CODEWITHSUNDEEP
pythonc++randomlcg
Axeon Thra
Axeon Thra

Reputation: 364

Mismatch in integer calculation results between C++ and Python (random number generator)

I needed a pseudo random number generation scheme (independent of standard libraries) for one of my projects and I tried a simple LCG based RNG. It seems to work fine except that it produces different values in C++ and Python. I am giving the relevant code for both below. I am not able to find the error. Any help will be greatly appreciated!

(c++)

// file: lgc.cc
// compile and run with: g++ lgc.cc -o lgc && ./lgc

#include <cstdio>
#include <cstdint>
#include <vector>

using namespace std;

uint64_t LGC(uint64_t x) {
  uint64_t A = 1103515245;
  uint64_t C = 12345;
  uint64_t M = (1 << 31);
  return (A * x + C) % M;
}

int main(int argc, char* argv[]) {

  for (auto x : {485288831, 485288832, 10, 16, 255, 756}) {
    uint64_t y = LGC(x);
    printf("%u %u\n", (uint32_t)x, (uint32_t) y);
  }

  return 0;
}

(python)

# file: lgc.py
# run with: python3 lgc.py

def LGC(x):
    A = 1103515245;
    C = 12345;
    M = int(2 ** 31);
    return (A * x + C) % M;

for x in [485288831, 485288832, 10, 16, 255, 756]:
    y = LGC(x)
    print(x, y)

(Results: c++)

485288831 3822790476
485288832 631338425
10 2445230203
16 476387081
255 2223525580
756 1033882141

(Results: python)

485288831 1675306828
485288832 631338425
10 297746555
16 476387081
255 76041932
756 1033882141

Upvotes: -1

Views: 145

Answers (1)

drescherjm
drescherjm

Reputation: 10837

The problem is with the shift in: uint64_t M = (1 << 31); the (1<<31) is a negative number -2147483648 for 32 bit integers because this is signed integer math. To fix you can use uint64_t M = (1U << 31); to make the shift use unsigned 32 bit integers. You could also use uint64_t M = (1UL << 31); to have the calculation use 64 bit unsigned integers

The following link shows printing the output of: (1 << 31), (1U << 31) and (1UL << 31) on a system with 32 bit int: https://ideone.com/vU0eyX

Upvotes: 1

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