How to distinguish whether a variable is a pure numeric string or a number, such as "123" and "123".

Look at $IsValidNum in the documentation 

Hi.

There is a COS function $ISVALIDNUM where you can check if the variable is valid number.

Regards,
Matjaž

In addition to the methods already mentioned, a lot of the %Library classes also have a class method called IsValid that you can use. For instance if ##class(%Numeric).IsValid(n) returns a 1, then n is a valid numeric value.

The original ANSI M (also known as ANSI MUMPS) standard did not distinguish between a canonical numeric string versus a number.  Many early implementations of this standard used strings of numeric characters as the internal representation of a number and did arithmetic on character strings instead of converting the numeric string to a binary representation and then using the hardware binary arithmetic instructions.  The result of a numeric operation would always be a character string using the canonical numeric representation.

In such an implementation there was no way to tell the difference between the character string "123" and the literal 123 since "123" was the canonical numeric string representation for that value.  For performance reasons, InterSystems ObjectScript has several different internal representations it can use for the number 123.  It can use the three character strintg "123"; it can use the 32-bit binary integer 123; it can use one of several representations of 123 in ObjectScripts' decimal floating point representation which has a 64-bit binary integer significand combined with an 8-bit power-of-10 exponent.  Writing 12300E-2 will usually be represented with the decimal floating-point representation and not with the integer repersentation.

The command WRITE $LISTSAME($LISTBUILD("123"),$LISTBUILD(12300E-2)) will write 1 because the string "123" and the numeric literal 12300E-2 are the same value.  However WRITE $LISTBUILD("123")=$LISTBUILD(12300E-2)) will write 0 because the $LISTBUILD will will generate different string encodings in this situation even though the values are considered to be identical.  Although $LISTBUILD could convert different internal representations into identical $LIST strings, it is faster just to place the internal representation into the generated binary string.  This is why you must use $LISTSAME when you want to check if two $LIST strings contain the same value.

There are a very few functions built into ObjectScript that have a behavior that depends on the internal representation of the argument.  All of these function were inherited when InterSystems extended ObjectScript to have a compatible feature with the language implemented by other vendors who produced extended M/MUMPS systems.  The most common function is $ZHEX(x) which turns a hex string to a decimal number and which turns a decimal number into a string.  Thus, WRITE $ZHEX("10") will write number 16 while WRITE $ZHEX(10) will write the string "A".  If you want to convert variable 'x' from an integer to hex then you need to write $ZHEX(+x) to make sure 'x' is using an internal numeric representation.  If you want to convert variable 'x' from hex to an integer then you need to write $ZHEX(x_"") to make sure 'x' is using the string internal representation.

One additional note:  The literal 123.0 will use an internal numeric representation so WRITE "123.0"=123.0 will WRITE 0 since a string equality comparison operator will be done between the literal string "123.0" and the canonical numeric string "123".  However, WRITE +"123.0"=123.0 will WRITE 1 because the unary plus operator converts the first operand to a numeric representation and then the string equality operator converts both numeric operands into their canonical numeric string representations making the string equality operation be the same as "123"="123".

I was told,  it's illegal to use data structure information, which doesn't were changed in the last 25 years (and after this many years, one could think to have the right given by "customary law" to use it), hence I decided for a more "legal" solution for the above problem - although this solution will work for IRIS (and recent Cache systems) only:

Class DC.Test Extends %RegisteredObject
{

/// Return TRUE if val contains a string
ClassMethod IsString2(val) As %Boolean
{
    quit {"a":(val)}.%GetTypeOf("a")="string"
}

/// Return TRUE if val contains a number (int, real or double)
ClassMethod IsNumber2(val) As %Boolean
{
    quit {"a":(val)}.%GetTypeOf("a")="number"
}

}