Computer Science
@CompSciFact

Daily tweets about computer science and related stuff @JohnDCook.

JohnDCook.com
Joined November 2010
679 Photos and videos
'If rounding errors vanished, 95% of numerical analysis would remain.' -- Nick Trefethen
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On false proofs of P=NP, or how not to use induction farkasdilemma.wordpress.com/…

On false proofs of P=NP, or how not to use induction

The power set of an $latex n$-set has $latex 2^n$ elements. This isn’t that complicated. However, mimicking some proofs purporting to show P=NP, I can (incorrectly!) show that the power set g…

farkasdilemma.wordpress.com
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De Bruijn sequences and security johndcook.com/blog/2019/10/2…

Cracking pass codes with De Bruijn sequences

How would you attack a pass code entered into a keypad without an "enter" key? Would requiring an enter key improve security more than requiring another digit?

johndcook.com
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Computer Science retweeted
NIK
@ns123abc
Sep 24
BREAKING: Google launching TPUs in space NEXT WEEK on SpaceX falcon 9 to test AI data centers orbit ITS HAPPENING
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The number of digits in an integer n in base b is 1 + floor( log_b(n) ). Simple but useful.
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The Bw-Tree: A B-tree for New Hardware microsoft.com/en-us/research…

The Bw-Tree: A B-tree for New Hardware Platforms - Microsoft Research

The emergence of new hardware and platforms has led to reconsideration of how data management systems are designed. However, certain basic functions such as key indexed access to records remain...

microsoft.com
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'Any sufficiently advanced technology is indistinguishable from a rigged demo.' -- Andy Finkel
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Leibniz (1646-1716) studied symbolic logic and built a sort of computer. johndcook.com/blog/2013/04/0…

Baroque computers

From an interview with Neal Stephenson, giving some background for his Baroque Cycle: Leibniz [1646-1716] actually thought about symbolic logic and why it was powerful and how it could be put to use....

johndcook.com
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Goodhart's law: When a metric becomes a target, it ceases to be a good metric.
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The original Lisp paper. McCarthy 1960 www-formal.stanford.edu/jmc/…
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How to compute the square root of a complex number, avoiding floating point problems. johndcook.com/blog/2020/06/0…

How to compute the square root of a complex number

Finding a square root of a complex number, avoiding overflow in floating point arithmetic.

johndcook.com
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'Projects promoting programming in 'natural language' are intrinsically doomed to fail.' -- Edsger Dijkstra
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Sleep sort beust.com/weblog/2011/06/15/…
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Popcount: Counting 1's in a bitstream johndcook.com/blog/2020/02/2…

Popcount: counting the number of 1's in a bit stream

The function popcount(x) counts the number of 1's in the binary representation of x. Comes up occasionally in applications. How to compute in C.

johndcook.com
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Computer Science retweeted
Arika
@Arikalabs
Sep 17
"Java is pass-by-value." "But when I pass an object, it modifies the original object!" "Yes, because the value being passed is the pointer to the object." "So it's pass-by-reference?" "No, the pointer is passed by value." [C programmer laughing in the distance]
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'If it's your decision, it's design; if not, it's a requirement.' -- Alistair Cockburn
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“It shouldn’t be Reverse Polish Notation, it should be German Notation because the verb goes at the end.” — Colin Beveridge
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Denotational Semantics: A methodology for language development scss.tcd.ie/Andrew.Butterfie… [pdf] by David A. Schmidt
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Programming on Parallel Machines: GPU, Multicore, Clusters and More heather.cs.ucdavis.edu/parpr… // free ebook
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