# NetRexx

## Technical detail

Numbers and Arithmetic

NetRexx arithmetic attempts to carry out the usual operations (including addition, subtraction, multiplication, and division) in as 'natural' a way as possible. What this really means is that the rules followed are those that are conventionally taught in schools and colleges. However, it was found that unfortunately the rules used vary considerably (indeed much more than generally appreciated) from person to person and from application to application and in ways that are not always predictable. The NetRexx arithmetic described here is therefore a compromise which (although not the simplest) should provide acceptable results in most applications.

Introduction

Numbers can be expressed in NetRexx very flexibly (leading and trailing blanks are permitted, exponential notation may be used) and follow conventional syntax. Some valid numbers are:

 12 /* A whole number */ /* A signed whole number */ /* Some decimal places */ /* Blanks around the sign, etc. */ /* Equal to 17 */ /* Equal to 0.5 */ /* Exponential notation */ /* Exponential notation */

(Exponential notation means that the number includes a sign and a power of ten following an 'E' that indicates how the decimal point will be shifted. Thus 4E+9 above is just a short way of writing 4000000000, and 0.73e-7 is short for 0.000000073.)

The arithmetic operators include addition (indicated by a '+'), subtraction ('-'), multiplication ('*'), power ('**'), and division ('/'). There are also two further division operators: integer divide ('%') which divides and returns the integer part, and remainder ('//') which divides and returns the remainder. Prefix plus ('+') and prefix minus ('-') operators are also provided.

When two numbers are combined by an operation, NetRexx uses a set of rules to define what the result will be (and how the result is to be represented as a character string). These rules are defined in the next section, but in summary:

Definition

This definition describes arithmetic for NetRexx strings (type Rexx). The arithmetic operations are identical to those defined in the ANSI standard for Rexx.[1]

Numbers

A number in NetRexx is a character string that includes one or more decimal digits, with an optional decimal point. The decimal point may be embedded in the digits, or may be prefixed or suffixed to them. The group of digits (and optional point) thus constructed may have leading or trailing blanks, and an optional sign ('+' or '-') which must come before any digits or decimal point. The sign may also have leading or trailing blanks. Thus:

 sign digit ::= + | - ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 ::= digit [digit]... ::= digits . [digits]| [.] digits ::= [blank]... [sign [blank]...]numeric [blank]...

where if the implementation supports extra digits these are also accepted as digits, providing that they represent values in the range zero through nine. In this case each extra digit is treated as though it were the corresponding character in the range 0-9.

Note that a single period alone is not a valid number.

Precision

The maximum number of significant digits that can result from an arithmetic operation is controlled by the digits keyword on the numeric instruction:

numeric digits [expression];

The expression is evaluated and must result in a positive whole number. This defines the precision (number of significant digits) to which arithmetic calculations will be carried out; results will be rounded to that precision, if necessary.

If no expression is specified, then the default precision is used. The default precision is 9, that is, all implementations must support at least nine digits of precision. An implementation-dependent maximum (equal to or larger than 9) may apply: an attempt to exceed this will cause execution of the instruction to terminate with an exception. Thus if an algorithm is defined to use more than 9 digits then if the numeric digits instruction succeeds then the computation will proceed and produce identical results to any other implementation.

Note that numeric digits may set values below the default of nine. Small values, however, should be used with care -- the loss of precision and rounding thus requested will affect all NetRexx computations, including (for example) the computation of new values for the control variable in loops.

In the remainder of this section, the notation digits refers to the current setting of numeric digits. This setting may also be referred to in expressions in programs by using the digits special word.

Arithmetic operators

NetRexx arithmetic is effected by the operators '+', '-', '*', '/', '%', '//', and '**' (add, subtract, multiply, divide, integer divide, remainder, and power) which all act upon two terms, together with the prefix operators '+' and '-' (plus and minus) which both act on a single term. The result of all these operations is a NetRexx string, of type Rexx. This section describes the way in which these operations are carried out.

Before every arithmetic operation, the term or terms being operated upon have any extra digits converted to the corresponding Arabic numeral (the digits 0-9). They then have leading zeros removed (noting the position of any decimal point, and leaving just one zero if all the digits in the number are zeros) and are then truncated to digits+1 significant digits[2] (if necessary) before being used in the computation. The operation is then carried out under up to double that precision, as described under the individual operations below. When the operation is completed, the result is rounded if necessary to the precision specified by the numeric digits instruction.

Rounding is done in the 'traditional' manner, in that the extra (guard) digit is inspected and values of 5 through 9 are rounded up, and values of 0 through 4 are rounded down.[3]

A conventional zero is supplied preceding a decimal point if otherwise there would be no digit before it. Trailing zeros are retained for addition, subtraction, and multiplication, according to the rules given below, except that a result of zero is always expressed as the single character '0'. For division, insignificant trailing zeros are removed after rounding.

The format method is defined to allow a number to be represented in a particular format if the standard result provided by NetRexx does not meet requirements.

Arithmetic operation rules -- basic operators

The basic operators (addition, subtraction, multiplication, and division) operate on numbers as follows:

If either number is zero then the other number, rounded to digits   digits if necessary, is used as the result (with sign adjustment as   appropriate). Otherwise, the two numbers are extended on the right   and left as necessary up to a total maximum of digits+1 digits.

The number with smaller absolute value may therefore lose some or all of its digits on the right.[4] The numbers are then added or subtracted as appropriate. For example:

xxxx.xxx + yy.yyyyy

becomes:

xxxx.xxx00
+ 00yy.yyyyy
---------------
zzzz.zzzzz

The result is then rounded to digits digits if necessary, taking into account any extra (carry) digit on the left after an addition, but otherwise counting from the position corresponding to the most significant digit of the terms being added or subtracted. Finally, any insignificant leading zeros are removed.

The prefix operators are evaluated using the same rules; the operations '+number' and '-number' are calculated as '0+number' and '0-number', respectively.

Multiplication

The numbers are multiplied together ('long multiplication') resulting in a number which may be as long as the sum of the lengths of the two operands. For example:

xxx.xxx * yy.yyyyy

becomes:

zzzzz.zzzzzzzz

and the result is then rounded to digits digits if necessary, counting from the first significant digit of the result.

Division

For the division:

yyy / xxxxx

the following steps are taken: first, the number 'yyy' is extended with zeros on the right until it is larger than the number 'xxxxx' (with note being taken of the change in the power of ten that this implies). Thus in this example, 'yyy' might become 'yyy00'. Traditional long division then takes place, which can be written:

zzzz
.------
xxxxx | yyy00

The length of the result ('zzzz') is such that the rightmost 'z' will be at least as far right as the rightmost digit of the (extended) 'y' number in the example. During the division, the 'y' number will be extended further as necessary, and the 'z' number (which will not include any leading zeros) may increase up to digits+1 digits, at which point the division stops and the result is rounded. Following completion of the division (and rounding if necessary), insignificant trailing zeros are removed.

Examples:

## /* With 'numeric digits 5' */

 12+7.00 1.3-1.07 == 19 == 0.23 == -0.77 == 3.6 == 21 == 0.72 == 0.33333 == 0.66667 == 2.5 == 0.1 == 1 == 4

Note: With all the basic operators, the position of the decimal point in the terms being operated upon is arbitrary. The operations may be carried out as integer operations with the exponent being calculated and applied afterwards. Therefore the significant digits of a result are not in any way dependent on the position of the decimal point in either of the terms involved in the operation.

Arithmetic operation rules -- additional operators

The operation rules for the power ('**'), integer division ('%'), and remainder ('//') operators are as follows:

Power

The '**' (power) operator raises a number (on the left of the operator) to a power (on the right of the operator). The term on the right is rounded to digits digits (if necessary), and must, after any rounding, be a whole number, which may be positive, negative, or zero. If negative, the absolute value of the power is used, and then the result is inverted (divided into 1).

For calculating the power, the number is effectively multiplied by itself for the number of times expressed by the power, and finally trailing zeros are removed (as though the result were divided by one).

In practice (see note below for the reasons), the power is calculated by the process of left-to-right binary reduction. For 'x**n': 'n' is converted to binary, and a temporary accumulator is set to 1. If 'n' has the value 0 then the initial calculation is complete. Otherwise each bit (starting at the first non-zero bit) is inspected from left to right. If the current bit is 1 then the accumulator is multiplied by 'x'. If all bits have now been inspected then the initial calculation is complete, otherwise the accumulator is squared by multiplication and the next bit is inspected. When the initial calculation is complete, the temporary result is divided into 1 if the power was negative.

The multiplicati ons and division are done under the normal arithmetic operation rules, detailed earlier in this section, using a precision of digits+elength+1 digits. Here, elength is the length in decimal digits of the integer part of the whole number 'n' (i.e., excluding any sign, decimal part, decimal point, or insignificant leading zeros, as though the operation n%1 had been carried out and any sign removed). Finally, the result is rounded to digits digits, if necessary, and insignificant trailing zeros are removed.

Integer division

The '%' (integer divide) operator divides two numbers and returns the integer part of the result. The result returned is defined to be that which would result from repeatedly subtracting the divisor from the dividend while the dividend is larger than the divisor. During this subtraction, the absolute values of both the dividend and the divisor are used: the sign of the final result is the same as that which would result if normal division were used.

The result returned will have no fractional part (that is, no decimal point or zeros following it). If the result cannot be expressed exactly within digits digits, the operation is in error and will fail -- that is, the result cannot have more digits than the current setting of numeric digits. For example, 10000000000%3 requires ten digits to express the result exactly (3333333333) and would therefore fail if digits were 9 or smaller.

Remainder

The '//' (remainder) operator will return the remainder from integer division, and is defined as being the residue of the dividend after the operation of calculating integer division as just described. The sign of the remainder, if non-zero, is the same as that of the original dividend.

This operation will fail under the same conditions as integer division (that is, if integer division on the same two terms would fail, the remainder cannot be calculated).

Examples:

## /* Again with 'numeric digits 5' */

 2**3 2**-3 == 8 == 0.125 == 69.758 == 0 == 2.1 == 3 == 1 == -1 == 0.2 == 0.1 == 1

Notes:

1. A particular algorithm for calculating powers is described, since it is efficient (though not optimal) and considerably reduces the number of actual multiplications performed. It therefore gives better performance than the simpler definition of repeated multiplication. Since results could possibly differ from those of repeated multiplication, the algorithm must be defined here so that different implementations will give identical results for the same operation on the same values. Other algorithms for this (and other) operations may always be used, so long as they give identical results to those described here.
2. The integer divide and remainder operators are defined so that they may be calculated as a by-product of the standard division operation (described above). The division process is ended as soon as the integer result is available; the residue of the dividend is the remainder.

Numeric comparisons

Any of the comparative operators may be used for comparing numeric strings. However, the strict comparisons (for example, '==' and '>>') are not numeric comparative operators and should not normally be used for comparing numbers, since they compare from left to right and leading and trailing blanks (and leading zeros) are significant for these operators.

Numeric comparison, using the normal comparative operators, is effected by subtracting the two numbers (calculating the difference) and then comparing the result with '0' -- that is, the operation:

A ? B

where '?' is any normal comparative operator, is identical to:

(A - B) ? '0'

It is therefore the difference between two numbers, when subtracted under NetRexx subtraction rules, that determines their equality.

Exponential notation

The definition of numbers above describes 'pure' numbers, in the sense that the character strings that describe numbers can be very long.

Examples:

say 10000000000 * 10000000000
/* would display: 100000000000000000000 */

say 0.00000000001 * 0.00000000001
/* would display: 0.0000000000000000000001 */

For both large and small numbers some form of exponential notation is useful, both to make such long numbers more readable and to make evaluation possible in extreme cases. In addition, exponential notation is used whenever the 'pure' form would give misleading information. For example:

numeric digits 5
say 54321*54321

would display '2950800000' if long form were to be used. This is misleading, as it appears that the result is an exact multiple of 100000, and so NetRexx would express the result in exponential notation, in this case '2.9508E+9'.

The definition of number (see above) is therefore extended by replacing the description of numeric by the following:

 mantissa ::= digits . [digits] | [.] digits ::= mantissa [E sign digits]

In other words, the numeric part of a number may be followed by an 'E' (indicating an exponential part), a sign, and an integer following the sign that represents a power of ten that is to be applied. The 'E' may be in uppercase or lowercase. Note that no blanks are permitted within this part of a number, but the integer may have leading zeros.

Examples:

12E+11 = 1200000000000
12E-5 = 0.00012
12e+4 = 120000

All valid numbers may be used as data for arithmetic. The results of calculations will be returned in exponential form depending on the setting of numeric digits. If the number of places needed before the decimal point exceeds digits, or if the absolute value of the result is less than 0.000001, then exponential form will be used. The exponential form generated by NetRexx always has a sign following the 'E'. If the exponent is 0 then the exponential part is omitted -- that is, an exponential part of 'E+0' will never be generated.

If the default format for a number is not satisfactory for a particular application, then the format method may be used to control its format. Using this, numbers may be explicitly converted to exponential form or even forced to be returned in 'pure' form. -

Different exponential notations may be selected with the numeric form instruction. This instruction allows the selection of either scientific or engineering notation. Scientific notation adjusts the power of ten so there is a single non-zero digit to the left of the decimal point. Engineering notation causes powers of ten to be expressed as a multiple of three -- the integer part may therefore range from 1 through 999.

Examples:

numeric form scientific
say 123.45 * 1e11
/* would display: 1.2345E+13 */

numeric form engineering
say 123.45 * 1e11
/* would display: 12.345E+12 */

The default exponential notation is scientific.

Whol numbers

Within the set of numbers understood by NetRexx it is useful to distinguish the subset defined as whole numbers.

A whole number in NetRexx is a number that has a decimal part which is all zeros (or that has no decimal part).

Numbers used directly by NetRexx

As discussed above, the result of any arithmetic operation is rounded (if necessary) according to the setting of numeric digits. Similarly, when a number (which has not necessarily been involved in an arithmetic operation) is used directly by NetRexx then the same rounding is also applied, just as though the operation of adding the number to 0 had been carried out. After this operation, the integer part of the number must have no more digits than the current setting of numeric digits.

In the following cases, the number used must be a whole number and an implementation restriction on the largest number that can be used may apply:

Implementation minimum: A minimum length of 9 digits must be supported for these uses of whole numbers by a NetRexx language processor.

Implementation independence

The NetRexx arithmetic rules are defined in detail, so that when a given program is run the results of all computations are sufficiently defined that the same answer will result for all correct implementations. Differences due to the underlying machine architecture will not affect computations.

This contrasts with most other programming languages, and with binary arithmetic in NetRexx, where the result obtained may depend on the implementation because the precision and algorithms used by the language processor are defined by the implementation rather than by the language.

Exceptions and errors

The following exceptions and errors may be signalled during arithmetic:

In the reference implementation, the exceptions and error types used for these three cases are DivideException, ExponentOverflowException, and OutOfMemoryError, respectively.

Footnotes:

[1] American National Standard for Information Technology -- Programming Language REXX, X3.274-1996, American National Standards Institute, New York, 1996.

[2] That is, to the precision set by numeric digits, plus one extra 'guard' digit.

[3] Even/odd rounding would require the ability to calculate to arbitrary precision (that is, to a precision not governed by the setting of numeric digits) at any time and is therefore not the mechanism defined for NetRexx.

[4] In the example, the number yy.yyyyy would have three digits truncated if digits were 5.

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