Python Loops

Python Loops

The flow of the programs written in any programming language is sequential by default. Sometimes we may need to alter the flow of the program. The execution of a specific code may need to be repeated several numbers of times.

For this purpose, The programming languages provide various types of loops which are capable of repeating some specific code several numbers of times. Consider the following diagram to understand the working of a loop statement.

Why we use loops in python?

The looping simplifies the complex problems into the easy ones. It enables us to alter the flow of the program so that instead of writing the same code again and again, we can repeat the same code for a finite number of times. For example, if we need to print the first 10 natural numbers then, instead of using the print statement 10 times, we can print inside a loop which runs up to 10 iterations.

Advantages of loops

There are the following advantages of loops in Python.

1. It provides code re-usability.
2. Using loops, we do not need to write the same code again and again.
3. Using loops, we can traverse over the elements of data structures (array or linked lists).

There are the following loop statements in Python.

Loop Statement	Description
for loop	The for loop is used in the case where we need to execute some part of the code until the given condition is satisfied. The for loop is also called as a per-tested loop. It is better to use for loop if the number of iteration is known in advance.
while loop	The while loop is to be used in the scenario where we don't know the number of iterations in advance. The block of statements is executed in the while loop until the condition specified in the while loop is satisfied. It is also called a pre-tested loop.
do-while loop	The do-while loop continues until a given condition satisfies. It is also called post tested loop. It is used when it is necessary to execute the loop at least once (mostly menu driven programs).

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Python Loops

Python Loops

The flow of the programs written in any programming language is sequential by default. Sometimes we may need to alter the flow of the program. The execution of a specific code may need to be repeated several numbers of times.

For this purpose, The programming languages provide various types of loops which are capable of repeating some specific code several numbers of times. Consider the following diagram to understand the working of a loop statement.

Why we use loops in python?

The looping simplifies the complex problems into the easy ones. It enables us to alter the flow of the program so that instead of writing the same code again and again, we can repeat the same code for a finite number of times. For example, if we need to print the first 10 natural numbers then, instead of using the print statement 10 times, we can print inside a loop which runs up to 10 iterations.

Advantages of loops

There are the following advantages of loops in Python.

1. It provides code re-usability.
2. Using loops, we do not need to write the same code again and again.
3. Using loops, we can traverse over the elements of data structures (array or linked lists).

There are the following loop statements in Python.

Loop Statement	Description
for loop	The for loop is used in the case where we need to execute some part of the code until the given condition is satisfied. The for loop is also called as a per-tested loop. It is better to use for loop if the number of iteration is known in advance.
while loop	The while loop is to be used in the scenario where we don't know the number of iterations in advance. The block of statements is executed in the while loop until the condition specified in the while loop is satisfied. It is also called a pre-tested loop.
do-while loop	The do-while loop continues until a given condition satisfies. It is also called post tested loop. It is used when it is necessary to execute the loop at least once (mostly menu driven programs).

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Python If-else statements

Decision making is the most important aspect of almost all the programming languages. As the name implies, decision making allows us to run a particular block of code for a particular decision. Here, the decisions are made on the validity of the particular conditions. Condition checking is the backbone of decision making.

In python, decision making is performed by the following statements.

Statement	Description
If Statement	The if statement is used to test a specific condition. If the condition is true, a block of code (if-block) will be executed.
If - else Statement	The if-else statement is similar to if statement except the fact that, it also provides the block of the code for the false case of the condition to be checked. If the condition provided in the if statement is false, then the else statement will be executed.
Nested if Statement	Nested if statements enable us to use if ? else statement inside an outer if statement.

Indentation in Python

For the ease of programming and to achieve simplicity, python doesn't allow the use of parentheses for the block level code. In Python, indentation is used to declare a block. If two statements are at the same indentation level, then they are the part of the same block.

Generally, four spaces are given to indent the statements which are a typical amount of indentation in python.

Indentation is the most used part of the python language since it declares the block of code. All the statements of one block are intended at the same level indentation. We will see how the actual indentation takes place in decision making and other stuff in python.

The if statement

The if statement is used to test a particular condition and if the condition is true, it executes a block of code known as if-block. The condition of if statement can be any valid logical expression which can be either evaluated to true or false.

The syntax of the if-statement is given below.

1. if expression:  
2.     statement  

Example 1

1. num = int(input("enter the number?"))  
2. if num%2 == 0:  
3.     print("Number is even")  

Output:

enter the number?10
Number is even

Example 2 : Program to print the largest of the three numbers.

1. a = int(input("Enter a? "));  
2. b = int(input("Enter b? "));  
3. c = int(input("Enter c? "));  
4. if a>b and a>c:  
5.     print("a is largest");  
6. if b>a and b>c:  
7.     print("b is largest");  
8. if c>a and c>b:  
9.     print("c is largest");  

Output:

Enter a? 100
Enter b? 120
Enter c? 130
c is largest

---

The if-else statement

The if-else statement provides an else block combined with the if statement which is executed in the false case of the condition.

If the condition is true, then the if-block is executed. Otherwise, the else-block is executed.

The syntax of the if-else statement is given below.

1. if condition:  
2.     #block of statements   
3. else:   
4.     #another block of statements (else-block)   

Example 1 : Program to check whether a person is eligible to vote or not.

1. age = int (input("Enter your age? "))  
2. if age>=18:  
3.     print("You are eligible to vote !!");  
4. else:  
5.     print("Sorry! you have to wait !!");  

Output:

Enter your age? 90
You are eligible to vote !!

Example 2: Program to check whether a number is even or not.

1. num = int(input("enter the number?"))  
2. if num%2 == 0:  
3.     print("Number is even...")  
4. else:  
5.     print("Number is odd...")  

Output:

enter the number?10
Number is even

---

The elif statement

The elif statement enables us to check multiple conditions and execute the specific block of statements depending upon the true condition among them. We can have any number of elif statements in our program depending upon our need. However, using elif is optional.

The elif statement works like an if-else-if ladder statement in C. It must be succeeded by an if statement.

The syntax of the elif statement is given below.

1. if expression 1:   
2.     # block of statements   
3.   
4. elif expression 2:   
5.     # block of statements   
6.   
7. elif expression 3:   
8.     # block of statements   
9.   
10. else:   
11.     # block of statements  

Example 1

1. number = int(input("Enter the number?"))  
2. if number==10:  
3.     print("number is equals to 10")  
4. elif number==50:  
5.     print("number is equal to 50");  
6. elif number==100:  
7.     print("number is equal to 100");  
8. else:  
9.     print("number is not equal to 10, 50 or 100");  

Output:

Enter the number?15
number is not equal to 10, 50 or 100

Example 2

1. marks = int(input("Enter the marks? "))  
2. f marks > 85 and marks <= 100:  
3.    print("Congrats ! you scored grade A ...")  
4. lif marks > 60 and marks <= 85:  
5.    print("You scored grade B + ...")  
6. lif marks > 40 and marks <= 60:  
7.    print("You scored grade B ...")  
8. lif (marks > 30 and marks <= 40):  
9.    print("You scored grade C ...")  
10. lse:  
11.    print("Sorry you are fail ?")  

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Python Comments

Python Comment is an essential tool for the programmers. Comments are generally used to explain the code. We can easily understand the code if it has a proper explanation. A good programmer must use the comments because in the future anyone wants to modify the code as well as implement the new module; then, it can be done easily.

In the other programming language such as C++, It provides the // for single-lined comment and /*.... */ for multiple-lined comment, but Python provides the single-lined Python comment. To apply the comment in the code we use the hash(#) at the beginning of the statement or code.

Let's understand the following example.

1. # This is the print statement  
2. print("Hello Python")  

Here we have written comment over the print statement using the hash(#). It will not affect our print statement.

Multiline Python Comment

We must use the hash(#) at the beginning of every line of code to apply the multiline Python comment. Consider the following example.

1. # First line of the comment   
2. # Second line of the comment  
3. # Third line of the comment  

Example:

1. # Variable a holds value 5  
2. # Variable b holds value 10  
3. # Variable c holds sum of a and b  
4. # Print the result  
5. a = 5  
6. b = 10  
7. c = a+b  
8. print("The sum is:", c)  

Output:

The sum is: 15

The above code is very readable even the absolute beginners can under that what is happening in each line of the code. This is the advantage of using comments in code.

We can also use the triple quotes ('''''') for multiline comment. The triple quotes are also used to string formatting. Consider the following example.

Docstrings Python Comment

The docstring comment is mostly used in the module, function, class or method. It is a documentation Python string. We will explain the class/method in further tutorials.

Example:

1. def intro():  
2.   """ 
3.   This function prints Hello Joseph 
4.   """  
5.   print("Hi Joseph")              
6. intro()  

Output:

Hello Joseph

We can check a function's docstring by using the __doc__ attribute.

Generally, four whitespaces are used as the indentation. The amount of indentation depends on user, but it must be consistent throughout that block.

1. def intro():  
2.   """ 
3.   This function prints Hello Joseph 
4.   """  
5.   print("Hello Joseph")              
6. intro.__doc__  

Output:

Output:
'\n  This function prints Hello Joseph\n  '

Note: The docstring must be the first thing in the function; otherwise, Python interpreter cannot get the docstring.

Python indentation

Python indentation uses to define the block of the code. The other programming languages such as C, C++, and Java use curly braces {}, whereas Python uses an indentation. Whitespaces are used as indentation in Python.

Indentation uses at the beginning of the code and ends with the unintended line. That same line indentation defines the block of the code (body of a function, loop, etc.)

Generally, four whitespaces are used as the indentation. The amount of indentation depends on user, but it must be consistent throughout that block.

1. for i in range(5):  
2.     print(i)  
3.     if(i == 3):  
4.         break  

To indicate a block of code we indented each line of the block by the same whitespaces.

Consider the following example.

1. dn = int(input("Enter the number:"))  
2. if(n%2 == 0):  
3.     print("Even Number")  
4. else:  
5.     print("Odd Number")  
6.      
7. print("Task Complete")  

Output:

Enter the number: 10
Even Number
Task Complete

The above code, if and else are two separate code blocks. Both code blocks are indented four spaces. The print("Task Complete") statement is not indented four whitespaces and it is out of the if-else block.

If the indentation is not used properly, then that will result in IndentationError.

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