Just subtract 1 and add 1 afterwards. In most programming languages, you need to watch out when finding a “previous” value, because for negative numbers, modulo does not work as you want in this case: it returns a negative number. Here’s the C/C++ version: int next(int i, int m, int n) { return (i + … Read more
When you have the expression: a % b = c It really means there exists an integer n that makes c as small as possible, but non-negative. a – n*b = c By hand, you can just subtract 2 (or add 2 if your number is negative) over and over until the end result is … Read more
Actually, it’s not true that 3.5 % 0.1 is 0.1. You can test this very easily: >>> print(3.5 % 0.1) 0.1 >>> print(3.5 % 0.1 == 0.1) False In actuality, on most systems, 3.5 % 0.1 is 0.099999999999999811. But, on some versions of Python, str(0.099999999999999811) is 0.1: >>> 3.5 % 0.1 0.099999999999999811 >>> repr(3.5 % … Read more
You can use the division version of Russian Peasant Multiplication. To find the remainder, execute (in pseudo-code): X = B; while (X <= A/2) { X <<= 1; } while (A >= B) { if (A >= X) A -= X; X >>= 1; } The modulus is left in A. You’ll need to implement … Read more
You probably had a typo when you first ran it. evaluating 0.5 % 0.3 returns ‘0.2’ (A double) as expected. Mindprod has a good overview of how modulus works in Java.
C has div and ldiv. Whether these generate separate instructions for the quotient and remainder will depend on your particular standard library implementation and compiler and optimization settings. Starting with C99, you also have lldiv for larger numbers.
If your modulus / divisor is a known constant, and you care about performance, see this and this. A multiplicative inverse is even possible for loop-invariant values that aren’t known until runtime, e.g. see https://libdivide.com/ (But without JIT code-gen, that’s less efficient than hard-coding just the steps necessary for one constant.) Never use div for … Read more
The C++ Standard(2003) says in ยง5.6/4, […] If the second operand of / or % is zero the behavior is undefined; […] That is, following expressions invoke undefined-behavior(UB): X / 0; //UB X % 0; //UB Note also that -5 % 2 is NOT equal to -(5 % 2) (as Petar seems to suggest in … Read more
Imagine that you have an elapsed time in seconds and you want to convert this to hours, minutes, and seconds: h = s / 3600; m = (s / 60) % 60; s = s % 60;
My general advice is as follows. Use whichever version you think is easier on the eye, and then profile your entire system. Only optimize those parts of the code that the profiler flags up as bottlenecks. I’ll bet my bottom dollar that the modulo operator isn’t going to be among them. As far as the … Read more