for doing something for every item on the list.
2. How is a for loop different from a while loop in Python?
A for loop iterates over a fixed collection, whereas a while loop iterates through the iterations until some specified condition is fulfilled. For loops are applied when iterations are known.
3. Can we use else with for loops in Python?
Yes, Python also supports the use of the else block for ‘for loops’. If the for loop goes through all the iterations without encountering the break statement, the code under the else block is executed.
4. How do you reverse a for loop in Python?
You can invert the for loop in Python using the reversed() function or by providing the range() function with a decrement value.
5. What is the role of the range() function in Python for loops?
The range() function generates a range of numbers and is generally used for restricting the iterations for ‘for loops’. It can set the start, the stopping value, and the step value.
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