Python 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 −
Types of Loops
Python programming language provides following types of loops to handle looping requirements.
Python For Loops
A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).
This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.
With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
for var in iterable: # statements
str = “AskAtul”
for i in str:
A s k A t u l
Python While Loop
In Python, While Loops is used to execute a block of statements repeatedly until a given condition is satisfied. And when the condition becomes false, the line immediately after the loop in the program is executed.
The syntax of a while loop in Python programming language is −
while expression: statement(s)
#The while loop will iterate until condition becomes false.
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Python nested Loop
Python programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.
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.
The following program uses a nested for loop to find the prime numbers from 2 to 100 −
#!/usr/bin/python i = 2 while(i < 100): j = 2 while(j <= (i/j)): if not(i%j): break j = j + 1 if (j > i/j) : print i, " is prime" i = i + 1 print "Good bye!"
2 is prime 3 is prime 5 is prime 7 is prime 11 is prime 13 is prime 17 is prime 19 is prime 23 is prime 29 is prime 31 is prime 37 is prime 41 is prime 43 is prime 47 is prime 53 is prime 59 is prime 61 is prime 67 is prime 71 is prime 73 is prime 79 is prime 83 is prime 89 is prime 97 is prime Good bye!
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.
Python Continue statement
It returns the control to the beginning of the loop.
#loop statements continue #the code to be skipped
i = 0 while(i < 10): i = i+1 if(i == 5): continue print(i)
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Python break statement
It terminates the current loop and resumes execution at the next statement, just like the traditional break statement in C.
The syntax for a break statement in Python is as follows −
#!/usr/bin/python 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!"
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!