3. Conditionals¶

This is one possible solution. Note that it is not particularly efficient, as the same code appears in the if and elif blocks. This can be improved once variable scope and functions are understood.:

# Add/Subtract Two Numbers
# Dan Schellenberg
# Oct 23, 2017

operation_chosen = input("Would you like to add or subtract? ")

if operation_chosen == "add":
    #get user input
    first_number = input("Please enter the first number: ")
    second_number = input("Please enter the second number: ")

    #convert input into numbers
    first_number = float(first_number)
    second_number = float(second_number)

    the_answer = first_number + second_number
    print("The answer when you add is", the_answer)

elif operation_chosen == "subtract":
    #get user input
    first_number = input("Please enter the first number: ")
    second_number = input("Please enter the second number: ")

    #convert input into numbers
    first_number = float(first_number)
    second_number = float(second_number)

    the_answer = first_number - second_number
    print("The answer when you subtract is", the_answer)

else:
    #neither add or subtract was entered
    print("Error. I don't know what that means. Please enter 'add' or 'subtract'.")

conditionals11: What does the following code print (choose from output a, b, c or nothing)?

if 4 + 5 == 10:
    print("TRUE")
else:
    print("FALSE")

• TRUE
• TRUE is printed by the if-block, which only executes if the conditional (in this case, 4+5 == 10) is true. In this case 5+4 is not equal to 10.
• FALSE
• Since 4+5==10 evaluates to False, Python will skip over the if block and execute the statement in the else block.
• TRUE on one line and FALSE on the next
• Python would never print both TRUE and FALSE because it will only execute one of the if-block or the else-block, but not both.
• Nothing will be printed
• Python will always execute either the if-block (if the condition is true) or the else-block (if the condition is false). It would never skip over both blocks.

conditionals12: What does the following code print?

if 4 + 5 == 10:
    print("TRUE")
else:
    print("FALSE")
print("TRUE")

a. TRUE

b.
   TRUE
   FALSE

c.
   FALSE
   TRUE
d.
   TRUE
   FALSE
   TRUE

• Output a
• Although TRUE is printed after the if-else statement completes, both blocks within the if-else statement print something too. In this case, Python would have had to have skipped both blocks in the if-else statement, which it never would do.
• Output b
• Because there is a TRUE printed after the if-else statement ends, Python will always print TRUE as the last statement.
• Output c
• Python will print FALSE from within the else-block (because 5+4 does not equal 10), and then print TRUE after the if-else statement completes.
• Output d
• To print these three lines, Python would have to execute both blocks in the if-else statement, which it can never do.

conditionals14: What does the following code print?

x = -10
if x < 0:
    print("The negative number ",  x, " is not valid here.")
print("This is always printed")

a.
This is always printed

b.
The negative number -10 is not valid here
This is always printed

c.
The negative number -10 is not valid here

• Output a
• Because -10 is less than 0, Python will execute the body of the if-statement here.
• Output b
• Python executes the body of the if-block as well as the statement that follows the if-block.
• Output c
• Python will also execute the statement that follows the if-block (because it is not enclosed in an else-block, but rather just a normal statement).
• It will cause an error because every if must have an else clause.
• It is valid to have an if-block without a corresponding else-block (though you cannot have an else-block without a corresponding if-block).