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We are going to create a simple tutorial for a basic programming concept: Variables and Data Types.
This tutorial will be in Python and will cover:
What are variables?
How to declare variables?
Different data types in Python (int, float, string, boolean)
Basic operations on these data types.
Let's break it down step by step.
Python Tutorial: Variables and Data Types
1. What is a Variable?
A variable is a named storage location in memory to store data. Think of it as a labeled box where you can keep information.
# Example: Assigning values to variables
age = 25 # Integer
name = "Alice" # String
price = 19.99 # Float
is_student = True # Boolean
2. Rules for Naming Variables
Must start with a letter or underscore (_)
Can contain letters, numbers, and underscores
Case-sensitive: age ≠ Age
Cannot use Python keywords (e.g., if, for, class)
# Valid names
user_name = "Bob"
first_name = "Charlie"
_counter = 100
# Invalid names (will cause errors)
2nd_place = "Runner-up" # Starts with a number
my-var = "test" # Contains a hyphen
3. Python Data Types
Python has built-in data types:
Type
Example
Description
int
10, -5
Whole numbers
float
3.14, -0.5
Numbers with decimal points
str
"Hello"
Text (strings)
bool
True, False
Truth values
list
[1, 2, 3]
Ordered, mutable collection
dict
{"key": "value"}
Key-value pairs
4. Check Data Type
Use type() to check a variable's type:
age = 25
print(type(age)) # Output: <class 'int'>
name = "David"
print(type(name)) # Output: <class 'str'>
5. Type Conversion
Convert between types using built-in functions:
# Convert to string
str(123) # Returns "123"
str(True) # Returns "True"
# Convert to integer
int("45") # Returns 45
int(3.9) # Returns 3 (truncates decimals)
# Convert to float
float("7.5") # Returns 7.5
float(10) # Returns 10.0
# Convert to boolean
bool(0) # Returns False
bool(42) # Returns True
bool("") # Returns False (empty string)
6. Basic Operations
# Arithmetic (numbers)
x = 10
y = 3
print(x + y) # 13 (Addition)
print(x - y) # 7 (Subtraction)
print(x * y) # 30 (Multiplication)
print(x / y) # 3.333... (Division)
print(x // y) # 3 (Floor division)
print(x % y) # 1 (Modulus)
print(x y) # 1000 (Exponent)
# String operations
greeting = "Hello"
name = "Eve"
print(greeting + " " + name) # "Hello Eve"
print(greeting * 3) # "HelloHelloHello"
# Boolean operations
is_adult = True
is_student = False
print(is_adult and is_student) # False (AND)
print(is_adult or is_student) # True (OR)
print(not is_adult) # False (NOT)
7. Dynamic Typing
Python variables can change type during runtime:
data = 42 # Integer
print(type(data)) # <class 'int'>
data = "Forty-two" # String
print(type(data)) # <class 'str'>
8. Best Practices
Use descriptive names:
✅ user_age ❌ x
Constants: Use uppercase for constants:
MAX_SPEED = 100
Avoid reserved words:
Don’t use if, for, class, etc.
Initialize variables:
Always assign a value before using.
9. Common Errors
# Error: Using undefined variable
print(undefined_var) # NameError: name 'undefined_var' is not defined
# Error: Type mismatch
result = "10" + 5 # TypeError: can only concatenate str (not "int") to str
# Fix: Convert to same type
result = int("10") + 5 # 15
10. Quick Practice
# Task: Create variables for a person's info
name = "Fiona"
age = 30
height = 1.65
is_employed = True
# Calculate retirement age (65)
years_to_retirement = 65 - age
# Print a summary
print(f"{name} is {age} years old and {height}m tall.")
print(f"Years to retirement: {years_to_retirement}")
print(f"Employed: {is_employed}")
Output:
Fiona is 30 years old and 1.65m tall.
Years to retirement: 35
Employed: True
Next Steps:
Explore lists, dictionaries, and control flow (if/else, loops) to build more complex programs! 🚀
Okay, so I need to figure out how to solve this problem. Let me start by reading it again carefully. The problem says: "A car travels from City A to City B at an average speed of 60 miles per hour. On the return trip, the car travels at an average speed of 40 miles per hour. What is the average speed for the entire round trip?" Hmm, average speed for the entire trip. I remember that average speed isn't just the average of the two speeds. Instead, you have to consider the total distance and the total time taken. Let me recall the formula for average speed. It's total distance divided by total time, right?
So, the car goes from City A to City B at 60 mph and comes back at 40 mph. Let me assume the distance between City A and City B is a certain value. Since the distance isn't given, maybe I can call it 'd' miles. That way, the round trip distance would be 2d miles. But I need to find the total time taken for the trip.
For the trip from A to B, the distance is d and the speed is 60 mph, so the time taken would be distance divided by speed, which is d/60 hours. Similarly, on the return trip from B to A, the distance is also d, but the speed is 40 mph, so the time taken is d/40 hours. Therefore, the total time for the round trip is d/60 + d/40.
Now, to find the average speed, I need to divide the total distance by the total time. The total distance is 2d, and the total time is d/60 + d/40. Let me write that out: average speed = 2d / (d/60 + d/40).
Hmm, let me simplify this. Both terms in the denominator have a 'd', so I can factor that out. So, the denominator becomes d(1/60 + 1/40). Then, the average speed becomes 2d / [d(1/60 + 1/40)]. The 'd' cancels out, which is good because we don't know its value. So now it's 2 / (1/60 + 1/40).
Now, I need to compute 1/60 + 1/40. To add these fractions, I need a common denominator. The least common denominator for 60 and 40 is 120. So, converting both fractions: 1/60 = 2/120 and 1/40 = 3/120. Adding them together gives 2/120 + 3/120 = 5/120. Simplifying 5/120, that's 1/24.
So, the denominator is 1/24, and the average speed is 2 divided by 1/24. Dividing by a fraction is the same as multiplying by its reciprocal, so 2 * 24/1 = 48. Therefore, the average speed for the entire round trip is 48 miles per hour.
Wait, let me check that again. If I go somewhere at 60 mph and come back at 40 mph, the average speed is 48? That seems right because the harmonic mean of 60 and 40 is 48. The formula for the harmonic mean of two speeds is 2ab/(a + b). Let me plug in a = 60 and b = 40: 26040/(60+40) = 4800/100 = 48. Yep, that matches. So that's correct.
But just to make sure I didn't make a mistake in my initial calculation, let me go through it again. Total distance is 2d. Total time is d/60 + d/40. Let's compute that: d/60 is approximately 0.0167d and d/40 is 0.025d. Adding them gives 0.0417d. Then, 2d divided by 0.0417d is 2 / 0.0417 ≈ 48. So that checks out.
Another way to think about it: suppose the distance between City A and City B is 120 miles. Then, the trip from A to B takes 120/60 = 2 hours. The return trip takes 120/40 = 3 hours. Total distance is 240 miles, total time is 5 hours. Average speed is 240/5 = 48 mph. Yep, that works too.
So, regardless of the method—using variables, harmonic mean formula, or plugging in actual numbers—the average speed comes out to 48 mph. Therefore, the answer should be 48 miles per hour.
The average speed for the entire round trip is calculated using the harmonic mean formula for two speeds. Here's the step-by-step solution:
Define Variables:
Let ( d ) be the one-way distance between City A and City B.
Total round trip distance = ( 2d ).
Calculate Time for Each Leg:
Time from A to B: ( \frac{d}{60} ) hours.
Time from B to A: ( \frac{d}{40} ) hours.
Total time: ( \frac{d}{60} + \frac{d}{40} ).
Simplify Total Time:
Common denominator for 60 and 40 is 120:
[
\frac{d}{60} + \frac{d}{40} = \frac{2d}{120} + \frac{3d}{120} = \frac{5d}{120} = \frac{d}{24}.
]
Compute Average Speed:
Average speed = ( \frac{\text{Total Distance}}{\text{Total Time}} = \frac{2d}{\frac{d}{24}} = 2 \times 24 = 48 ) mph.
Final Answer: The average speed for the entire round trip is (\boxed{48}) miles per hour.
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发表于 2025-11-6 17:36:32