Control Flow: Conditionals#

Goals of this lecture#

What is “control flow”?
What is a conditional?
Conditionals in action (if/elif/else).
Complex and nested conditional statements.

What is control flow?#

By default, Python commands are executed in a linear order, i.e., line by line.

Unless we tell Python otherwise, each line will be executed once.

But sometimes it’s useful to control:

Which lines get executed?
How many times do those lines get executed?

These control “parameters” correspond, roughly, to:

Conditional statements (if/elif/else).
Loops (e.g., for or while).

What is a conditional?#

In a nutshell, a conditional is a statement that checks for whether some condition is met.

We can use the if command to control which lines of code are executed.

condition = True
if condition:
    print("This code will only run if the condition is True.")

This code will only run if the condition is True.

condition = False
if condition:
    print("This code will only run if the condition is True.")

Check-in#

Consider the code block below. Which part is the conditional statement?

x = 10
y = 5
if x > y:
    print("X is bigger than Y.")

X is bigger than Y.

Check-in#

Consider the code block below. Why won’t the print statement run?

x = "One string"
y = "Another string"
if x == y:
    print("These strings are the same.")

What belongs in an `if` statement?#

An if statement should evaluate to True or False.
- This includes the outcome of any comparison operation (>, ==, etc.).
- Technically, it also includes numbers/strings (which evaluate to True) and NoneType (which is equivalent to False).
An if statement is extremely useful for modifying the behavior of a program, depending on some condition.

if None:
    print("This won't print")

Check-in#

What happens if our if statement evaluates to False (e.g., the statement 4 > 5 would evaluate to False)?

Conditionals with operators#

Conditional statements can be used with operators. This is really useful if you want to modify your program based on whether two variables are equal (==), or one is larger (>) or smaller (<) than the others, and so on.

checking_account = 1000
if checking_account > 200:
    print("Withdrawal allowed.")

Withdrawal allowed.

`else` statement#

An else statement tells Python what to do if an if statement evaluates to False.

condition = False
if condition:
    print("Condition is TRUE.")
else:
    print("Condition is FALSE.")

Condition is FALSE.

When to use an `else` statement?#

An else statement can only be used after an if statement (see the SyntaxError below).

else:
    print("test")

  Input In [8]
    else:
    ^
SyntaxError: invalid syntax

An else statement is most useful if you want two different things to happen, depending some condition:

If condition == True, execute Action A.
else, execute Action B.

condition = False
if condition:
    print("Do this if the condition is TRUE.")
else:
    print("Do this if the condition is FALSE.")

Do this if the condition is FALSE.

Quick note on indentation#

Notice that the code below an if or else statement must be indented, if you want it to be associated with that statement.

If there is no indented code below an if statement, you’ll get an IndentationError.

if 3 > 2:
print("No idententation")

  Input In [10]
    print("No idententation")
    ^
IndentationError: expected an indented block

However, you can still have un-indented code below an if or else statement, as long as there’s also indented code.

if 3 > 2:
    print("This will execute if the condition is met.")
print("This will execute regardless.")

This will execute if the condition is met.
This will execute regardless.

Check-in#

Which lines in the code below would actually print?

condition = False
if condition:
    print("Do this if the condition is TRUE.")
else:
    print("Do this if the condition is FALSE.")
print("Also do this.")

`elif` statement#

An elif statement tells Python what to do if an if statement evaluates to False, and some other condition is met.

This is kind of a combination of an if and else statement.

condition1 = False
condition2 = True
if condition1:
    print("Condition 1 is true.")
elif condition2:
    print("Condition 2 is true.")

Condition 2 is true.

When will an `elif` statement run?#

An elif statement will only run if the if statement evaluates to False––even if the elif statement would’ve evaluated to True!

condition1 = True
condition2 = True
if condition1:
    print("Condition 1 is true.")
elif condition2:
    print("Condition 2 is also true, but this won't print.")

Condition 1 is true.

`if` vs. `elif`#

The key difference between two if statements in a row vs. an if/elif statement is:

The code under both if statements can run if both statements are True.
The code under an elif statement will only run if the if statement is False.

condition1 = True
condition2 = True
if condition1:
    print("Condition 1 is true.")
if condition2:
    print("Condition 2 is also true, and this will also print.")

Condition 1 is true.
Condition 2 is also true, and this will also print.

`elif` vs. `else`#

An elif statement cannot be placed after an else statement.
- This will generate a SyntaxError.
It also just doesn’t make sense logically. If elif were at the end, it’d never be evaluated anyway, since else covers everything other than the if statement.

if 2 > 3:
    print("True")
else:
    print("False")
elif 2 > 1:
    print("True?")

  Input In [15]
    elif 2 > 1:
    ^
SyntaxError: invalid syntax

Check-in#

What do you expect the value of x to be if the following code is run? (Try to figure it out before running the code to check what x is.)

y = 1
x = 0
if y >= 1:
    x -= 2
elif y >= 1:
    x -= 1
else:
    x += 1

Check-in#

What do you expect the value of x to be if the following code is run? (Try to figure it out before running the code to check what x is.)

y = 1
x = 0
if y >= 1:
    x -= 2
if y >= 0:
    x -= 1
else:
    x += 1

Check-in#

Why did those two different code blocks behave differently?

Both `elif` and `else` “attach” to the nearest `if` statement#

Any given else or elif statement is attached/associated with exactly one if statement (the one immediately above).

This means that we must be very careful to think about what each else statement is actually comparing against.

Check-in#

The following code ends up printing a contradiction (e.g., A is True, followed by Neither A nor B are True). Why is this happening?

Hint: Think about what we just discussed––an else attaches to the nearest if statement.

A = True
B = False
C = True
if A:
    print("A is True")
if B:
    print("B is True")
else:
    print("Neither A nor B are True.")

A is True
Neither A nor B are True.

More complex conditionals#

So far, we’ve dealt with fairly limited conditional statements:

Each if checks only a single condition.
Relatively linear ordering: if, elif, then else.

But conditional statements can be considerably more complex:

Each if statement can check multiple conditions using logical operators like or and and.
Conditional statements can be nested.

Using `and` and `or`#

Recall that and and or can be used to evaluate multiple statements.

and returns True if all statements are True.
or returns True if at least one statement is True.

We can use these to check for more complex conditions.

a = 20
b = 30
c = 40
if b > a and c > b:
    print("Both conditions are True.")

Both conditions are True.

Check-in#

Why does the top code block execute the code under the if statement, while the bottom one doesn’t?

a = 20
b = 30
c = 25
if b > a or c > b:
    print("At least one condition is True.")

At least one condition is True.

a = 20
b = 30
c = 25
if b > a and c > b:
    print("Both conditions are True.")

A simple use-case for `and`#

is_password = True
checking_account = 1000
withdrawal = 500
if is_password and (withdrawal < checking_account):
    print("Withdrawal permitted.")
    checking_account -= withdrawal
    print(str(checking_account) + " left in checking.")

Withdrawal permitted.
500 left in checking.

A simple use-case for `or`#

is_dog = True
is_cat = False
if is_dog or is_cat:
    print("This is a dog or cat.")
else:
    print("This is neither a dog nor cat.")

This is a dog or cat.

Check-in: `and` vs. `else`#

How would an else statement behave following an if statement using an and (e.g., X and Y)? (Choose either (1) or (2).)

The else statement will run if both X and Y are False.
The else statement will run if at least one of X and Y is `False.

Check-in: `or` vs. `else`#

How would an else statement behave following an if statement using an or (e.g., X or Y)? (Choose either (1) or (2).)

The else statement will run if both X and Y are False.
The else statement will run if at least one of X and Y is `False.

Using nested conditionals#

A nested conditional is one that contains at least one if statement “nested” within another conditional statement.

a = 8
if a > 5:
    if a >= 10:
        print("A is greater than or equal to 10.")
    else:
        print("A is bigger than 5, but smaller than 10.")
else:
    print("A is smaller than or equal to 5.")

A is bigger than 5, but smaller than 10.

Nested `if` vs. `and`#

A nested if statement functions similarly to an and statement.
In both cases, some block of code will only run if both conditions are met.
The key difference is that a nested if statement allows you more granularity in terms of evaluating which conditions are met, and what to do in each case.

Code Style: Indentation#

With conditionals, it’s hugely important that you keep track of your indentation.
It’s easy to introduce bugs by making something indented where it shouldn’t be, or the other way around.
Debugging practice:
- As before, read each line carefully.
- Track the state of each variable.
- Track whether a given conditional statement evaluates to True or False, and what would happen next.

Conclusion#

Conditional statements give you extra control over which lines in your Python program get run.
This is very useful for modifying the behavior of your program, depending on some other condition.
- When we discuss functions (and later on, filtering datasets), this will become even clearer.
Important to keep track of your indentation when dealing with if statements, especially nested statements.

CSS 1

Control Flow: Conditionals

Contents

Control Flow: Conditionals#

Goals of this lecture#

What is control flow?#

What is a conditional?#

Check-in#

Check-in#

What belongs in an `if` statement?#

Check-in#

Conditionals with operators#

`else` statement#

When to use an `else` statement?#

Quick note on indentation#

Check-in#

`elif` statement#

When will an `elif` statement run?#

`if` vs. `elif`#

`elif` vs. `else`#

Check-in#

Check-in#

Check-in#

Both `elif` and `else` “attach” to the nearest `if` statement#

Check-in#

More complex conditionals#

Using `and` and `or`#

Check-in#

A simple use-case for `and`#

A simple use-case for `or`#

Check-in: `and` vs. `else`#

Check-in: `or` vs. `else`#

Using nested conditionals#

Nested `if` vs. `and`#

Code Style: Indentation#

Conclusion#

CSS 1

Control Flow: Conditionals

Contents

Control Flow: Conditionals#

Goals of this lecture#

What is control flow?#

What is a conditional?#

Check-in#

Check-in#

What belongs in an if statement?#

Check-in#

Conditionals with operators#

else statement#

When to use an else statement?#

Quick note on indentation#

Check-in#

elif statement#

When will an elif statement run?#

if vs. elif#

elif vs. else#

Check-in#

Check-in#

Check-in#

Both elif and else “attach” to the nearest if statement#

Check-in#

More complex conditionals#

Using and and or#

Check-in#

A simple use-case for and#

A simple use-case for or#

Check-in: and vs. else#

Check-in: or vs. else#

Using nested conditionals#

Nested if vs. and#

Code Style: Indentation#

Conclusion#

What belongs in an `if` statement?#

`else` statement#

When to use an `else` statement?#

`elif` statement#

When will an `elif` statement run?#

`if` vs. `elif`#

`elif` vs. `else`#

Both `elif` and `else` “attach” to the nearest `if` statement#

Using `and` and `or`#

A simple use-case for `and`#

A simple use-case for `or`#

Check-in: `and` vs. `else`#

Check-in: `or` vs. `else`#

Nested `if` vs. `and`#