Conditional Processing and loop concept in python !

In the programs we have seen till now, there has always been a series of statements and Python faithfully executes them in the same order. What if you wanted to change the flow of how it works? This is fine for very simple programs, but most programs, like the driving directions example, aren’t that simple. The driving directions included statements like, Continue on Main Street until you see a church on your right and If the street is blocked because of construction. . . . These statements are known as control structures, and they change the flow of the program’s execution from a simple sequential order to a more complex and more useful flow.

In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program  accordingly. Conditional statements give us this ability.

The simplest form is the if statement :
if d1+d2 == 7 or d1+d2 == 11:
print "winner", d1+d2

                                          

                      

The boolean expression after the if statement is called the condition. If it is true, then the indented statement gets executed. If not, nothing happens.

Alternative execution:
A second form of the if statement is alternative execution, in which there are two possibilities and the condition determines which one gets executed. The syntax looks like this:
if x%2 == 0:
print x, "is even"
else:
print x, "is odd"

Chained conditionals:
Sometimes there are more than two possibilities and we need more than two branches. One way to express a computation like that is a chained conditional:
point= None
if d1+d2 == 7 or d1+d2 == 11:
print "winner"
elif d1+d2 == 2 or d1+d2 == 3 or d1+d2 == 12:
print "loser"
else:
point= d1+d2
print "point is", point
elif is an abbreviation of “else if.” Again, exactly one branch will be executed.

The return statement allows you to terminate the execution of a function before you reach the end. One reason to use it is if you  detect an error condition:
import math
def printLogarithm(x):
if x <= 0:
print "Positive numbers only, please."
return
result = math.log(x)
print "The log of x is", result

The pass Statement:
The pass statement does nothing. Use the pass statement to fill in the required suite of statements.
if n%2 == 0:
pass # Ignore even values
else:
count += 1 # Count the odd values

Keyboard input :
The programs we have written so far are a bit rude in the sense that they accept no input from the user. They just do the same thing every time.
Python provides built-in functions that get input from the keyboard. The simplest is called raw input. When this function is called, the program stops and waits for the user to type something.
#!/usr/bin/python
number = 23
guess = int(raw_input('Enter an integer : '))
if guess == number:
print 'Congratulations, you guessed it.'

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