Realizing x != x for NaN triggers the cat-stare during code review

Description

The meme is split into a text header and a two-panel photo. Header text reads: "Teacher: When is x!=x ?\nMe: When x equals to 1 or 2.\nTeacher: Then why you did you write NaN on the paper?\nMe:". In the left photo a person (face blurred) sits at a dining table holding printed papers while a grey Scottish-fold cat sits beside them, staring at the sheet. A closed space-grey MacBook rests on a red placemat in the foreground. The right photo zooms in on the cat’s wide-eyed, mildly panicked expression as it confronts the paper. The joke relies on IEEE-754 floating-point semantics where the special value NaN is the only value for which the comparison x != x evaluates to true, often surprising students and developers. It humorously captures the classic floating-point edge case and the confusion it causes during teaching or code reviews

Common Tags

FloatingPointArithmetic CSFundamentals CodingHumor DeveloperHumor FloatingPointPrecision BugsInSoftware

Comments

Anonymous ★ Top Pick When I hit `if (x != x)` in the hottest loop: sure, it’s the IEEE-754-certified NaN check, but now both the branch predictor and every reviewer look exactly like that cat

Anonymous ★ Top Pick

When I hit `if (x != x)` in the hottest loop: sure, it’s the IEEE-754-certified NaN check, but now both the branch predictor and every reviewer look exactly like that cat
Anonymous

After 20 years in the industry, I've learned that explaining IEEE 754 floating-point edge cases to non-programmers is harder than explaining to the board why our microservices need another orchestration layer - at least the board pretends to understand
Anonymous

The beauty of NaN is that it's the only value in programming that's so special, it refuses to be equal to itself - a perfect metaphor for that one microservice in your architecture that never agrees with its own health checks. IEEE 754 basically said 'undefined behavior deserves undefined equality,' and now we all get to explain to junior devs why `NaN === NaN` returns false while they question their entire understanding of mathematics and reality itself
Anonymous

IEEE 754's eternal gift: NaN == NaN? False - because even bits need plausible deniability in prod
Anonymous

x != x - the day IEEE‑754 teaches you equality isn’t reflexive and your dedupe job quietly hoards NaNs
Anonymous

Math: x! = x -> {1,2}. JavaScript: x!=x -> NaN. Classic postmortem: notation leaked across bounded contexts
@ImJmik 5y

Never Oh, stop......
@tarasssssssssssssss 5y

JS jokes or what idk
Deleted Account 5y

i dont get the maths bit
1. @bit69tream 5y
  
  x! is factorial of x
  
  Deleted Account 5y
  
  oh
  
  @metal_Dash 5y
  
  Гений
@TTpocT 5y

А, я думал не равен
@null_po 5y

If you say a number loudly, then its value will increase: 5 = 5 5! = 120
1. @freeapp2014 5y
  
  But if you say it even more loudly, it will go back down 5!! = 15
  
  @x_Arthur_x 5y
  
  How is that so?
  
  @freeapp2014 5y
  
  https://en.m.wikipedia.org/wiki/Double_factorial