Programming Fundamentals编程基础

Pseudocode (using a temporary variable).伪代码（使用临时变量）。

SET a TO 10
SET b TO 20
SET temp TO a      -- save a's value
SET a TO b         -- a now holds 20
SET b TO temp      -- b now holds 10
OUTPUT a, b        -- prints: 20 10

Without temp, the first assignment SET a TO b would overwrite a's original value before b could capture it.若没有 temp，第一条赋值 SET a TO b 会在 b 捕获 a 的原始值之前将其覆盖。

Python (two ways).Python（两种方式）。

# Classic temp variable
a = 10
b = 20
temp = a
a = b
b = temp
print(a, b)   # 20 10

# Python shortcut (tuple unpacking)
a, b = b, a
print(a, b)   # 20 10

The tuple-unpacking shortcut evaluates both sides before any assignment, so no temp is needed. Both produce identical results.元组解包方式在任何赋值之前就对两侧求值，因此不需要临时变量。两种方式产生相同结果。

age    = 17           # int
gpa    = 3.85         # float
name   = "Alex"       # str
passed = True         # bool

print(type(age))      # <class 'int'>
print(type(gpa))      # <class 'float'>
print(type(name))     # <class 'str'>
print(type(passed))   # <class 'bool'>

Python infers the type from the literal value written on the right-hand side: integer literal → int, decimal literal → float, quoted text → str, True/False → bool. No explicit type declaration is needed in Python (unlike Java's int age = 17;).Python 从右侧写的字面值推断类型：整数字面值 → int，十进制字面值 → float，引号文本 → str，True/False → bool。Python 不需要显式类型声明（不同于 Java 的 int age = 17;）。

Pseudocode.伪代码。

INPUT width
INPUT height
SET area TO width * height
OUTPUT area

Python.Python。

width  = float(input("Enter width:  "))
height = float(input("Enter height: "))
area   = width * height
print("Area =", area)

We use float() so the program handles decimal dimensions (e.g., 3.5 metres). If we used int(), entering 3.5 would raise a ValueError. Always choose the conversion that matches what the user might realistically type.使用 float() 是为了让程序处理小数尺寸（如 3.5 米）。如果使用 int()，输入 3.5 会引发 ValueError。始终选择与用户实际可能输入相匹配的转换。

a = 17
b = 5

print(a + b)   # 22    (addition / 加法)
print(a - b)   # 12    (subtraction / 减法)
print(a * b)   # 85    (multiplication / 乘法)
print(a / b)   # 3.4   (true divide — always float / 真除——始终浮点数)
print(a // b)  # 3     (floor divide — integer quotient / 整除——整数商)
print(a % b)   # 2     (remainder: 17 = 3*5 + 2 / 余数：17 = 3*5 + 2)
print(a ** 2)  # 289   (power: 17² / 幂：17²)

print(a > b)   # True  (relational / 关系)
print(a == 17) # True
print(a != b)  # True
print(b >= 10) # False

Key distinction: a / b gives 3.4 (float). a // b gives 3 (int, rounds toward zero). a % b gives the leftover 2. Knowing // and % is essential for problems like "how many whole hours in 200 minutes?" (200 // 60 = 3) and "how many minutes left over?" (200 % 60 = 20).关键区别：a / b 给出 3.4（浮点数）。a // b 给出 3（整数，向零取整）。a % b 给出余数 2。掌握 // 和 % 对解决"200 分钟有几个完整小时？"（200 // 60 = 3）和"剩余几分钟？"（200 % 60 = 20）等问题至关重要。

Bug 1 — adding string to int.错误 1——将字符串与整数相加。

# BUG: input() returns a string
age = input("Enter age: ")
new_age = age + 1   # TypeError: can only concatenate str (not "int") to str

# FIX:
age = int(input("Enter age: "))
new_age = age + 1   # works correctly

Bug 2 — int() on a decimal string.错误 2——对小数字符串使用 int()。

# BUG:
price = int(input("Enter price: "))  # user types "9.99" → ValueError

# FIX: use float() first, then convert if needed
price = float(input("Enter price: "))   # works for "9.99"

Bug 3 — concatenating number into message.错误 3——将数字拼接到消息中。

# BUG:
score = 95
message = "Your score is " + score   # TypeError

# FIX:
message = "Your score is " + str(score)   # works
# OR use an f-string (simpler):
message = f"Your score is {score}"        # also works

The pattern is: whenever you mix types in an expression, Python raises a TypeError unless you explicitly convert. Type errors are among the most common beginner mistakes — and the easiest to fix once you know what the message means.规律是：每当你在表达式中混合类型，除非你显式转换，否则 Python 会引发 TypeError。类型错误是最常见的初学者错误之一——一旦你理解了错误消息的含义，也是最容易修复的。

Version 1 — hard to read ("magic numbers").版本 1——难以阅读（"魔法数字"）。

p = float(input())
t = p * 0.13
f = p + t
print(f)

What is 0.13? What is p, t, f? A reader cannot tell without context.0.13 是什么？p、t、f 是什么？读者没有上下文无法判断。

Version 2 — clear names and a named constant.版本 2——清晰命名与具名常量。

TAX_RATE = 0.13   # Ontario HST rate — a constant

price_before_tax = float(input("Enter price before tax: "))
tax_amount        = price_before_tax * TAX_RATE
final_price       = price_before_tax + tax_amount
print("Final price:", final_price)

Now the code documents itself. If the tax rate changes to 0.15, you change one line (TAX_RATE = 0.15) and every calculation updates automatically. This is why constants matter more than just style — they prevent "change in one place, miss it in ten" bugs.现在代码自我说明。如果税率变为 0.15，只需更改一行（TAX_RATE = 0.15），所有计算自动更新。这就是常量比样式更重要的原因——它们防止了"改一处、漏十处"的错误。

Step 1 — IPO.步骤 1——IPO。

• Input: weight in kg (float), height in m (float).输入：体重（千克，浮点数），身高（米，浮点数）。
• Process: BMI = weight / height².处理：BMI = 体重 / 身高²。
• Output: BMI rounded to 1 decimal place.输出：BMI 保留 1 位小数。

Step 2 — Pseudocode.步骤 2——伪代码。

INPUT weight_kg
INPUT height_m
SET bmi TO weight_kg / (height_m * height_m)
OUTPUT bmi (rounded to 1 decimal)

Step 3 — Python translation.步骤 3——Python 翻译。

weight_kg = float(input("Weight in kg:  "))
height_m  = float(input("Height in m:   "))
bmi       = weight_kg / (height_m ** 2)
print(f"BMI = {bmi:.1f}")

The :.1f format specifier rounds the float to 1 decimal place in the output string.:.1f 格式说明符在输出字符串中将浮点数舍入到 1 位小数。

Step 4 — run with known input.步骤 4——用已知输入运行。

# weight = 70 kg, height = 1.75 m
# BMI = 70 / (1.75)^2 = 70 / 3.0625 ≈ 22.9
# Expected output: BMI = 22.9   ✓

Step 5 — edge case: height = 0.步骤 5——边界情况：身高 = 0。

# height_m = 0 → division by zero → ZeroDivisionError
# Guard: add an if-check before the calculation (covered in Control Flow guide)

Spotting the edge case now — before writing any more code — is exactly what AB CSE1110 outcome 2.4.1 ("maintain the IPO structure") and ON ICS3U A2.1 ("incorporate user input, processing, and screen output") require.现在就发现边界情况——在写更多代码之前——正是 AB CSE1110 结果 2.4.1（"维护 IPO 结构"）和 ON ICS3U A2.1（"包含用户输入、处理和屏幕输出"）所要求的。