Python 3 - Loops

In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on. There may be a situation when you need to execute a block of code several number of times.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times. The following diagram illustrates a loop statement −

Python programming language provides following types of loops to handle looping requirements.

Loop Type	Description
while loop	Repeats a statement or group of statements while a given condition is TRUE. It tests the condition before executing the loop body. A while loop statement in Python programming language repeatedly executes a target statement as long as a given condition is true. Syntax The syntax of a while loop in Python programming language is − while expression: statement(s) Here, statement(s) may be a single statement or a block of statements with uniform indent. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true. When the condition becomes false, program control passes to the line immediately following the loop. In Python, all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code. Python uses indentation as its method of grouping statements. Flow Diagram Here, key point of the while loop is that the loop might not ever run. When the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed. Example #!/usr/bin/python3 count = 0 while (count < 9): print ('The count is:', count) count = count + 1 print ("Good bye!") When the above code is executed, it produces the following result − The count is: 0 The count is: 1 The count is: 2 The count is: 3 The count is: 4 The count is: 5 The count is: 6 The count is: 7 The count is: 8 Good bye! The block here, consisting of the print and increment statements, is executed repeatedly until count is no longer less than 9. With each iteration, the current value of the index count is displayed and then increased by 1. The Infinite Loop A loop becomes infinite loop if a condition never becomes FALSE. You must use caution when using while loops because of the possibility that this condition never resolves to a FALSE value. This results in a loop that never ends. Such a loop is called an infinite loop. An infinite loop might be useful in client/server programming where the server needs to run continuously so that client programs can communicate with it as and when required. #!/usr/bin/python3 var = 1 while var == 1 : # This constructs an infinite loop num = int(input("Enter a number :")) print ("You entered: ", num) print ("Good bye!") When the above code is executed, it produces the following result − Enter a number :20 You entered: 20 Enter a number :29 You entered: 29 Enter a number :3 You entered: 3 Enter a number :11 You entered: 11 Enter a number :22 You entered: 22 Enter a number :Traceback (most recent call last): File "examples\test.py", line 5, in num = int(input("Enter a number :")) KeyboardInterrupt Above example goes in an infinite loop and you need to use CTRL+C to exit the program. Using else Statement with Loops Python supports to have an else statement associated with a loop statement. If the else statement is used with a for loop, the else statement is executed when the loop has exhausted iterating the list. If the else statement is used with a while loop, the else statement is executed when the condition becomes false. The following example illustrates the combination of an else statement with a while statement that prints a number as long as it is less than 5, otherwise else statement gets executed.p> #!/usr/bin/python3 count = 0 while count < 5: print (count, " is less than 5") count = count + 1 else: print (count, " is not less than 5") When the above code is executed, it produces the following result − 0 is less than 5 1 is less than 5 2 is less than 5 3 is less than 5 4 is less than 5 5 is not less than 5 Single Statement Suites Similar to the if statement syntax, if your while clause consists only of a single statement, it may be placed on the same line as the while header. Here is the syntax and example of a one-line while clause − #!/usr/bin/python3 flag = 1 while (flag): print ('Given flag is really true!') print ("Good bye!") Above example goes into infinite loop and you need to press CTRL+C keys to exit.
for loop	Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable. The for statement in Python has the ability to iterate over the items of any sequence, such as a list or a string. Syntax for iterating_var in sequence: statements(s) If a sequence contains an expression list, it is evaluated first. Then, the first item 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. Flow Diagram The range() function The built-in function range() is the right function to iterate over a sequence of numbers. It generates an iterator of arithmetic progressions. >>> range(5) range(0, 5) >>> list(range(5)) [0, 1, 2, 3, 4] range() generates an iterator to progress integers starting with 0 upto n-1. To obtain a list object of the sequence, it is typecasted to list(). Now this list can be iterated using for statement >>> for var in list(range(5)): print (var) This will produce following output 0 1 2 3 4 Example #!/usr/bin/python3 for letter in 'Python': # traversal of a string sequence print ('Current Letter :', letter) print() fruits = ['banana', 'apple', 'mango'] for fruit in fruits: # traversal of List sequence print ('Current fruit :', fruit) print ("Good bye!") When the above code is executed, it produces the following result − Current Letter : P Current Letter : y Current Letter : t Current Letter : h Current Letter : o Current Letter : n Current fruit : banana Current fruit : apple Current fruit : mango Good bye! Iterating by Sequence Index An alternative way of iterating through each item is by index offset into the sequence itself. Following is a simple example − #!/usr/bin/python3 fruits = ['banana', 'apple', 'mango'] for index in range(len(fruits)): print ('Current fruit :', fruits[index]) print ("Good bye!") When the above code is executed, it produces the following result − Current fruit : banana Current fruit : apple Current fruit : mango Good bye! Here, we took the assistance of the len() built-in function, which provides the total number of elements in the tuple as well as the range() built-in function to give us the actual sequence to iterate over. Using else Statement with Loops Python supports to have an else statement associated with a loop statement If the else statement is used with a for loop, the else block is executed only if for loops terminates normally (and not by encountering break statement). If the else statement is used with a while loop, the else statement is executed when the condition becomes false. The following example illustrates the combination of an else statement with a for statement that searches for even number in given list. #!/usr/bin/python3 numbers=[11,33,55,39,55,75,37,21,23,41,13] for num in numbers: if num%2==0: print ('the list contains an even number') break else: print ('the list doesnot contain even number') When the above code is executed, it produces the following result − the list doesnot contain even number
nested loops	You can use one or more loop inside any another while,or for or loop. Python programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept. Syntax for iterating_var in sequence: for iterating_var in sequence: statements(s) statements(s) The syntax for a nested while loop statement in Python programming language is as follows − while expression: while expression: statement(s) statement(s) A final note on loop nesting is that you can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa. Example The following program uses a nested for loops to display multiplication tables from 1-10 − #!/usr/bin/python3 import sys for i in range(1,11): for j in range(1,11): k=i*j print (k, end=' ') print() The print() function inner loop has end=' ' which appends a space instead of default newline. Hence numbers will appear in one row. Last print() will be executed at the end of inner for loop When the above code is executed, it produces following result − 1 2 3 4 5 6 7 8 9 10 2 4 6 8 10 12 14 16 18 20 3 6 9 12 15 18 21 24 27 30 4 8 12 16 20 24 28 32 36 40 5 10 15 20 25 30 35 40 45 50 6 12 18 24 30 36 42 48 54 60 7 14 21 28 35 42 49 56 63 70 8 16 24 32 40 48 56 64 72 80 9 18 27 36 45 54 63 72 81 90 10 20 30 40 50 60 70 80 90 100

Loop Control Statements

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

Python supports the following control statements. Click the following links to check their detail.

Control Statement	Description
break statement	Terminates the loop statement and transfers execution to the statement immediately following the loop. The break statement is used for premature termination of current loop. After abandoning the loop, execution at the next statement is resumed, just like the traditional break statement in C. The most common use for break is when some external condition is triggered requiring a hasty exit from a loop. The break statement can be used in bothwhile and for loops. If you are using nested loops, the break statement stops the execution of the innermost loop and start executing the next line of code after the block. Syntax The syntax for a break statement in Python is as follows − break Flow Diagram Example #!/usr/bin/python3 for letter in 'Python': # First Example if letter == 'h': break print ('Current Letter :', letter) var = 10 # Second Example while var > 0: print ('Current variable value :', var) var = var -1 if var == 5: break print ("Good bye!") When the above code is executed, it produces the following result − Current Letter : P Current Letter : y Current Letter : t Current variable value : 10 Current variable value : 9 Current variable value : 8 Current variable value : 7 Current variable value : 6 Good bye! Following program demonstrates use of break in a for loop iterating over a list. User inputs a number which is searched in the list. If found, loop terminates with 'found' message #!/usr/bin/python3 no=int(input('any number: ')) numbers=[11,33,55,39,55,75,37,21,23,41,13] for num in numbers: if num==no: print ('number found in list') break else: print ('number not found in list') Above program will produce following output- any number: 33 number found in list any number: 5 number not found in list
continue statement	Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. The continue statement in Python returns the control to the beginning of current loop. When encountered, the loop starts next iteration without executing remaining statements in the current iteration. The continue statement can be used in both while and for loops. Syntax continue Flow Diagram Example #!/usr/bin/python3 for letter in 'Python': # First Example if letter == 'h': continue print ('Current Letter :', letter) var = 10 # Second Example while var > 0: var = var -1 if var == 5: continue print ('Current variable value :', var) print ("Good bye!") When the above code is executed, it produces the following result − Current Letter : P Current Letter : y Current Letter : t Current Letter : o Current Letter : n Current variable value : 9 Current variable value : 8 Current variable value : 7 Current variable value : 6 Current variable value : 4 Current variable value : 3 Current variable value : 2 Current variable value : 1 Current variable value : 0 Good bye!
pass statement	The pass statement in Python is used when a statement is required syntactically but you do not want any command or code to execute. It is used when a statement is required syntactically but you do not want any command or code to execute. The pass statement is a null operation; nothing happens when it executes. Thepass is also useful in places where your code will eventually go, but has not been written yet (e.g., in stubs for example): Syntax pass Example #!/usr/bin/python3 for letter in 'Python': if letter == 'h': pass print ('This is pass block') print ('Current Letter :', letter) print ("Good bye!") When the above code is executed, it produces following result − Current Letter : P Current Letter : y Current Letter : t This is pass block Current Letter : h Current Letter : o Current Letter : n Good bye!

Let us go through the loop control statements briefly −

Iterator and Generator

Iterator is an object which allows a programmer to traverse through all the elements of a collection, regardless of its specific implementation. In Python iterator object implements two methods : iter() and next()

String, List or Tuple object can be used to create an Iterator

list=[1,2,3,4]
it = iter(list) # this builds an iterator object
print (next(it)) #prints next available element in iterator

Iterator object can be traversed using regular for statement
!usr//bin/python3
for x in it:
   print (x, end=" ")

or using next() function
while True:
   try:
      print (next(it))
   except StopIteration:
      sys.exit() #you have to import sys module for this

A generator is a function that produces or yields a sequence of values using yield method.

When a generator function is called, it returns an generator object without even beginning execution of the function. When next() method is called for the first time, the function starts executing until it reaches yield statement which returns the yielded value. The yield keeps track of i.e. remembers last execution. And second next() call continues from previous value.

Following example defines a generator which generates an iterator for all the Fibonacci numbers. 
!usr//bin/python3
import sys
def fibonacci(n): #generator function
    a, b, counter = 0, 1, 0
    while True:
        if (counter > n): 
            return
        yield a
        a, b = b, a + b
        counter += 1
f = fibonacci(5) #f is iterator object

while True:
   try:
      print (next(f), end=" ")
   except StopIteration:
      sys.exit()

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