CODEWITHSUNDEEP
c#
Colin Banbury
Colin Banbury

Reputation: 891

"Substring" a Numeric Value

In C#, what is the best way to "substring" (for lack of a better word) a long value.

I need to calculate a sum of account numbers for a trailer record but only need the 16 least significant characters.

I am able to this by converting the value to string but wondered if there is a better way in which it can be done.

long number = 1234567890123456789L;
const long _MAX_LENGTH = 9999999999999999L;

if (number > _MAX_LENGTH)
{
  string strNumber = number.ToString();
  number = Convert.ToInt64(strNumber.Substring(strNumber.Length - 16));
}

This will return the value 4567890123456789.

Upvotes: 3

Views: 3188

Answers (3)

Michael Buen
Michael Buen

Reputation: 39393

Complete Code, use modulo operator:

long number = 1234567890123456789L;
const long _MAX_LENGTH = 9999999999999999L;

number = number % (_MAX_LENGTH + 1);                

Console.WriteLine (number);

Live test: http://ideone.com/pKB6w

Until you are enlightened with modulo approach, you can opt for this one instead for the meantime:

long number = 1234567890123456789L;
const long _MAX_LENGTH = 9999999999999999L;


if (number > _MAX_LENGTH) {             
    long minus = number / (_MAX_LENGTH +  1) * (_MAX_LENGTH +  1);              
    number = number - minus;
}

Console.WriteLine(number);

Live test: http://ideone.com/oAkcy

Note:

Strongly recommended, use modulo approach, don't use subtraction. Modulo approach is the best, no corner case, i.e. no need to use if statement.

Upvotes: 1

Daren Thomas
Daren Thomas

Reputation: 70324

You could do:

long number = 1234567890123456789L;
long countSignificant = 16;
long leastSignificant = number % (long) Math.Pow(10, countSignificant);

How does this work? Well, if you divide by 10, you drop off the last digit, right? And the remainder will be that last digit? The same goes for 100, 1000 and Math.Pow(1, n).

Let's just look at the least significant digit, because we can do this in our head:

1234 divided by 10 is 123 remainder 4

In c#, that would be:

1234 / 10 == 123;
1234 % 10 == 4;

So, the next step is to figure out how to get the last n significant digits. It turns out, that that is the same as dividing by 10 to the power of n. Since c# doesn't have an exponent operator (like ** in python), we use a library function:

Math.Pow(10, 4) == 1000.0; // oops: a float!

We need to cast that back to a long:

(long) Math.Pow(10, 4) == 1000;

I think now you have all the pieces to create a nice function of your own ;)

Upvotes: 8

Thorsten Dittmar
Thorsten Dittmar

Reputation: 56697

You could use modulo (the % operator in C#). For example:

123456 % 100000 = 23456

or

123456 % 1000 = 456

As a quick guide I keep remembering that you get as many digits as there are zeros in the divisor. Or, vice versa, the divisor needs as many zeros as you want to keep digits.

So in your case you'd need:

long number = 1234567890123456789L;
long divisor = 10000000000000000L;
long result = number % divisor;

Upvotes: 7

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