What this session is for

• Get fluent with f-strings for decimals and alignment
• Handle what input() gives you correctly
• Write functions that take arguments and return values
• Settle the difference between print() and return for good

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Part A — required

3-1 Temperature conversion

Implement c_to_f(c) using F = C * 9/5 + 32, returning a float rounded to one decimal place.

c_to_f(0)     →  32.0
c_to_f(37)    →  98.6
c_to_f(-40)   →  -40.0

3-2 Masking a student number

Implement mask_id(sid): keep the first four and last four characters and star out everything in between.

mask_id("526030910001")  →  "5260****0001"
mask_id("12345678")      →  "12345678"

Slicing and concatenation are enough — no loop.

3-3 An aligned results table

Implement format_row(name, score), returning one formatted line: the name left-aligned in 10 characters, the score right-aligned in 6 with one decimal place.

format_row("Alex", 87.5)    →  "Alex        87.5"
format_row("Blake", 92.0)   →  "Blake       92.0"
format_row("Chen", 65.25)   →  "Chen        65.2"

Use an f-string with :<10 and :>6.1f.

3-4 Find and fix a print/return mistake

The code below is meant to average two numbers and double the result. It raises TypeError. Say why, fix it, and explain in a sentence or two on the answer sheet.

def average(a, b):
    print((a + b) / 2)

result = average(10, 20) * 2
print(result)

3-5 Reading a number safely

Implement read_int(prompt): show the prompt, read a line, convert it to an integer and return it.

You do not have to handle non-numeric input yet — that needs the exception handling from L11 — but try it once: type abc and see which kind of error comes back. The answer sheet asks what input() returns, which is the same root cause.

3-6 Returning several values

Implement stats(a, b, c) returning the minimum, maximum and mean of the three numbers.

stats(3, 9, 5)  →  (3, 9, 5.666666666666667)

The caller should be able to write:

low, high, mean = stats(3, 9, 5)

Do not round the mean — return it as it comes.

3-7 Circumcircle and incircle (marked by the platform)

In Lab 2 you computed a triangle’s area with Heron’s formula as straight-line code. This time write it as a function, then build two more on top of it.

def area(a, b, c): ...
def circles(a, b, c): ...    # returns (circumradius R, inradius r)

The formulas:

S = the area, from Heron's formula
s = (a + b + c) / 2          circumradius R = a·b·c / (4S)
                             inradius      r = S / s

area(3, 4, 5)      →  6.0
circles(3, 4, 5)   →  (2.5, 1.0)
circles(5, 5, 6)   →  (3.125, 1.5)

circles must call area rather than repeating Heron’s formula. That is exactly what this problem is about, and the marking checks for it.

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Part B — going deeper: turning a raw line into an aligned table

Real data is rarely ready to use. It usually arrives as a line of text, and you have to break it apart, convert it to the right types, and lay it out so a person can read it. Four steps, each building on the last, none of them needing a loop.

The raw data, one record per line:

Alex,87.5
Blake,65.25
Christopher,92

B-1 Break one apart

Implement parse(record), splitting a record into a name and a score, the score as a float:

parse("Alex,87.5")        →  ("Alex", 87.5)
parse("Christopher,92")   →  ("Christopher", 92.0)

B-2 Lay one out

Feed the result of parse into your format_row from Part A: name left-aligned in 10 columns, score right-aligned in 6 with one decimal place.

Do it for all three records (write three calls — no loop) and paste the output onto the answer sheet.

B-3 Wide characters do not line up

Now run the same code over these three records instead:

张三,87.5
李四,65.25
欧阳修远,92

The columns go crooked. Answer:

1. What is len("张三")? What is len("Alex")?
2. Do they take up the same width on screen?
3. So what exactly is f"{name:<10}" counting when it pads?

B-4 A workaround

Without reaching for anything you have not been taught, find a way to get those three rows roughly aligned. Write it, and say where it breaks down.

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Part C — fundamentals: running Python from the terminal, and redirecting output

Week 1 was moving around (pwd ls cd); week 2 was creating and changing things (mkdir touch cp mv rm). This week: the two ways to run Python from a terminal, and sending output somewhere other than the screen.

Interactive versus script

Typing python on its own opens the interactive interpreter (the >>> prompt), where you try one line at a time — good for checking an idea:

python
>>> 2 ** 100
>>> exit()          # or press Ctrl+D

Typing python file.py runs a script from top to bottom. That is how you run programs.

Redirection: sending output to a file

python hello.py > out.txt      # write output to the file, replacing it
python hello.py >> out.txt     # append to the end of the file
python guess.py < input.txt    # feed the file to the program as if typed

C-1 Walk through it

Copy the commands and their output out of the terminal onto the answer sheet (no screenshot needed):

1. Open the interactive interpreter, evaluate 2 ** 100, and leave
2. Redirect your B-2 output into table.txt, then look at it with cat table.txt
3. Create input.txt with two numbers on two lines, and feed it to your 3-5 program with <

C-2 Think it through

1. What is the difference between > and >>? How would you demonstrate it?
2. After using >, is the output still on screen? Why?

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Before you submit

• Every function returns rather than prints (except 3-5’s prompt)
• 3-1 returns a float, not a string
• 3-7’s circles calls area rather than repeating Heron’s formula
• The Part A answer sheet is submitted (3-2’s edge case, 3-4’s explanation and the three-inputs exercise are all on it)
• The Part B answer sheet is submitted
• The Part C answer sheet is submitted