# The Fork in the Road: logic.

# 1. Introducing a new type of value: BOOLEAN.
# There are only TWO bollean values, True and False (note the capital letters!)

true = True # True is a reserved keyword in python, true isn't 
# so we can use it as a name of a variable
false = False # same here!
print("Value of variable true:")
print(true)
print("Value of variable false:")
print(false)

# There are expressions which can be evaluated as boolean, for example
# comparisons: <, >, <= (less or equal), >= (greater or equal)

print("Evaluating boolean expressions:")
print(5<6)
print(5>6)
print(5<=6)
print(5>=6)
print(5<=5)
print(5>=5)

# Another important comparison operation is 'equal to': ==
# Be very careful not to confuse it with the assignment operator!!!
# Assignment: a = 13  (variable a gets value 13, this is NOT an expression)
# Comparison: a == 13 (a boolean expression: it
# evaluates to True is a has value 13, and to False in ALL other cases)

a = 13
print("Is a == 13?", a==13)
print("Is a == 14?", a==14)

#-------------------------------------------------------------

# 2. IF-ELIF-ELSE: Operator if is used to choose between available
# actions depending on some condition. Here's an example:
print("\nAn example of how 'if' operator works:")
a = 10
if a>10 : # keyword "if" followed by a boolean expression followed by ":"
    print("a is greater than 10") # note that this clause is moved to the right!
                                 # This is how python deals with hierarchy
                                 # of blocks of code - more on this later.
else: # keyword "else" and a semicolon. It's on the same level as your 'if'!
    print("a is not greater than 10") # moved to the right again!

# When the boolean expression after 'if' is evaluated to True, the first
# block of code will be executed; if it evaluates to False, then the
# second block will be executed. 
# You can also have more than one line of code in your clauses:

print("\nLet's try again with different a:")
a = 11
if a>10 : 
    print("a is greater than 10")
    print("this is another line of code")
else: 
    print("a is not greater than 10")
    print("the more lines of code you have, the better!")

# If you have more than two outcomes to take care of, use 'elif'
# (it is short for else if):
print("\nAn example of how 'elif' operator works:")

if a<=5:
    print("a is less than 5")
elif a<=10:  # a is more than 5 already because otherwise
            # the previous branch would've been used
    print("a is greater than 5 and less than or equal to 10")
elif a<=20: # similarly this branch will be executed if a is
            # greater than 10 but less than or equal to 20
    print("a is greater than 10 and less than or equal to 20")
else:
    print("all other values of a")  # if none of the three conditions
                                    # are true, you'll end up here.

print("\nAnother example (Amy's idea):")
print("Are you happy? y/n")

answer = input()

if answer=="y":
    print(":)")
elif answer=="n":
    print(":(")
else:
    print(":|")

#------------------------------------------------------------------    
# 3. Converting values:
# you can convert strings into numbers by using function int() or float()
# (obviously your string should only contain numbers and a floating point!)
print("\nConverting strings to numbers")
a = "34"
b = int(a)
c = float(a)
print("b:", b)
print("c:", c)
print("b+c+1:", b+c+1)

# You can also convert numbers to string using str():
print("Converting numbers to strings:")
x = str(5)
y = str(10)
print("x+y?", x+y)

# And both numbers and string can by converted to boolean!
# bool(0) and bool('') are evaluated to False, everything else to True:

print("Converting numbers and strings to boolean:")
print("bool(0):", bool(0))
print("bool(100):", bool(100))
print("bool(''):", bool(''))
print("bool('anything else'):", bool('anything else'))


# Now you have enough material to write a program with logical forks.
# Try a variation hello_age.py or hello_language.py

#----------------------------------------------------------------------

# 4. Additional information:

# 4.1 Logical operators 'or' and 'and' evaluate as you would expect:
# True and True -> True, any other combinations is False
# False or False -> False, any other combination will be True.
# Examples:
# 10<5 and 10>5 -> False and True -> False
# 10<5 or !0>5 -> False or True -> True
print("\nAn example of and/or operators:")
a = 10
b = 20
c = 30

if a<20 and b<20: # False and False evaluate to False
    print("a<20 and b<20")
elif c==30 or b>20:
    print("c==30 or b>20")  # True and False evaluate to True so this branch
                            # will be executed
else:
    print("something")


# 4.2 You can stack your ifs:
# (just don't forget to shift your blocks of code to the right!)

print("\nAn example of stacking if operators:")

if a == 10: 
    if b == 20: 
        print("branch 1")
    else:
        print("branch 2")
else:
    if c < 100:
        print("branch 3")
    else:
        print("branch 4")

# try this code with other values of a, b and c, and see what happens!