Concatenate integers in C#

Question

Is there an inexpensive way to concatenate integers in csharp?

Example: 1039 & 7056 = 10397056

Answer 1

If you can find a situation where this is expensive enough to cause any concern, I'll be very impressed:

int a = 1039;
int b = 7056;

int newNumber = int.Parse(a.ToString() + b.ToString())

Or, if you want it to be a little more ".NET-ish":

int newNumber = Convert.ToInt32(string.Format("{0}{1}", a, b));

int.Parse is not an expensive operation. Spend your time worrying about network I/O and O^N regexes.

Other notes: the overhead of instantiating StringBuilder means there's no point if you're only doing a few concatenations. And very importantly - if you are planning to turn this back into an integer, keep in mind it's limited to ~2,000,000,000. Concatenating numbers gets very large very quickly, and possibly well beyond the capacity of a 32-bit int. (signed of course).

Answer 2

I'm a bit late at the party, but recently I had to concatenate integers. With 0 < a,b < 10^9 it can be done quite fast.

static ulong concat(uint a, uint b)
{
    if (b < 10U) return 10UL * a + b;
    if (b < 100U) return 100UL * a + b;
    if (b < 1000U) return 1000UL * a + b;
    if (b < 10000U) return 10000UL * a + b;
    if (b < 100000U) return 100000UL * a + b;
    if (b < 1000000U) return 1000000UL * a + b;
    if (b < 10000000U) return 10000000UL * a + b;
    if (b < 100000000U) return 100000000UL * a + b;
    return 1000000000UL * a + b;
}

Edit: the version below might be interesting (platform target: x64).

static ulong concat(ulong a, uint b)
{
    const uint c0 = 10, c1 = 100, c2 = 1000, c3 = 10000, c4 = 100000,
        c5 = 1000000, c6 = 10000000, c7 = 100000000, c8 = 1000000000;
    a *= b < c0 ? c0 : b < c1 ? c1 : b < c2 ? c2 : b < c3 ? c3 :
         b < c4 ? c4 : b < c5 ? c5 : b < c6 ? c6 : b < c7 ? c7 : c8;
    return a + b;
}

Answer 3

1.  string ConcatInt(int x,int y){return String.Format("{0}{1}",x,y);} 

2.  int ConcatInt(int x,int y){ return (x * Math.Pow(10, y.length)) + y; } 

Edit Note: Fixes some mistypes. There are more type issues left. I'm just giving an outline of the answer

The second method should actually be:

static int ConcatInt2(int x, int y) {
   return (int)(x * Math.Pow(10, y.ToString().Length)) + y;
}

Answer 4

inexpensive? String concatenation or formatted string is probably going to be considerably faster.

Otherwise you can do something like:

Math.Pow(10,Math.Ceiling(Math.Log10(second)))*first+second

provided first and second are integers. This is about the only way you'll do it not involving converting to a string and back, but I am extremely doubtful that it will be faster.

Answer 5

I don't think you can get any simpler than this:

static uint Concat (uint a, uint b)
{
  uint
    pow = 1;

  while (pow < b)
  {
    pow = ((pow << 2) + pow) << 1;
    a = ((a << 2) + a) << 1;
  }

  return a + b;
}

which has no memory allocations, string conversions or multiplies; or maybe:

static uint Concat (uint a, uint b)
{
  uint
    pow = 1;

  while (pow < b)
  {
    pow = ((pow << 2) + pow) << 1;
  }

  return a * pow + b;
}

If you want to concatenate two binary numbers:

static uint Concat (uint a, uint b)
{
  uint
    mask = uint.MaxValue;

  while ((mask & b) != 0)
  {
    mask <<= 1;
    a <<= 1;
  }

  return a | b;
}

Answer 6

If you want to concatenate many ints to a String

StringBuilder sb = new StringBuilder(1039);
sb.Append(7056);
sb.Append(1234);
sb.Append(1235);
....
sb.Append(9999);
sb.ToString();

Answer 7

If we want integer result then:

int result = int.Parse(input1.ToString() + input2.ToString());

For a string result do this:

string result = input1.ToString() + input2.ToString();

Answer 8

The "Mathy" and "No String" method follows:

    int a = 1039;
    int b = 7056;

    int bLen = (int)Math.Ceiling(Math.Log10(b));
    int ab = (a * ((int)Math.Pow(10, bLen))) + b;

Note that it may still be slow because of the Log10 call.

Answer 9

how about this?

int c = b;
while(c > 0) {
   a *= 10;
   c /= 10;
}
a += b;

Answer 10

Not really inpexpensive, but:

string con = string.Format("{0}{1}",int1,int2);

or

string con = int1.ToString() + int2.ToString();

If you use this in a loop, I think I would use Option 1, which uses a StringBuilder internally.

Answer 11

public int ConcatInts(int int1, int int2)
{
    return (int)(int1 * Math.Pow(10, int2.ToString().Length)) + int2;
}

Edit: Guess I wasn't the first with this solution!

Answer 12

// Concatenating two numbers program//

        Console.WriteLine("Enter a number for a");
        int a = int.Parse(Console.ReadLine());

        Console.WriteLine("Enter a number for b");
        int b = int.Parse(Console.ReadLine());

        int Concatenating = Convert.ToInt32(string.Format("{0}{1}", a, b));
        Console.WriteLine(Concatenating);
        Console.ReadKey();