64 Bit Long Integer Parameter

Question

I am doing computer science course which it is required for my Information Technology major. So I am trying to understanding this steps by step. I don't know how I did it wrong or isn't what the expected output.

Any suggestion or help? Thank you.

My code:

/**
 * Create a function called count that takes a 64 bit long integer parameter (n)
 * and another integer pointer (lr) and counts the number of 1 bits in n and
 * returns the count, make it also keep track of the largest run of
 * consecutive 1 bits and put that value in the integer pointed to by lr.
 * Hint: (n & (1UL<<i)) is non-zero when bit i in number n is set (i.e. a 1 bit)
 */


/* 1 point */
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

int count (uint64_t n)
{       
  int ret = 0;
  long x = n;
  if (x < 0)
    x = -x;
  while (x != 0)
    {
      ret += x % 2;
      x /= 2;
    }
  return ret;     //done summing when n is zero.
}

/**
 * Complete main below and use the above function to get the count of 1 bits
 * in the number passed to the program as the first command line parameter.
 * If no command line parameter is provided, print the usage:
 *   "Usage: p3 <int>\n"
 * Hints:
 * - Use atoll to get a long long (64 bit) integer from the string.
 * - Remember to use & when passing the integer that will store the longest
 *   run when calling the count function.
 *
 * Example input/output:
 * ./p3 -1
 * count = 64, largest run = 64
 * ./p3 345897345532
 * count = 17, largest run = 7
 */
int main (int argc, char *argv[])
{
  if (argc < 2)
    {
      printf ("Usage: p3 <int>\n");
    }
  int n = atoll(argv[1])
  printf("count = %d, largest run = %d\n", n, count(n));

}

When I run the compile to see the output but it doesn't seems correct to match the example output.

Where is n coming from? printf("count = %d, largest run = %d\n", n, count(n));. Not from argv. — Fiddling Bits
– Fiddling Bits, Commented Oct 31, 2018 at 19:15

Yunbin Liu · Accepted Answer · 2018-10-31 20:29:06Z

2

use atoll get int64_t from argv[1]
use (n&(1UL<<i)) define each bit is 1 or 0
use var to record the current consecutive 1 bits count

explain:

temp means current consecutive 1 bits count

if n&(1UL<<i) == 1, current bit is 1, so current consecutive 1 bits count add 1, so ++temp;
if n&(1UL<<i) == 0, current bit is 0, so current consecutive 1 bits count is 0, so temp = 0;

The following code could work:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

int count(int64_t n, int* lr) {
    *lr = 0;

    int temp = 0;
    int ret  = 0;

    for (int i = 0; i != 64; ++i) {
        if (n&(1UL<<i)) {
            ++ret;
            ++temp;
            if (temp > *lr)
                *lr = temp;
        } else {
            temp = 0;
        }
    }
    return ret;
}

int main (int argc, char *argv[]) {
    if (argc != 2) {
        printf ("Usage: p3 <int>\n");
        return -1;
    }

    int64_t n = atoll(argv[1]);
    int k;

    int sum = count(n, &k);

    printf("count = %d, largest run = %d\n", sum, k);

    return 0;
}

edited Oct 31, 2018 at 20:29

answered Oct 31, 2018 at 20:01

Yunbin Liu

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3 Comments

Mike ODell Over a year ago

Can you explain more about Pointer Integer?

Yunbin Liu Over a year ago

@MikeODell I add a simple explain.

Andrew Henle Over a year ago

Never use atoll(), or any of the other ato*() functions. Per 7.22.1 Numeric conversion functions of the C standard: "The functions atof, atoi, atol, and atoll need not affect the value of the integer expression errno on an error. If the value of the result cannot be represented, the behavior is undefined." You can't tell if atoll() works or not as there's no error return. Not only that, if "the value ... cannot be represented", then the behavior is undefined. Use strtoll() instead.

Fiddling Bits · Accepted Answer · 2018-10-31 19:36:24Z

1

What you've posted results in a compilation error:

main.c:52:44: error: ‘n’ undeclared (first use in this function)
   printf("count = %d, largest run = %d\n", n, count(n));
                                            ^

As the comments in your code suggest, you need to add the following line:

int n = n = atoll(argv[1]);

Modify your main function to look something like this:

int main (int argc, char *argv[])
{
    if(argc < 2)
    {
        printf ("Usage: p3 <int>\n");
    }
    else
    {
        int n = atoll(argv[1]);
        printf("count = %d, largest run = %d\n", n, count(n));
    }

    return 0;
}

If your count function is supposed to return the number of 1 bits in n your implementation won't work. Change the while body to the following:

ret += x % 2;
x /= 2;

answered Oct 31, 2018 at 19:36

Fiddling Bits

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3 Comments

Mike ODell Over a year ago

Updated my code, but it's only show: count = -1, largest run = 1

Fiddling Bits Over a year ago

That's because you're changing -1 to 1 here: if (x < 0) x = -x;. What answer do you expect?

Mike ODell Over a year ago

I have 'if (x < 0) x = -x;' in function. So when I input -1 then the output should be 64 for count and 64 for largest run.. So here is for example, if it is 345897345532 then it would be 17 for count and 7 for largest run... Does this makes sense?

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