Python - Loops

Repeating code over and over again known as a loop. And is also a fairly common scenario in programming. Python provide couple of ways to doing that.

Why we use loop in programming?

There may be some situation where we need to execute a block of code again and again or we want to execute some lines of code repeatedly. A loop in programming languages allows us to execute a statement or block of statement multiple times.

Like other programming languages, python provide following types of loops to handle such type of scenario where we need to execute code or block of code repeatedly.

1. While Loop
2. For Loop

While Loop

To repeat some lines of code or block of code while a given condition is TRUE. In while loop the controls goes to loop body after testing some condition.

Example:

 

def main():
    x = 0
    # define while loop
    while(x < 5):
        print("value of x: ",x)
        x = x +1
if __name__ == "__main__": main()       
 

Code Description:

The above code snippet is a while loop and the content of while loop is indented that indicates that these line of code belong to the while loop and not to the main part of the function. So while loop executes while the condition contain inside the parenthesis is TRUE. So guys while the condition x  is less than five we are going to print the value of x inside the while loop, and also we are going to increment the value of x by one (1). Other programming languages like C, C++, C# etc provide a bunch of ways to do these simple kinds of things using (while, do while loops etc). Python likes to keep things simple. In python there are two ways to doing this while and for loop.

Output

In initial state the value of x is 0 so the output of the above code starts with 0 and prints the value until the condition in parenthesis no longer TRUE. So the output is 0,1,2,3,4 and so the loop terminate.

For Loop

For loop in python is little bit different like other programming languages (JavaScript, C++ and Java). These types of language have the concept of index variable that counts the number of iteration in for loop.

So for example in JavaScript the syntax of for loop is looks like this.

 

for (i=0,i<10,i++){
    // your logic goes here
}      
 

In  above code a counter variable i=0 is used to control the execution of the loop. That’s not is used in python to operate the loops in python. Python for loops are called iterator.  So in this case if I want have x to loop over the range of numbers than I need to use python built in range() function. So I have a range going to 4 to 10 in for loop and inside the loop I have a print statement that will prints the value of x.

#!/usr/bin/python3

def main():
    x = 0
    # define a for loop
    for x in range(4,10):
        print("Value of x: ",x)
if __name__ == "__main__": main()
       
 

Output

In output window you can see that its printed out 4 to 9 so x within the range which is not obviously is not inclusive of 10. So it exclude the number 10 right here. That’s why it printed out the number 4 to 9. For loops not only operates on number it also operates on sets of things such as collections, objects and lists. Here collections mean a set of or a list of objects for example a list of or a collection of day's name.

Object of data comes from database

For example we have a database of a university and in that database we have a table named "tbl_student" and in that table we have 3 records. So the object looks like "tbl_student(1,2,3)". So here we operates or execute our for loop in "tbl_student" instead of number or range of numbers.

Data comes from Collections/List

For example a list of day's, list of months and list of data which we need to loop through.

Days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]

Use a for loop over a collection/list

#!/usr/bin/python3

def main():
    # use a for loop over a collection/list
    list_of_days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"]
    for day in list_of_days:
        print("Day Name: ",day)
if __name__ == "__main__": main()   
 

Code Description

In the above code I declared a variable name list_of_days and I am initializing this to a list of days using opening and closing square bracket. And inside of the bracket I have my day name. Now I am going to loop through the list and printing out the day. Here we are not using any number instead we are iterating the for loop for each member of the list item. And the print statement will prints out the day name.

 

So its looping over the content of the list item, again here no indexed counter involved just iterating over the member of a list.

Loop control statements

Python loop control statement will change the execution of a loop. For example on some condition we want to stop or continue the loop iteration. In python we have "break" and "continue" key word to stop/break or continue the loop execution.

Break statement

The break statement is used to break the execution of a loop if a condition is met.

#!/usr/bin/python3

def main():
    # use of break loop control statement
    for x in range(5,10):
        if x == 7: break
        print("x: ",x)
if __name__ == "__main__": main()   
       
 

In above code If x is equal to 7 than break, the break statement is here will cause to terminate the for loop and fall through the next block of code. When we run this program this will print out only 5 and 6 because when the condition is met the break statement will kicks out the loop.

Continue statement

The continue statement skips the rest of the statement when it is encounter.

 

#!/usr/bin/python3

def main():
    # use of continue loop control statement
    for x in range(5,10):
        if (x % 2 == 0): continue
        print("x: ",x)
if __name__ == "__main__": main()  
       
 

In above code our condition is if x modulus 2 is equal to 0 or in other word take x and divide it by 0 if the value left over is 0 than continue. Its mean if the condition is met don't go to the next line (in our case the next line is print statement) just go back up to the start of the loop. In simple word if I come across in the even number doesn't print out that number. The continue statement will skip the next line if the condition will met. This will print out 5,6 and 9.