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The shuffle everyone writes — sorting with a random comparator — is biased, and not slightly. A million measured runs show eight of the twenty-four orderings turning up twice as often as the other sixteen.
Lena Hoffmann · 13 min read
Twenty percent off and then a further ten percent off is not thirty percent off — it is twenty-eight, because the second discount applies to the already-reduced price. The general rule, the cases where order matters, and how sales tax changes the answer.
Lena Hoffmann · 13 min read
Moving from 4% to 5% is a rise of one percentage point and a rise of twenty-five percent. Both statements are true, which is exactly why the ambiguity is so easy to exploit — and why basis points exist.
Lena Hoffmann · 14 min read
Divisible by four, except centuries, except centuries divisible by four hundred. The three-line rule exists because the tropical year is not 365.25 days, and the arithmetic behind it explains ten deleted days in 1582 and a calendar that repeats exactly every 400 years.
Lena Hoffmann · 15 min read
"One month later" has no single correct answer, and every date library has had to pick one. Month addition is neither associative nor invertible, a day is not always 24 hours, and age is not days divided by 365.25.
Lena Hoffmann · 15 min read
An annuity's price is a present value over a probability-weighted term. On a stated mortality at 4 percent, $100,000 at 65 buys $7,492 a year — 4.00 points of interest, 1.78 of returned capital and 1.71 of mortality credit.
Camille Laurent · 14 min read
Reinvesting a 3 percent yield for 30 years turns 100 shares into 242.7 and multiplies the final position by exactly that factor: $32,434 becomes $78,726. Tax at 30 percent on each dividend costs $18,223 of it — nearly two and a half times the tax actually paid.
Camille Laurent · 12 min read
(1 + r/n)^n rises with n but converges on e^r. At 6 percent, monthly and continuous compounding differ by $1.59 on $10,000 over a year. At 24 percent the same gap is $30.07, and over thirty years it is 7.4 percent of the balance.
Camille Laurent · 11 min read
A link becomes text with its destination deleted, a nested list loses its hierarchy, a table becomes a row of words. Then the technical half: markdown has no single spec, and a regex stripper mangles a filename, a multiplication sign and the inside of a code block — all shown against a real parser.
Daniel Okonkwo · 12 min read
Case, whitespace, punctuation and diacritics: four policies, six real sentences, and the answer changes with each one. Then the harder half — reversing a string is itself undefined, and code-unit reversal breaks emoji and detaches accents, demonstrated in Node.
Daniel Okonkwo · 12 min read
Greedy against lazy on the same string, the dot that skips newlines, $& and $$ in the replacement, a reused /g regex that silently skips a row, and why /i knows nothing about Turkish i. Every failure run in Node, with a count-then-replace routine that catches them.
Daniel Okonkwo · 14 min read
Code-unit order, natural order and locale collation run on the same list in Node, with the outputs printed. Why Zebra beats apple, why item10 beats item9, and why ä sits next to a in German but after z in Swedish.
Daniel Okonkwo · 12 min read