If can be understood as metaphorical English what if which most people use in their day to day life. What if this doesn't happen? If I were to become a billionaire. If this match is drawn we are out of the champions' league. If Churchill had not come to power Battle of Britain would have been lost. If this car doesn't start, use the other one. We are loaded with numerous examples from across the globe on usage of if. Same is the case with programming languages. Majority of the programming languages have control statements and you will find majority of them use the keyword if in their control statements. Python is no different and also facilitates the usage of if. Let's understand with an example:
password= raw_input("Enter the passwordt")
if password=="MI6":
print "Welcome Mr. Bond."
Any function or control statement block in Python is started by placing the colon at the end of the line. Colon here marks the beginning of the if block and print statement begins after one tab space right after the colon. Programmers usually make a mistake by mixing space with tab right after the colon.
In the preceding example, after password is entered, the interpreter checks for the entered string and compares with "MI6"; if password entered is correct, it prints Welcome Mr. Bond.. If the password is wrong, it will skip the if block and terminate the program:
But merely using only if doesn't give much of the choice to the interpreter and it has to terminate the program. What if the interpreter is given an alternative for a failed if test. In that case, else can be used to give choice to the interpreter.
if condition :
statements-1
else:
statements-2
Let's take an example to understand if and else conditions:
password= raw_input("Enter the passwordt")
if password=="MI6":
print "Welcome Mr. Bond."
else:
print "Access Denied."
Here, if the password is entered wrongly, the interpreter will immediately execute statement inside the else block. Here, the control statement is divided into two blocks one is the if block and the other is the else block. At one time, the interpreter will execute either of the two blocks:
