Conditions

# Conditions

Week 4

NOTES:
New rule: closed laptops _unless_ we are doing an exercise.

---

## Review

![](https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExMW50M2hxbHh6eHUxa29mbWE0bzN2aG9wdjkwdTB0OGNwc205Y2NtdyZlcD12MV9naWZzX3NlYXJjaCZjdD1n/777Aby0ZetYE8/giphy.gif)

What is a variable?

A named container for data

What are the five basic data types in Python?

- Integer 
- Floating-point number 
- String 
- Boolean 
- NoneType

What data type would you use for a price?

Floating-point number

What data type would you use for a name?

String

What data type would you use for a true/false fact?

Boolean

What does `print()` do?

outputs text to the terminal

What type of data does `input()` always return?

String

How do you convert a string to an integer?

`int()`

```py [2]
age_string  = input("How old are you? ")
age_integer = int(age_string)
```

How can you add to a string?

Concatenation (or f-string)

```py [3]
first_name = "Rylan"
last_name  = "Schubkegel"
full_name  = first_name + " " + last_name
```

What is an f-string?

A string that can insert variables

```py [2]
name = input("Enter your name: ")
print(f"Hello, {name}")
```

What are the two boolean values?

`True` and `False`

NOTE: Booleans are a primitive type from week 2. Today they become useful.

---

## Making Decisions

So far, programs run **top to bottom**

Every line happens, every time

Real programs **choose** what to do

- If the password is correct, log in 
- If the answer is right, add a point 
- If you are 18 or older, you can vote

A **condition** is a yes/no question

The computer's answers are `True` and `False`

```py
if True:
    print("Pinocchio's nose will grow")
```

---

## Comparisons

| Operator | Meaning |
| --- | --- |
| `==` | equal to |
| `!=` | not equal to |

`=` **assigns** a value

`==` **compares** two values

Comparison operators always result in a `True`/`False` value

What is the output of each line?

```py
x = 2
print(x == 2) # 1
print(x == 3) # 2
print(x != 3) # 3
```

1. True 
2. False 
3. True

| Operator | Meaning |
| --- | --- |
| `<` | less than |
| `>` | greater than |
| `<=` | less than or equal to |
| `>=` | greater than or equal to |

What is the output of each line?

```py
print(10 > 3) # 1
print(2 < 2)  # 2
print(7 >= 7) # 3
print(4 <= 3) # 4
```

1. True 
2. False 
2. True 
3. False

What's wrong here?

```py
age = 18
can_vote = age = 18
print("You are 18 years old:", can_vote)
```

`=` assigns, it does not compare

---

## The `if` Statement

Three parts:

1. `if`
2. A condition that is `True` or `False`
3. A **colon** `:` then an indented block

```py [|2|3]
age = int(input("Enter your age: "))
if age >= 18:
    print("You can vote!")
```

Python indentation

```py
if True:
 print('1 space is fine')
if True:
        print('8 spaces is fine')
if True:
    print()
     print('4 then 5 spaces is NOT FINE')
```

NOTE: indentation is one of the most common problems I see

If the condition is `True`, run the indented lines

If the condition is `False`, skip them

What's wrong here?

```py
if age >= 18
    print("You can vote.")
```

Missing colon `:`

---

## `else` and `elif`

`else` (the other path)

```py [5-6]
age = int(input("Enter your age: "))

if age >= 18:
    print("You can vote.")
else:
    print("You cannot vote yet.")
```

Exactly one of these two messages will print

`elif` (else if)

Check another condition when the first one was `False`

```py [5-6]
age = int(input("Enter your age: "))

if age < 5:
    price = 5
elif age < 16:
    price = 10
else:
    price = 18

print(f"You'll pay ${price} for the ticket")
```

NOTE:
- Conditions are checked **in order**
- The first `True` one wins; the rest are skipped
- `else` is optional, but useful as a fallback

Order matters!

```py
score = 95

if score >= 60:
    print("Pass")
elif score >= 90:
    print("A")
```

What prints?

`Pass` — the first condition is already `True`

NOTE: Put the most specific checks first. For grades, check A before Pass.

Separate `if`s can **all** run

`elif` is only tried if the previous check failed

```py
score = 95

if score >= 90:
    print("A")
if score >= 60:
    print("Pass")
```

```txt
A
Pass
```

---

## Checking Answers

```py
answer = input("What is the capital of France? ")

if answer == "Paris":
    print("Correct!")
else:
    print("Incorrect.")
```

`"Paris"` and `"paris"` are **not** equal

Tell the user the expected format

NOTE: This is in the quiz assignment. You can also convert the answer with `.lower()` if you want to accept any capitalization.

Optional: ignoring capitalization

```py
answer = input("What is the capital of France? ")

if answer.lower() == "paris":
    print("Correct!")
else:
    print("Incorrect.")
```

Example: keeping score

```py
score = 0

answer = input("2 + 2 = ")
if answer == "4":
    print("Correct!")
    score = score + 1
else:
    print("Incorrect.")

print(f"Your score: {score}")
```

NOTE: `score += 1` means "add 1 to score." Same as `score = score + 1`. This is exactly what the quiz assignment needs.

### Exercise

Make a "guess the number" game!

1. Pick a secret number (e.g., 7)
2. Ask the user to guess the number
3. If the guess is correct, print "You got it!" Otherwise, print "Nice try."

---

![Jimmy Fallon thats it](https://media4.giphy.com/media/v1.Y2lkPTc5MGI3NjExd2NmamM3NmR2c3RtenR3bGV5dnR1MnNyM3BzNnh3ZGNwYTFyZ2V6YyZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/uSZQ6WPIcpouUEpNTe/giphy.gif)