convert subset of vector to int

Question

I am having trouble figuring out how to convert an vector of hex values to a decimal long int.

    vector<uint8_t>v;

    v.push_back(0x02);
    v.push_back(0x08);
    v.push_back(0x00);
    v.push_back(0x04);
    v.push_back(0x60);
    v.push_back(0x50);
    v.push_back(0x58);
    v.push_back(0x4E);
    v.push_back(0x01);
    v.push_back(0x80);

//How would I achieve this:
    long time =  0x00046050584E0180; //1,231,798,102,000,000

How would I get elements 2-9 for the vector v into an long int like represented above with the long 'time'.

Thanks!

Seems something wrong, your example is missing first 2 bytes (0x02 and 0x08) — Galimov Albert, Jul 23 '13 at 17:40
@PSIAlt I didn't elaborate. I only care about the last 8. In this scenario 0x02 is tag, and 0x08 is the length of the value. This is just one field if many that I am parsing. — JonnyCplusplus, Jul 23 '13 at 20:28

Mats Petersson · Accepted Answer · 2013-07-23T15:03:03.487

4

The basic principler here would be:

int x = 0; 
for(uint8_t i : v)
{
   x <<= 8; 
   x |= i;
}

edited Jul 23 '13 at 15:03

answered Jul 23 '13 at 14:44

Mats Petersson

126,704
14
140
227

The original code uses unsigned integers whose upper bytes are filled with zero on upcast. – David Foerster Jul 23 '13 at 14:59
Ah, that makes more sense. – Mats Petersson Jul 23 '13 at 15:02
Any decent compiler would have optimized that away anyway... :-] – David Foerster Jul 23 '13 at 15:06

score 1 · Answer 2 · answered Jul 23 '13 at 14:43

1

Not tested:

int time = 0;
for (int i:v) time = time << 8 + i;

answered Jul 23 '13 at 14:43

hivert

10,579
3
31
56

David Foerster · Answer 3 · 2014-11-18T21:48:06.353

1

Since std::vector contains its data in a memory vector, you can use a pointer cast, e. g.:

vector<uint8_t> v;
assert(v.size() >= 2 + sizeof(long)/sizeof(uint8_t));
long time = *reinterpret_cast<const long*>(&v[2]);

Make sure, the vector contains enough data though, and beware of different endianness types.

edited Nov 18 '14 at 21:48

answered Jul 23 '13 at 14:57

David Foerster

1,461
1
14
23

score 1 · Answer 4 · answered Jul 23 '13 at 15:40

You can of course do this algorithmically with the appropriately defined function:

long long f( long long acc, unsigned char val )
{
   return ( acc << 8 ) + val;
}

the value is computed by:

#include <numeric>

long long result = std::accumulate( v.begin() + 2, v.end(), 0ll, f );

score 0 · Answer 5 · answered Jul 23 '13 at 14:53

0

My solution is as below and I've already tested. You may have a try.

long long res = 0;
for (int i = 2; i < 10; ++i)
    res = (res << 8) + v[i];

answered Jul 23 '13 at 14:53

Annie Kim

895
6
17

convert subset of vector to int

5 Answers5

Linked