Beginner's Guide On For Loop in Python

 

Beginner's Guide On For Loop in Python

Introduction

This post is for beginners who are trying to learn Python and they will find this step-by-step guide very easy to understand. 

Here, I will be first introducing the concept of the 'for' loop in Python, and thereafter, I will proceed to explain the significance and methods of using this loop in your Python projects with several easy-to-understand examples and source code.

 

What is a 'for' loop

A 'for' loop in Python is used to loop over a sequence and it is very useful when you perform tasks in your python functions. The 'for' loop could be used for looping or iterating over a list, tuple, set, dictionary or a string. We will see the examples below.

During the phase of looping through a list for example, after creating the variable name to specify each item in the list, we will decide what is the action that is to be executed in the phase of this iteration whereby each and every item specified inside the list is accessed one by one in the exact order that you want it to be performed. We will see more in detail when we go through the examples one by one. The only difference in the case of dictinaries is that through such iteration, is looping through the dictionaries will first provide access with the keys which in turn, will provide access with the individual items as required.

 

How to Create a for Loop

In order to create a 'for' loop, we need to create a variable and an object through which we want to iterate this loop.

 

Looping through a List

The process of looping through a list involves iterating through all the items contained inside the list and thereafter execute some task on every item in the list, one by one. Therefore, we use a 'for' loop in the case, you wish to carry out some task on each and every item inside the list, in order to get the desired output.

In the following example, we have a list called items with a list of items, namely, "table", "desk" and "sofa" and we want to loop through these items one by one and then print out the items in the list. The code for this example is indicated below. When we write, 'for item in items', we mean for iterating through or going through the items, item by item, or in other words, one by one.

 #Code:

items = ["table", "desk", "sofa"]

for item in items:

       print(item)

 

In the second example for looping through a list, we consider a list called my_list with numbers from 1 to 5. Here we wish to loop through all the numbers 1,2,3,4 and 5 in this list and then first multiply each of the numbers by 2, and thereafter, print out the result of this multiplication. So, the first item is number 1 and when multiplied by 2 will yield result of 2, then the next number in the sequence is 2 which when multiplied by 2 will give you 4 as a resultant number, then, the next number 3 when multiplied by 2 will give 6 as a resultant number, then, 4 multiplied by 2 will give you 8 as a resultant number and the last number in the list is 5 which when multiplied by 2 will give you the number 10 as a resultant number in the list. 

So, now, after performing the multiplication by 2 on each of the existing numbers of the list will give you a resultant items of 2,4,6,8,10 respectively in the list. Hence when you print out the result of the multiplication as specified in the task in the for loop, you will get the outcome with the new numbers as mentioned above in the list.

 #Code:

my_list = [1, 2, 3, 4, 5]

    for item in my_list:

        print(item * 2)

 

 Looping through String

In order to access each of the character contained in a string, we could use the 'for' loop for the purpose of iteration. Take a look at the example code sample given below where we consider the string 'meerateachestech' in which we would like to access all the characters in this string and it is for this purpose that we create a 'for' loop here by using the variable x to iterate over each of the characters in this string.
After the iteration, we perform the task of printing out the characters and in the outcome 
In the second example as shown below, we have another string called my_string, which contains the string, "Hello" and here we loop through each and every character in the string by saying for char in my_string, meaning, for every character in my_string, we will get all the characters, as shown in the code block below.

#Code:

for x in "meerateachestech":

      print(x)

 

 

#loop through every character in the string and print out the characters one by one.

 Code:

 my_string="Hello"

 for char in my_string:

        print(char)

 

Using the for loop with break statement

The break statement stops the loop before looping through all the items in the list. If we use the break statement, we will be stopping the loop right at the point, we use the break statement, thereby stopping the iteration through all of the items or rather preventing it from looping through the rest of the items in the list since the iteration had to stop exactly the list since the iteration had to stop exactly the time the break statement was declared.
 
In the below code, we notice that fruits is the list with three fruits namely 'orange',  'grapes', and 'berry' When we write, for fruit in fruits, we mean, loop through every fruit in the fruits  and then print out the fruit in the list, one b one. Then we use a conditional statement that  if a fruit while iteration happens to be 'grapes', then use the break statement. When the  break  statement is used, immediately the iteration of the items in the loop is stopped at the point when the items are reaching the value of grapes. So looping will stop after fruit reaches a value of grapes.
In the output, we will only see 'orange' and 'grapes' because the iteration stops at grapes, it will not loop further through the rest of the fruits, that is berry and berry will not be printed out. We can clearly see that berry comes after grapes.   

#Code:

fruits = ["orange","grapes","berry"]

        for fruit in fruits:

           print(fruit)

           if fruit == "grapes":

           break

#output     orange 

                   grapes

 

fruits = ["orange","grapes","berry"]

 

        for fruit in fruits:

          #print(fruit)

          if fruit == "grapes":

            break

          print(fruit)

#output orange

#reason break is before print

  

In the second case, when the break statement comes before the print statement, the iteration is stopped in the loop and the code block after the break statement is not able to run with the result that we do not get orange and berry in the output as the code block stops executing when the break is called before the print statement right after the first item in the list.
The reason for this changed behaviour is because the break statement comes before the print statement in this code block as shown below.

 

Using the for loop with continue statement

When we use the for loop with the continue statement, we will observe that we can skip or miss out on a specific iteration and then move on to the next iteration. So the continue statement is responsible for stopping the loop at that specific point of using the continue statement and then jump to the next iteration whereby the code block in the previous block below the continue statement, will not be carried out and then it stops the current iteration of loop and continues with the next iteration.
In the below code, we find that the iteration stops at berry so it will not print out berry but it will print out the rest of the fruits. In other words it will skip the fruit 'grapes' as we used the continue statement at grapes.

 

#Code:
fruits = ["orange", "grapes", "berry"]

for fruit in fruits:

      if fruit == "grapes":

          continue

      print(fruit)

#output orange

                berry

     

Using the for loop with the range function

When looping through a range function, the range function will help in repetitive action for a specified number of times. That is, for example, if we want to iterate through a code block, for a specified number of times, then the range function combined with a for loop is very useful. The range function returns a output sequence of numbers that starts from 0 by default and keeps incrementing or increasing by 1 as a default option an thereafter ends at a specific number that is mentioned in the range.
The range function can use parameters to specify a start value and an end value , for example, range(2,7) means a range of numbers starting from 2 (not the default 0) and ends at 7, but not inclusive of 7. So, the output will carry numbers, 2,3,4,5,6 since end point is not inclusive by the rule of a range function.

Using the for loop with a range function having a start parameter 

The default start parameter of a range function is 0 but if we use a different value other than the default value of 0, range function will yield an output with a start value that is provided in the parameter as a start value and if no parameter is provided as a start value, then then the output range will start with a default value of 0 as shown in the code block provided.

 #Code:

for x in range(7):

      print(x)

#outcome 0 1 2 3 4 5 6

 

for x in range(2,7):

       print(x)

#output 2 3 4 5 6

 

Using the for loop with a range function having the Increment Parameter 

The range function defaults  to incrementing by 1 but it is possible to include the incrementing value by adding a third parameter in the range function parenthesis as shown below in the  code block.

   

for x in range(2,30,3):

     print(x)

#output 2 5 8 11 14 17 20 23 26 29

 

Using for loop with else loop

When we use the else condition for a range function, the for loop states a block of code or rather a task to be executed once the loop is completed.
 
Example 
Print all numbers from 0 to  5 , then,print a message  when the loop has completed as shown in the code block below. In the output we get numbers from 0 to 6 since the range function has just one parameter value , that is 7.
So the output will give resultant numbers from default 0 and go on to print numbers until 6 and then also print out "done" after finishing the loop and it doesn't print 7 which is the range function parameter in this case. Because, by default, the resultant end number will be 1 less than the provided number here. So , after printing the last number 6, it will print "done" as shown in the outcome of the code block.

#Code:         

for x in range(7):

     print(x)

else:

     print("done!")

#output 0 1 2 3 4 5 6 Done

         

The else loop will not be executed if  the if loop is stopped by a break  statement , as shown in the code block below.

 #Code:                        

for x in range(7):

     if x == 3:

        break

        print(x)

    else:

        print("finally done!")

#output 0 1 2

#if loop breaks the else

 loop is not executed

                                

Using nested for loops

When you need to  iterate over multiple collections or carry out tasks that require nested loops or iterations,then you could use the nested "for loops"  

 #Code:

sizes = ["big", "small", "medium"]

fruits = ["oranges", "grapes", "berry"]

for size in sizes:

    for fruit in fruits:

         print(size, fruit)

 

#output big oranges big grapes big berry

small oranges small grapes small berry

medium oranges medium grapes medium berry

 

Using the for loop in Iterating through dictionaries

We can loop through the keys, values, or key-value  pairs of a dictionary using  a for loop as shown in the example below. my_dict is a dictionary with a key-value pairs as shown below whereby a,b,c are the keys and the valued are 1,2 and 3 respectively. Hence in the for loop we need to mention both the key as well as value and we also write dot items() in order to loop through the items in this dictionary. Then inside the print() function we need to write both the parameters, the key and the value.

 #Code:

my_dict = {"a": 1, "b": 2, "c": 3}

    for key, value in my_dict.items():

        print(key, value)

   

Using the for loop in Iterating through a tuple or set

It is possible to use the for loops to iterate through tuples as well as sets. Let us consider an example of a tuple as shown below. my_tuple contains three numbers, namely, 10, 20 and 30.

We iterate through items in the tuple by saying for item in my_tuple , print each item in the tuple. Ofcourse, this is the English way of putting the execution shown below in the for loop in the code block below. please make a note here that whatever term is used in the for loop iteration it is the same term that needs to be printed out.

Below, had we written items instead of item in the for loop, then we need to type items as the parameter in the print() function.

Hope, it is clear now.

#Code: 

my_tuple = (10, 20, 30)

    for item in my_tuple:

        print(item)

                          

When to use the for loops

In general, use a for loop when you know how many times the loop should run. If the  number of iterations is not known in advance  or depends on a condition, a while loop might  be more appropriate.

 

Conclusion

For loops are used when you know how many iterations you want to go through. While loops are used when you don't know the number of iterations you want to go through. loop is typically used when you have a known  sequence of items, such as a list, string, or range, and you need to iterate over each element in that sequence.

This is all about the basics for understanding the 'for loop' in Python for beginners. I hope, I have made myself clear and if you still have any questions, please do not hesitate to mention or shout out in the comment section below. 😀

 

 You may also like to read more tutorial posts on Python:

Python While Loops

Essentials About Python Return Statements

How To Use Python Print Function