编程基础

This is the first course in University Computer Science: programming fundamentals 程序设计基础, taught with Python 3. It assumes no prior programming. Every program in this handout is complete and runnable — type it, run it, change one line, and predict what changes before running again. That predict-then-run loop is the entire skill.

The course is a ladder:

• Units 1.1 to 1.3 build the mechanics: values and types, selection, iteration.
• Unit 1.4 packages work into functions.
• Units 1.5 and 1.6 add the standard collections and files with error handling.
• Unit 1.7 is the discipline on top: decomposing problems, testing, debugging.

How to read each unit: a short story says why it matters; a picture or a rule of thumb comes before any syntax; then the precise rules; then worked examples that get harder, each with a trace you can check by running the code; then the mistakes people actually make; then a three-line summary. Every example here can be checked a second way — running it. Reading code without running it is like reading sheet music without hearing it.

A shopping app shows "4.20" — is that text or a number? If it is text, adding two prices gives "4.203.50"; if it is a number, it gives 8.7. Which one happens is decided by the type 类型, and the whole of unit 1.1 is learning to see types before you run anything.

A value 值 has a type, and the type decides what you can do with it: int (whole numbers), float (decimals), str (text), bool (True/False). The type(...) function names any value's type — ask it freely while learning.

age = 21              # int
price = 4.20          # float
name = "Ada"          # str
open_ = True          # bool
print(type(age))      # <class 'int'>

Variables 变量 are names bound to values; assignment age = 22 rebinds the name — it never edits the old value. The mental picture, worth keeping for the whole degree:

Reading an expression is "open the boxes, compute, close a new box": count * 3.50 with count = 6 opens the int box 6 and the float box 3.50, and Python's mixing rule promotes the int — the result is the float 21.0, never the int 21.

Conversion is explicit: int("21") is 21, but "4" + 2 is a TypeError — Python refuses to guess whether you meant "42" or 6. Output with print(...); input with input(...), which always returns a string 字符串, so numbers need converting:

born = int(input("Birth year? "))
print("About", 2026 - born, "years old")

Worked example 1 (the café till). count = int(input("Coffees? ")), then print("Total:", count * 3.50). At 6 coffees this prints Total: 21.0 — the .0 marks a float, because int * float promotes to float. Watching which type comes out of an expression is unit 1.1's whole habit. Trace it the state way: input returns the string "6"; int(...) opens a new box holding the int 6; the multiplication makes a fresh float box 21.0; print shows it.

Worked example 2 (the classic accident, repaired). A banner printer joins a count onto a message:

count = int(input("Tickets? "))
print("Sold " + count + " today")

Run it and Python raises TypeError: can only concatenate str (not "int") to str — the rule refuses to guess. The repair names the conversion explicitly: print("Sold " + str(count) + " today"), or the cleaner print("Sold", count, "today"), which inserts the spaces for you. The lesson to carry: + means addition for numbers, joining for strings, and never a silent mix.

Common mistakes. Forgetting that input() always gives a string, so input() * 2 repeats text rather than doubling a number. Writing 4.20 + 6 and expecting an int answer — any float in the expression makes a float out. Trusting 0.1 + 0.2 == 0.3 (it is False; binary rounding — unit 1.2 shows the fix).

Summary: four basic types decide every operation; names are tags, assignment rebinds; expressions mix types by promotion with floats; conversions are explicit and input() needs one.

A theme park turnstile must charge children, adults and seniors differently — one program, several paths. Selection 选择 is how a program chooses: if, elif, else run a block only when a condition holds.

The condition is any boolean expression; comparisons (==, !=, <, >=) and logic (and, or, not) build it. Two rules make every decision readable:

1. Branches are tested top to bottom; the first true branch wins and the rest are skipped.
2. Indentation is not decoration — it IS the block. One extra space changes which lines belong to the branch.

temp = 37.5
if temp >= 38:
    print("Fever")
elif temp >= 36 and temp < 38:
    print("Normal")
else:
    print("Low")

Worked example 1 (the cinema price). Under 12 or 65+ pay 5, students pay 7, everyone else 10:

age = int(input("Age? "))
student = input("Student? y/n ") == "y"
if age < 12 or age >= 65:
    price = 5
elif student:
    price = 7
else:
    price = 10
print("Pay", price)

Order matters: the first true branch wins, which is why the age check precedes the student check — a 70-year-old student should pay 5, and this ordering says so. Run it with 70 and y to see the flowchart's leftmost path taken.

Worked example 2 (grade bands, escalated). The same skeleton, now with a chain where each elif only needs one comparison, because the tests above it have already failed:

score = int(input("Score? "))
if score >= 80:
    band = "A"
elif score >= 70:
    band = "B"
elif score >= 60:
    band = "C"
else:
    band = "F"
print("Band:", band)

At 79 the first test fails, the second succeeds: Band: B. The escalation to notice: the elif score >= 70 does not also need and score < 80 — reaching that line already proves the score is below 80. Trusting the fall-through like this is what makes chains short; re-testing everything is the beginner's longer, wronger version.

Floating-point comparisons need care: 0.1 + 0.2 == 0.3 is False (binary rounding), so compare against a tolerance: abs(x - y) < 1e-9.

Common mistakes. Writing = where == is meant — the classic bug, and Python refuses it in conditions (SyntaxError) precisely because it is so common. Testing the special case after the general one, so the general one always wins. Using elif with a condition that can never be reached because an earlier branch already caught it.

Summary: conditions choose paths, first-true-wins, indentation is the block; order tests specific-to-general; floats compare by tolerance, never by ==.

Paying 5,000 invoices by hand is a career; with a loop it is a program. Iteration 迭代 repeats work: a for loop walks a known sequence; a while loop runs while a condition holds. The design question is always: do you know how many trips, or does a condition decide?

total = 0
for i in range(1, 101):      # 1..100 — range stops BEFORE its end
    total += i
print(total)                  # 5050

Use for when the trip count is known; use while when a condition decides. Every while needs three things visible in the code: the variable's start, the condition, and something inside that moves toward exiting — otherwise it is an infinite loop 死循环.

n = 5
factorial = 1
while n > 1:
    factorial *= n
    n -= 1                    # the step toward exiting
print(factorial)              # 120

The trace table 跟踪表 is the course's checking habit for loops: follow the variables round by round, on paper first. For the factorial loop: (n=5, f=1) → (4, 5) → (3, 20) → (2, 60) → (1, 120). Build the table by hand, then add a print(n, factorial) inside the loop and let Python grade you.

Worked example 2 (a search with break, escalated). Find the first multiple of 7 that is greater than 100 — the trip count is unknown, the condition decides:

n = 100
while True:
    if n % 7 == 0:
        break                 # found it — leave the loop now
    n += 1
print(n)                      # 105

The trace is short: 100, 101, …, 104 all fail the test; 105 passes and break exits. break leaves the loop early; continue skips to the next round. Both are readability tools — a loop that must scan until it finds something, then stop, reads best with break. The further escalation, doubling money: at 5% a year, how long does 1000 take to reach 2000?

balance = 1000.0
years = 0
while balance < 2000.0:       # the condition itself is the goal
    balance *= 1.05
    years += 1
print(years, round(balance, 2))   # 15 2078.93

Fifteen years, and the loop stops the moment the condition flips — the trace table would show balance crossing the line at 2078.93, not landing on 2000 exactly.

Common mistakes. Editing the list (unit 1.5) or the loop variable you are for-looping over. range(1, 100) when 100 was meant — the end is excluded. A while whose exit step is missing or commented out: the condition never flips, and the program freezes.

Summary: for for known trip counts, while for conditions; every loop shows start, condition and step; trace on paper, then print-grade; break and continue for clean early exits.

The same price calculation appears in five places; the tax rule changes; four of the five get updated. A function 函数 packages a job behind a name so it is written once and trusted everywhere: parameters in, a return value 返回值 out. def defines; calling runs.

def total_price(count, unit=3.50):
    """Return the total for count items at unit price."""
    return count * unit

print(total_price(6))            # 21.0
print(total_price(6, unit=4.0))  # 24.0

The definition gives a parameter 参数 (count, unit); the call supplies an argument 实参 (6). A default value (unit=3.50) makes an argument optional. return hands a value back and ends the call; printing is not returning — a function without return returns None, and the caller that tries arithmetic on it gets a TypeError.

Each call gets its own local scope 局部作用域: names inside are invisible outside, and disappear when the call ends. A call trace of nested calls reads: caller pauses → callee runs → return value lands in the caller.

Worked example 1 (the fee-inclusive order).

def add_fee(amount):
    """Return amount plus a fixed 3 pence fee."""
    return amount + 3

def order_total(unit_pence, quantity):
    """Return the order total: subtotal plus the fixed fee."""
    return add_fee(unit_pence * quantity)

print(order_total(250, 4))   # 1003

The contract in the docstring matters: whole pence in, whole pence out. Tracing order_total(250, 4): subtotal 1000, add_fee returns 1003 — and the frame diagram above shows the pause and the hand-back.

Worked example 2 (a real algorithm inside a small function, escalated). is_prime(n) returns True when nothing from 2 up to $\sqrt{n}$ divides n:

def is_prime(n):
    if n < 2:
        return False
    d = 2
    while d * d <= n:        # only test up to the square root
        if n % d == 0:
            return False     # found a divisor — answer known, stop
        d += 1
    return True

print([p for p in range(2, 30) if is_prime(p)])
# [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

Two function habits meet here: an early return ends the call the moment the answer is known (no flag variable needed), and the loop bound d * d <= n comes from the derivation — if $n = a \times b$ with $a \le b$, then $a \le \sqrt{n}$, so testing beyond $\sqrt{n}$ re-tests divisors already seen. Run it: the list is exactly the primes under 30.

Common mistakes. Printing inside a function instead of returning, so the result cannot be reused. Forgetting the return on one branch — that path quietly yields None. Reaching into a function's locals from outside; they are gone when the call ends.

Summary: def once, call anywhere; parameters in, one value out via return; early returns end the call; locals live and die with the call; docstrings state the contract.

A café's till needs the day's orders in order, the menu by name, a location that must not drift, and today's unique drinks. Four jobs, four containers — choosing among them is the design decision of unit 1.5.

A list 列表 is an ordered, changeable sequence: marks = [72, 85, 61], indexed from 0, sliced with marks[1:3]. A dictionary 字典 maps keys to values: price = {"flat white": 3.5, "tea": 2.8}, read and written by key. A tuple 元组 is a fixed sequence (53.1, -1.5) — good for coordinates that should not change. A set 集合 holds unique items with no order: {"a", "b"}.

marks = [72, 85, 61]
marks.append(90)               # [72, 85, 61, 90]
best = max(marks)              # 90
average = sum(marks) / len(marks)   # 77.0

Assignment never copies: a = [1]; b = a; b.append(2) makes a [1, 2] — the list is shared, the names are not. This aliasing 别名 is unit 1.1's boxes again, now with two tags on one box, and it explains the next rule too: mutating a list while looping over it skips elements — loop over a copy (for m in marks[:]) or build a new list.

Worked example 1 (tally by category).

orders = ["tea", "flat white", "tea", "tea", "mocha"]
tally = {}
for item in orders:
    tally[item] = tally.get(item, 0) + 1
print(tally)   # {'tea': 3, 'flat white': 1, 'mocha': 1}

The dictionary's .get(key, 0) supplies 0 for a first sighting — the idiom that makes counting a one-liner. Verify by hand: "tea" appears three times in orders, and the printed tally says so.

Worked example 2 (the day's best seller, escalated). Building on the tally, report the top item and its count:

best = max(tally, key=tally.get)
print(best, tally[best])       # tea 3

The key=tally.get argument asks max to compare items by their counts rather than alphabetically — the same key= trick ranks any collection by any measure. The escalation matters: real programs chain these tools (count, then rank) rather than re-scanning orders twice.

Choosing the collection is the design decision: order + duplicates → list; lookup by name → dictionary; fixed grouping → tuple; uniqueness → set.

Common mistakes. tally[item] += 1 on a key that is not yet in the dictionary (KeyError) — .get is the fix. Expecting a set or dictionary to remember order. Assuming b = a copies a list; it aliases one.

Summary: four containers for four jobs; assignment shares, never copies; .get makes counting safe; key= makes ranking a one-liner.

The café closes, the power cuts — and the morning's tally survives only if the program wrote it to disk. A file 文件 outlives the program. with open(path) as f: opens, closes on exit, and reads text; open(path, "w") writes (and empties first — the classic accident).

with open("log.txt", "w") as f:
    f.write("opened\n")

with open("log.txt") as f:
    for line in f:
        print(line.rstrip())     # rstrip removes the trailing newline

The with block is the safety rail: whatever happens inside it, the file closes. Reading gives lines with their newline, so .rstrip() belongs in every loop.

Errors are values Python raises as exceptions 异常: FileNotFoundError, ValueError, KeyError, TypeError. try/except catches a named type — never a bare except: that hides real bugs.

Worked example 1 (the absent settings file). Reading a text size from a settings file that may be missing, or may not hold a whole number:

try:
    with open("settings.txt") as f:
        size = int(f.read().strip())
except FileNotFoundError:
    size = 12                    # no file yet: a sensible default
except ValueError:
    size = 12                    # the file holds "big", not a number
print("Text size", size)

Run it three times — with 16 in the file, with no file, and with big in the file — and it prints a usable size each time, from three different routes. That is the shape of robust code: one sensible outcome, several roads to it. Handle the error you expect, where you can respond to it; let the rest crash loudly.

Worked example 2 (a robust input loop, escalated). Ask until the user types a whole number:

def read_count():
    while True:
        try:
            return int(input("How many? "))
        except ValueError:
            print("Whole numbers only.")

print(read_count())

Feed it x then 4: the first trip raises, is caught, prints the message, and the while True asks again; the second trip's int(...) succeeds and return exits the loop and the function in one move. Three units combine here — the loop (1.3), the function contract (1.4) and the exception (1.6) — which is the point of the escalation.

Common mistakes. open(path, "w") on the wrong path: the file is created or emptied before you notice. Catching Exception broadly and continuing with broken state. Forgetting .rstrip(), so output shows double-spaced lines.

Summary: with open(...) for both reading and writing; "w" empties first; exceptions are values — catch the named type you expect, fall back sensibly, let the rest crash.

Two teams write the same app; one ships in a week, one drowns for a month. The difference is not typing speed — it is that the fast team's code is built from small pieces they can each prove correct while the slow team debugs a whole app at once. Unit 1.7 is that discipline.

Decomposition 分解 splits a problem into functions of one job each — each small enough to test alone. Testing 测试 compares a function's actual output with the expected, at the boundaries: zero, one, empty, negative, huge.

def add_fee(amount):
    return amount + 3

def order_total(unit_pence, quantity):
    return add_fee(unit_pence * quantity)

assert order_total(250, 4) == 1003
assert order_total(250, 0) == 3      # the zero-quantity boundary
assert order_total(0, 4) == 3

An assert that never fires is a passing test. Write the tests before you write the body when you can — they are the contract made executable, and the loop above (red → green → refactor) is that habit running continuously.

Worked example 1 (grow the tests with the function). Extend order_total with a 10% discount over £10 — the test is written first, at the new boundary:

def order_total(unit_pence, quantity):
    subtotal = add_fee(unit_pence * quantity)
    if subtotal >= 1000:             # pence: over £10
        subtotal = int(subtotal * 0.9)
    return subtotal

assert order_total(200, 4) == 803    # under £10 — unchanged
assert order_total(250, 4) == 902    # 1003 pence → 10% off → 902.7 → 902
assert order_total(250, 5) == 1127   # 1253 pence → 1127.7 → 1127

Check the middle test by hand: subtotal 1003, ten percent off gives 902.7, and int(...) rounds down to 902. Notice the first assert too: at 200 pence and four cups the total stays 803 — the boundary tests prove both that the discount fires and that it does not fire early.

Worked example 2 (bisect a bug with prints, escalated). The tally from 1.5 prints {'tea': 3} but the best-seller line prints the wrong drink. The method: (1) reproduce with the smallest failing input; (2) print the values at the boundary between the trusted part and the suspected part — here, the tally before the max; (3) fix one thing; (4) re-run the tests. Moving the print halfway through the code each run halves the search: four prints find a bug in a sixteen-line program, however well it hides.

Debugging 调试 is finding the difference between what you believe the code does and what it does — the print at the trust/suspect boundary is the instrument.

Common mistakes. Testing only the happy path: the boundaries (zero, one, empty, negative) are where programs break. Writing a week of code before the first test, then debugging a mountain. Fixing two things at once, so neither result means anything.

Summary: decompose until every piece tests alone; assert the boundaries the minute a function exists; write the test first when you can; debug by bisection with prints at trust boundaries — the habit the whole degree builds on.