Stanford CS106A - Python Programming Methodology Flashcards

Question 1

What is a variable in Python?

Accepted Answer

A named storage location that holds a value. Variables are dynamically typed in Python, meaning their type can change during execution. Example: `x = 10`, `name = "Alice"`.

Question 2

List the four primary built-in data types in Python.

Accepted Answer

1. `int`: Integers (e.g., `5`, `-100`)
2. `float`: Floating-point numbers (e.g., `3.14`, `-0.5`)
3. `str`: Strings (sequences of characters, e.g., `"hello"`)
4. `bool`: Booleans (either `True` or `False`)

Question 3

Explain type conversion (casting) in Python with an example.

Accepted Answer

Type conversion is changing a value from one data type to another. Python provides built-in functions like `int()`, `float()`, `str()`, and `bool()`.
Example:
```python
num_str = "123"
num_int = int(num_str) # Converts string "123" to integer 123
```

Question 4

Describe the basic structure of an `if`/`elif`/`else` statement.

Accepted Answer

Used for conditional execution. Code blocks are executed based on whether a condition evaluates to `True`.
```python
if condition1:
    # code if condition1 is True
elif condition2:
    # code if condition1 is False and condition2 is True
else:
    # code if all conditions are False
```

Question 5

How do you iterate a specific number of times using a `for` loop?

Accepted Answer

Use the `range()` function. `range(n)` generates numbers from 0 up to (but not including) $n$.
Example:
```python
for i in range(5):
    print(i) # Prints 0, 1, 2, 3, 4
```

Question 6

When would you use a `while` loop instead of a `for` loop?

Accepted Answer

A `while` loop is used when the number of iterations is not known beforehand and depends on a condition remaining `True`. A `for` loop is typically used when iterating over a sequence or a known range.
Example:
```python
count = 0
while count < 3:
    print(count)
    count += 1
```

Question 7

Define a simple Python function that takes two numbers and returns their sum.

Accepted Answer

```python
def add_numbers(a, b):
    return a + b

result = add_numbers(5, 3) # result will be 8
```

Question 8

Explain the difference between local and global scope in Python functions.

Accepted Answer

*   **Local scope**: Variables defined inside a function are local to that function and cannot be accessed from outside it.
*   **Global scope**: Variables defined outside any function are global and can be accessed (but not directly modified without the `global` keyword) from anywhere in the program.

Question 9

What is a Python list? Provide an example of creating and accessing elements.

Accepted Answer

An ordered, mutable collection of items. Items can be of different data types.
Example:
```python
my_list = [1, "hello", 3.14]
print(my_list[0]) # Output: 1
my_list.append(True) # Adds True to the end
```

Question 10

How do you add and remove elements from a Python list?

Accepted Answer

*   **Add**: `append(item)` adds to the end; `insert(index, item)` adds at a specific index.
*   **Remove**: `remove(item)` removes the first occurrence of a value; `pop(index)` removes and returns the item at a specific index (or the last item if no index is given).

Question 11

What is a Python dictionary? Give an example of creation and accessing values.

Accepted Answer

An unordered, mutable collection of key-value pairs. Keys must be unique and immutable (e.g., strings, numbers, tuples).
Example:
```python
my_dict = {"name": "Alice", "age": 30, "city": "NY"}
print(my_dict["name"]) # Output: Alice
my_dict["age"] = 31 # Modifies value
```

Question 12

How do you iterate through keys, values, and key-value pairs in a dictionary?

Accepted Answer

*   **Keys**: `for key in my_dict:` or `my_dict.keys()`
*   **Values**: `for value in my_dict.values():`
*   **Key-value pairs**: `for key, value in my_dict.items():`

Question 13

Explain string indexing and slicing in Python.

Accepted Answer

*   **Indexing**: Accessing individual characters by their position (0-based). `my_string[0]` gets the first character.
*   **Slicing**: Extracting a substring using `[start:end:step]`. `my_string[1:4]` gets characters from index 1 up to (but not including) 4.
Example: `s = "Python"`, `s[0]` is 'P', `s[1:3]` is "yt".

Question 14

How can you check if a substring exists within a string, and replace parts of a string?

Accepted Answer

*   **Check existence**: Use the `in` operator. Example: `"Py" in "Python"` returns `True`.
*   **Replace**: Use the `replace()` method. Example: `"hello world".replace("world", "Python")` returns `"hello Python"`.

Question 15

Describe the basic steps for reading data from a text file in Python.

Accepted Answer

1.  **Open the file**: Use `open("filename.txt", "r")` (the "r" mode is for reading). It's best to use a `with` statement for automatic closing.
2.  **Read data**: Use methods like `read()` (reads entire file), `readline()` (reads one line), or `readlines()` (reads all lines into a list).
3.  **Process data**.
4.  **Close the file**: (Automatically handled by `with` statement).
Example:
```python
with open("data.txt", "r") as file:
    content = file.read()
    print(content)
```

Question 16

How do you write data to a text file in Python?

Accepted Answer

1.  **Open the file**: Use `open("output.txt", "w")` for writing (overwrites existing file) or `open("output.txt", "a")` for appending.
2.  **Write data**: Use the `write()` method. Remember to add newline characters `
` manually if needed.
3.  **Close the file**: (Automatically handled by `with` statement).
Example:
```python
with open("output.txt", "w") as file:
    file.write("Hello, CS106A!
")
    file.write("This is a new line.")
```

Question 17

What is a "class" in Object-Oriented Programming (OOP)?

Accepted Answer

A blueprint or template for creating objects. It defines a set of attributes (data) and methods (functions) that the objects created from the class will possess.

Question 18

What is an "object" (or instance) in OOP?

Accepted Answer

An individual entity created from a class. It's a concrete realization of the class blueprint, possessing its own unique set of attribute values.
Example: If `Car` is a class, `my_car = Car("red", "Toyota")` creates an object `my_car`.

Question 19

Explain the purpose of the `__init__` method in a Python class.

Accepted Answer

The `__init__` method is a special method (often called a constructor) that is automatically called when a new object (instance) of the class is created. It's used to initialize the object's attributes.
Example:
```python
class Dog:
    def __init__(self, name, breed):
        self.name = name
        self.breed = breed
```

Question 20

Describe "problem decomposition" in the context of programming.

Accepted Answer

The process of breaking down a large, complex problem into smaller, more manageable sub-problems. Each sub-problem can then be solved independently, and their solutions combined to solve the original problem. This makes development easier and more organized.

Question 21

What is "algorithmic thinking"?

Accepted Answer

A problem-solving approach that involves defining a clear, step-by-step procedure (an algorithm) to solve a problem. It focuses on logical sequencing, identifying inputs/outputs, and designing efficient steps to achieve a desired outcome.

Stanford CS106A - Python Programming Methodology

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code if condition1 is True

code if condition1 is False and condition2 is True

code if all conditions are False

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