# This tutorial will introduce simple implementations of conditional if
# statements in Python.

# Here's a simple if statement.  If statements use the same indentation
# scheme as for loops.  Indented lines immediately following the if 
# statement are within the if control sturcture.  The if statement ends
# when the indented block of code ends.
ans = 'y'
if ans == 'y':
    print('yes')

# In the example above, ans was set equal to 'y' and the if condition was
# true such that 'yes' was printed.  In the example below, ans is not equal
# to 'y' and the if condition is false.  As a result, no output is printed.
ans = 'n'
if ans == 'y':
    print('yes')

# We can enhance the capability of our if statement by adding an else
# statement.  Anytime the if condition is false, the else statement will
# execute.

# True, so 'yes' output.
ans = 'y'
if ans == 'y':
    print('yes')
else:
    print('no')

# False, so 'no' output.
ans = 'n'
if ans == 'y':
    print('yes')
else:
    print('no')

# False, so 'no' output.
ans = 'x'
if ans == 'y':
    print('yes')
else:
    print('no')

# In the above if statement the only "y" produces a "yes", any other entry
# results in "no".  Suppose that you only want to accept "y" and "n" as
# valid responses.  Including an 'else if' condtion (using 'elif') as done 
# below produces the desired results.

# If condition true, so output 'yes'.
ans = 'y'
if ans == 'y':
    print('yes')
elif ans == 'n':
    print('no')
else:
    print('invalid response')

# Elseif condition true, so output 'no'.
ans = 'n';
if ans == 'y':
    print('yes')
elif ans == 'n':
    print('no')
else:
    print('invalid response')

# If and elseif conditions false, so output 'invalid response'.
ans = 'x';
if ans == 'y':
    print('yes')
elif ans == 'n':
    print('no')
else:
    print('invalid response')

# To complete this tutorial, here's how you can prompt the user for an
# input in Python.  All you need to do is use the 'input()' statement.
val = input("Enter your value: ") 
if val == 'y':
    print('yes')
elif val == 'n':
    print('no')
else:
    print('invalid response')

Enter your value: physics
invalid response