This page is for one line snippets of knowledge that I have learned, typically throughout the current day.

There is a parenthesis missing in the original Lisp paper, in the evcon function.

In GCC C/C++, the only case when sizeof is evaluated at run time instead of the usual compile time, is when its argument is a Variable-Length Array.

You can compute the diameter of a tree by finding the deepest leaf, then finding the deepest leaf from there, although I do not know of any proof.

In python, it is sometimes faster to do deepcopy = lambda x: pickle.loads(pickle.dumps(x)) than the original deepcopy.

In ISO C pointer arithmetic, &*x cancels out never truly referencing *x, making this safe even when x = NULL.

In ISO C pointer arithmetic, addition is commutative, making "string"[2] ≡ *("string" + 2) ≡ *(2 + "string") ≡ 2["string"].

A permutation is sorted in $O(n)$ by swapping values in-place, faster than the general comparison-based sorting optimum of $O(n \log n)$

GCC casts to unsigned when doing signed-unsigned comparisons, making -1 > 2U equivalent to (unsigned) -1 > 2U, which evaluates to 1 (true).