How do I compute C's % using Python's %?
The difference between the two is in the way they handle the case of negative arguments.
In both languages, the % is defined in such a way that this relationship (// being integer division) holds:
a // b * b + a % b == a
but the rounding of a // b is different in C and in Python, leading to a different definition of a % b.
For example, in C (where integer division is just / with int operands) we have:
int a = 31;
int b = -3;
a / b; // -10
a % b; // 1
while in Python:
a = 31
b = -3
a // b # -11
a % b # -2
I am aware of this question, which addresses the opposite (i.e. how to compute Python's % from C's %) and contains additional discussions.
I am also aware of Python 3.7 math module introducing remainder() but its result is a float, not an int and hence it will not enjoy arbitrary precision.
Some ways would be:
def mod_c0(a, b):
if b < 0:
b = -b
return -1 * (-a % b) if a < 0 else a % b
def mod_c1(a, b):
return (-1 if a < 0 else 1) * ((a if a > 0 else -a) % (b if b > 0 else -b))
def mod_c2(a, b):
return (-1 if a < 0 else 1) * (abs(a) % abs(b))
def mod_c3(a, b):
r = a % b
return (r - b) if (a < 0) != (b < 0) and r != 0 else r
def mod_c4(a, b):
r = a % b
return (r - b) if (a * b < 0) and r != 0 else r
def mod_c5(a, b):
return a % (-b if a ^ b < 0 else b)
def mod_c6(a, b):
a_xor_b = a ^ b
n = a_xor_b.bit_length()
x = a_xor_b >> n
return a % (b * (x | 1))
def mod_c7(a, b):
a_xor_b = a ^ b
n = a_xor_b.bit_length()
x = a_xor_b >> n
return a % ((-b & x) | (b & ~x))
def mod_c8(a, b):
q, r = divmod(a, b)
if (a >= 0) != (b >= 0) and r:
q += 1
return a - q * b
def mod_c9(a, b):
if a >= 0:
if b >= 0:
return a % b
else:
return a % -b
else:
if b >= 0:
return -(-a % b)
else:
return a % b
which all work as expected, e.g.:
print(mod_c0(31, -3))
# 1
Essentially, mod_c0() implements an optimized version of mod_c1() and mod_c2(), which are identical except that in mod_c1() the call to (relatively expensive) call to abs() is replaced by a ternary conditional operator with the same semantic.
Instead, mod_c3() and mod_c4() try to directly fix the a % b value for the cases where it is needed. The difference between the two is in how they detect opposite signs of the arguments: (a < 0) != (b != 0) versus a * b < 0.
The mod_c5() approach is inspired by @ArborealAnole's answer, and essentially uses the bit-wise xor to handle the cases correctly, while mod_c6() and mod_c7() are the same as @ArborealAnole's answer but using adaptive right shift with int.bit_length().
The mod_c8() approach uses a corrected definition of integer division to fix up the modulus value.
The mod_c9() method is inspired by @NeverGoodEnough's answer, and essentially goes full conditional.
Covering all sign cases:
vals = (3, -3, 31, -31)
s = '{:<{n}}' * 4
n = 14
print(s.format('a', 'b', 'mod(a, b)', 'mod_c(a, b)', n=n))
print(s.format(*(('-' * (n - 1),) * 4), n=n))
for a, b in itertools.product(vals, repeat=2):
print(s.format(a, b, mod(a, b), mod_c0(a, b), n=n))
a b mod(a, b) mod_c(a, b)
------------- ------------- ------------- -------------
3 3 0 0
3 -3 0 0
3 31 3 3
3 -31 -28 3
-3 3 0 0
-3 -3 0 0
-3 31 28 -3
-3 -31 -3 -3
31 3 1 1
31 -3 -2 1
31 31 0 0
31 -31 0 0
-31 3 2 -1
-31 -3 -1 -1
-31 31 0 0
-31 -31 0 0
A bit more tests and benchmarks:
import itertools
n = 100
l = [x for x in range(-n, n + 1)]
ll = [(a, b) for a, b in itertools.product(l, repeat=2) if b]
funcs = mod_c0, mod_c1, mod_c2, mod_c3, mod_c4, mod_c5, mod_c6, mod_c7, mod_c8, mod_c9
for func in funcs:
correct = all(func(a, b) == funcs[0](a, b) for a, b in ll)
print(f"{func.__name__} correct:{correct} ", end="")
%timeit -n 8 -r 8 [func(a, b) for a, b in ll]
# mod_c0 correct:True 8 loops, best of 8: 9.67 ms per loop
# mod_c1 correct:True 8 loops, best of 8: 11.1 ms per loop
# mod_c2 correct:True 8 loops, best of 8: 12.3 ms per loop
# mod_c3 correct:True 8 loops, best of 8: 10.3 ms per loop
# mod_c4 correct:True 8 loops, best of 8: 10 ms per loop
# mod_c5 correct:True 8 loops, best of 8: 10.1 ms per loop
# mod_c6 correct:True 8 loops, best of 8: 17.1 ms per loop
# mod_c7 correct:True 8 loops, best of 8: 20.3 ms per loop
# mod_c8 correct:True 8 loops, best of 8: 15.8 ms per loop
# mod_c9 correct:True 8 loops, best of 8: 9.29 ms per loop
Perhaps there are better (shorter?, faster?) ways, given that the implementation of Python's % using C's % seems much simpler:
((a % b) + b) % b
To get some feeling on how the C-style % computation (mod_c*() functions from above) stands against the usual % or the operations required to get Python-style % from C:
def mod_py(a, b):
return a % b
def mod_c2py(a, b):
return ((a % b) + b) % b
%timeit [mod_py(a, b) for a, b in ll]
# 100 loops, best of 3: 5.85 ms per loop
%timeit [mod_c2py(a, b) for a, b in ll]
# 100 loops, best of 3: 7.84 ms per loop
Note of course that mod_c2py() is only useful to get a feeling of what performances we could expect from a mod_c() function.
(EDITED to fix some of the proposed methods and include some timings)
(EDITED-2 to add the mod_c5() solution)
(EDITED-3 to add the mod_c6() to mod_c9() solutions)