CS101 Control Flow

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Conditional Control

Conditional statements or conditional control structures allow a program to have different behaviors depending on certain conditions being met.

Intuitively, this mimics the way humans make simple decisions and act upon them. For example, reasoning about whether to go outside might look like:

• Condition: Is it raining outside?
• If it is raining outside, then bring an umbrella.
• Otherwise, do not bring an umbrella.

We could keep adding clauses to make our reasoning more sophisticated, such as “If it is sunny, then wear sunscreen”.

Control Flow

In programming, control flow is the order in which statements and instructions are executed. Programmers are able to change a program’s control flow using control structures such as conditionals.

Being able to alter a program’s control flow is powerful, as it lets us adapt a running program’s behavior depending on the state of the program. For example, suppose a user is using a banking application and wants to withdraw \$500. We certainly want the application to behave differently depending on whether the user has \$20 or \$1000 in their bank account!

elif Statement

The Python elif statement allows for continued checks to be performed after an initial if statement. An elif statement differs from the else statement because another expression is provided to be checked, just as with the initial if statement.

If the expression is True, the indented code following the elif is executed. If the expression evaluates to False, the code can continue to an optional else statement. Multiple elif statements can be used following an initial if to perform a series of checks. Once an elif expression evaluates to True, no further elif statements are executed.

# elif Statement pet_type = "fish" if pet_type == "dog": print("You have a dog.") elif pet_type == "cat": print("You have a cat.") elif pet_type == "fish": # this is performed print("You have a fish") else: print("Not sure!")

or Operator

The Python or operator combines two Boolean expressions and evaluates to True if at least one of the expressions returns True. Otherwise, if both expressions are False, then the entire expression evaluates to False.

True or True # Evaluates to True True or False # Evaluates to True False or False # Evaluates to False 1 < 2 or 3 < 1 # Evaluates to True 3 < 1 or 1 > 6 # Evaluates to False 1 == 1 or 1 < 2 # Evaluates to True

Equal Operator ==

The equal operator, ==, is used to compare two values, variables or expressions to determine if they are the same.

If the values being compared are the same, the operator returns True, otherwise it returns False.

The operator takes the data type into account when making the comparison, so a string value of "2" is not considered the same as a numeric value of 2.

# Equal operator if 'Yes' == 'Yes': # evaluates to True print('They are equal') if (2 > 1) == (5 < 10): # evaluates to True print('Both expressions give the same result') c = '2' d = 2 if c == d: print('They are equal') else: print('They are not equal')

Not Equals Operator !=

The Python not equals operator, !=, is used to compare two values, variables or expressions to determine if they are NOT the same. If they are NOT the same, the operator returns True. If they are the same, then it returns False.

The operator takes the data type into account when making the comparison so a value of 10 would NOT be equal to the string value "10" and the operator would return True. If expressions are used, then they are evaluated to a value of True or False before the comparison is made by the operator.

# Not Equals Operator if "Yes" != "No": # evaluates to True print("They are NOT equal") val1 = 10 val2 = 20 if val1 != val2: print("They are NOT equal") if (10 > 1) != (10 > 1000): # True != False print("They are NOT equal")

Comparison Operators

In Python, relational operators compare two values or expressions. The most common ones are:

• < less than
• > greater than
• <= less than or equal to
• >= greater than or equal too

If the relation is sound, then the entire expression will evaluate to True. If not, the expression evaluates to False.

a = 2 b = 3 a < b # evaluates to True a > b # evaluates to False a >= b # evaluates to False a <= b # evaluates to True a <= a # evaluates to True

if Statement

The Python if statement is used to determine the execution of code based on the evaluation of a Boolean expression.

• If the if statement expression evaluates to True, then the indented code following the statement is executed.
• If the expression evaluates to False then the indented code following the if statement is skipped and the program executes the next line of code which is indented at the same level as the if statement.
# if Statement test_value = 100 if test_value > 1: # Expression evaluates to True print("This code is executed!") if test_value > 1000: # Expression evaluates to False print("This code is NOT executed!") print("Program continues at this point.")

else Statement

The Python else statement provides alternate code to execute if the expression in an if statement evaluates to False.

The indented code for the if statement is executed if the expression evaluates to True. The indented code immediately following the else is executed only if the expression evaluates to False. To mark the end of the else block, the code must be unindented to the same level as the starting if line.

# else Statement test_value = 50 if test_value < 1: print("Value is < 1") else: print("Value is >= 1") test_string = "VALID" if test_string == "NOT_VALID": print("String equals NOT_VALID") else: print("String equals something else!")

and Operator

The Python and operator performs a Boolean comparison between two Boolean values, variables, or expressions. If both sides of the operator evaluate to True then the and operator returns True. If either side (or both sides) evaluates to False, then the and operator returns False. A non-Boolean value (or variable that stores a value) will always evaluate to True when used with the and operator.

True and True # Evaluates to True True and False # Evaluates to False False and False # Evaluates to False 1 == 1 and 1 < 2 # Evaluates to True 1 < 2 and 3 < 1 # Evaluates to False "Yes" and 100 # Evaluates to True

Boolean Values

Booleans are a data type in Python, much like integers, floats, and strings. However, booleans only have two values:

• True
• False

Specifically, these two values are of the bool type. Since booleans are a data type, creating a variable that holds a boolean value is the same as with other data types.

is_true = True is_false = False print(type(is_true)) # will output: <class 'bool'>

not Operator

The Python Boolean not operator is used in a Boolean expression in order to evaluate the expression to its inverse value. If the original expression was True, including the not operator would make the expression False, and vice versa.

not True # Evaluates to False not False # Evaluates to True 1 > 2 # Evaluates to False not 1 > 2 # Evaluates to True 1 == 1 # Evaluates to True not 1 == 1 # Evaluates to False