We Keep Coding

algorithm-shorten +10 digits string

I'm using C# language for a project. I need to provide user with large (9+ digit) number, which they will have to reenter into another system (for later data correlation). Having a user enter a number that large (by hand) with no errors will be almost impossible.
I have been trying to come up with a solution to shorten that number using base64, but all the code I have found will create a string combination of character and digits. Is there a simple math algorithm I can use to make a large number smaller? The result should be numeric not alpha numeric.

You address the problem in a wrong way, instead of changing the number size just build a convenient way for the user to copy past the number , a simple key event wich will copy the number to the buffer, then the user will not have to write the number down.

Reducing a number using only numbers will never work.
What you really need is some form of error checking.
One that works very good is the Verhoeff Algorithm that will detect almost every typo. There are many examples to find online.
like:
https://www.codeproject.com/articles/15939/verhoeff-check-digit-in-c

You can use a Hash algorithm to hash your large number, but you need to deal with hash collision.
One of those very easy to implement is checksum sum16:
https://en.wikipedia.org/wiki/List_of_hash_functions
See sum16 you can only have 0-65536. Think about sum18 ?

How to format currency from database

The goal
I want to format correctly the currency from database.
The problem
I'm formatting the currency from database with this:
#String.Format("{0:C}", #Model["MinProductPrice"])
The problem is: 150 have to be 1,50, and not 150,00 — and this formatting is doing this.
What is the right formatting type to my case?

You probably want to divide the number by 100 first (remember to change the type), so 150 becomes 1.50, which gets converted to "1,50" depending on locale:
#String.Format("{0:C}", #Model["MinProductPrice"] / 100.0m)

I'll extend my comments into an answer, I think that's more appropriate. I think you should change the column type to a money or decimal type to prevent bugs by making the use of the column more obvious. Your output on your page will be correct and won't require any "magic numbers" to get it to print out properly.
Just a note but you can also print a currency string doing this:
#Model["MinProductPrice"].ToString("C")

I attributed the casting responsibility to database. I'm using MySQL and the query is like this:
ROUND(CAST(MIN(`map`.`Product_Price`) AS DECIMAL)/100,2) as `minProductPrice`
Anyway, I would to thanks jlafay and csharpler about their answers — they were very helpful and worked well for me.

Custom .Net library or existing library to handle sequence formatting?

I am thinking a library already exists for this, but I need allow my users to create a numbering format for their documents.
For example, let's say we have an RFI from and the user has a specific format the numbering sequence needs to be in. A typical RFI number looks like this for their system: R0000100. The next RFI in line would be R0000101.
Before I set out to creating a formatting engine for numbers such as these, does something already exist that can accommodate this?
Update:
I failed to save the edit to this question. Anyway, I also want to give the users the ability to create their own formats. So, I may have a form where they can input the format: R####### And also allow them to specify the starting integer: in the case 100. Also, I may want to allow them to specify how they want to increment. maybe only by 100s. So the next number may be R0000200. I know this may sound ridiculous, but you never know. That is why I asked if something like this already exists.

If you keep value and format separated, you won't need a library or such a thing.
The numbers would be simple, say, integers i, i.e. 100, 101, 102, that you manage/store however you see fit. The formatting part would simply be a matter of R + i.ToString("0000000"), or if you want to have the format as a string literal string.Format("R{0:0000000}", i).
I know, this might only be an example, but as your question stands, the formatting options, that .NET provides out of the box seem to suffice.

The incrementing of identity field values is most often handled in an RDBMS-style database. This comes with a few benefits, such as built-in concurrency handling. If you want to generate the values yourself, a simple class to get the last-issued value and increment by one would be very easy to create. Make it thread-safe so you don't get any duplicates or gaps and you'll be good to go.

sql type float, real, decimal?

well in my database i had a colum for price of one product
i had it as float, my problem is if i saved it since my c# application
as 10.50 .. in a query it returns 10,50 and if i update i get a error
10,50 cant convert to float ... or something so..
and if i saved it as decimal, in queries inside sql management .. are ok..
but in my c# application... i get the same error..
10.50 retuns as 10,50 i dont know why, and how to solved it.. my unique solution is saved it
as varchar...

That's a localisation problem of some sort. 10,50 is the "European" way of writing ten and a half. If you're getting that from your select statements then your database is probably configured incorrectly.

Generally speaking you should use the same type throughout your layers. So if the underlying types in the database are x, you should pass around those data with identical types in c#, too.
What type you choose depends on what you are storing--you shouldn't be switching around types just to get something to "work". To that end, storing numeric data in a non-numeric type (e.g. varchar) will come back to bite you very soon. It's good you've opened this question to fix that!
As others have miraculously inferred, you are likely running into a localization issue. This is a great example of why storing numbers as strings is a problem. If you properly accept user input in whatever culture/localization they want (or you want), and get it into a numeric-type variable, then the rest (talking to the DB) should be easy. More so, you should not do number formatting in the database if you can help it--that stuff is much better placed at the front end, closer to the users.

I think your setting in windows regional and language for decimal symbol is wrong.please set it to dot and again test it.

This may help out for temporary use but I wouldn't recommend it for permanent use:
Try making it so that just before you save the file, convert the number to a string, replace the commas with periods (From , to .) and then save it into the database as the string, hopefully it should see that it is in the correct format and turn it into what the database sees as "Decimal" or "Floating".
Hope this helps.

Yep, localization.
That said, I think your pice is being stored on a "money" field in SQLServer (I'm assuming it's SQLServer you're using). If that was a float in the DB, it would return it with a normal decimal point, and not the European money separator ",".
To fix:
Fist DO NO USE FLOAT in your c# code, unless you absolutely require a floating point number. Use the decimal type instead. That's not just in this case, but in all cases. Floating point numbers are binary (base-2), not decimal (base-10), so what you see in the interface is only a decimal approximation of the actual number. The result is that frequently (1 == 1) evaluates as false!
I've run into that problem myself, and it's maddening if you don't know that can happen. Always use decimal instead of float in c#.
Ok, after you've fixed that, then do this to get the right localization:
using System.Globalization;
...
NumberFormatInfo ni = new NumberFormatInfo();
ni.CurrencyDecimalSeparator = ",";
decimal price = decimal.Parse(dbPriceDataField, ni);
Note that "dbPriceDataField" must be a string, so you may have to do a ".ToString()" on that db resultset's field.
If you end up having to handle other "money" aspects of that money field, like currency symbols, check out: http://msdn.microsoft.com/en-us/library/system.globalization.numberformatinfo.aspx
If you need more robust error handling, either put that decimal.Parse in a try/catch, or use decimal.TryParse.
EDIT --
If you know what culture (really, country), the db is set to, you can do this instead:
using System.Globalization;
...
CultureInfo ci = new CultureInfo("fr-FR"); // fr-FR being "french France"
decimal price = decimal.Parse(dbprice, ci.NumberFormat);

Such problems were faced by me in my Web Apps... but i found the solution like I was fetching my price value in textbox. So I was have database attached with that. So when you attached your database with textbox... When you right click textbox and click Edit DataBinding.... in that you have to provide.... type like in Bind Property..... {0:N2}
This will work only for web apps or websites... not for desktop applications...

Suggest a good method for having a number with high precision in C#

I am programming on a project which I should store the key of the user to the initial configuration of a machine, I want to write it in C#.
I have an initial configuration which consists of two number R and X0, R = 3.9988 and X0 = 0.5. I want to add the user key to these numbers. for example:
Key: hos110 =>
R = 3.9988104111115049049048
X0 = 0.5104111115049049048
104111115049049048 are ASCII codes of the key which are concatenated.
How can I store these numbers?
Is there a better method for doing this?
Update: How about MATLAB?

You're not really "adding" numbers. You are concatenating strings.
Store them as strings. You can't get much more precise than that.
If you need to perform any arithmetic operations, it is easy enough to convert them to a decimal number on the fly.

I don't really follow why you're using a key as part of a number, but leaving that aside... System.Decimal (aka decimal) seems like the right tool for the job here.

If you need infinite precision you need something that is called BigInteger. However these classes are usually only used for scientific calculations (and usually unsuited for stroring the data) which doesn't really seem to match your code sample. If you need to do only general calculations use Strings and then convert them to Decimal for the calculations.
However if you are looking for such a BigInterger Class you can find one here.
.Net 4.0 will have a BigInteger built-in-class in the class libraries named System.Numerics.BigInteger.

Well, depending on the precision you are trying to achieve, you can probably save these as a pair of decimal values.
However, if this is an ASCII code, you may just want to save these as a string directly. This will avoid the numerical precision issues, especially if you're going to pull off the 104111... prior to using this information.

It seems that you are storing a "key", so why not use a String then?

Floating point numbers are inherently imprecise. I'm not sure what this 'initial configuration' is or why it's a float, but you're not going to be able to tack on a 'user key' (whatever that may be) and recover it later. Store the user key separately, in a string or something.

If these 'numbers' have no numeric value, i.e. you will not use them for mathematical computation then there is no need to store them in a numeric datatype. You can store them as strings.