Question

Just switch from Python to C++ and I begin to re-write my Python tools in C++ for better understanding, but can’t solve this one…

This function will generate range of random numbers, for example “randomRange(12)” may return range of 12 numbers like “823547896545”

Python:

  def randomRange(n):
        range_start = 10**(n-1)
        range_end = (10**n)-1
        return randint(range_start, range_end)

  number = randomRange(12)

C++:

  int n;
  int randomRange(n){
        int range_start = ?
        int range_end = ?
        int result = ?(range_start, range_end);
        return (result);
  };

  int number = randomRange(12);

I can’t find equivalent for question marks ” ? “

Answer 1

You’ll have trouble getting good randomness with high values of n, but:

#include <math.h>         // for pow()
#include <stdlib.h>       // for drand48()

long randomRange(int n)
{
    // our method needs start and size of the range rather 
    // than start and end.
    long range_start = pow(10,n-1);
    long range_size = pow(10,n)-range_start;
    // we expect the rand48 functions to offer more randomness
    // than the more-well-known rand() function. drand48()
    // gives you a double-precision float in 0.0-1.0, so we 
    // scale up by range_size and and to the start of the range.
    return range_start + long(drand48() * range_size);
};

Here’s another approach. On 32-bit platforms, you can only do 9 digits in an int, so we’ll make the function return a double, and generate a string of ASCII digits then convert:

#include <math.h>         // for pow()
#include <stdlib.h>       // for atof()

// arbitrary limit
const int MAX_DIGITS = 24;

double randomRange(int n)
{
    char bigNumString[ MAX_DIGITS+1 ];
    if (n > MAX_DIGITS)
    {
        return 0;
    }
    // first digit is 1-9
    bigNumString[0] = "123456789"[rand()%9];
    for (int i = 1; i < n; i++)
    {
        // subsequent digits can be zero
        bigNumString[i] = "0123456789"[rand()%10];
    }
    // terminate the string
    bigNumString[i] = 0;
    // convert it to float
    return atof(bigNumString);
};