2.4.12 The NUMERIC InstructionNUMERIC = DIGITS [ expr ] ; FORM [ SCIENTIFIC | ENGINEERING | [ VALUE ] expr ] ; FUZZ [ expr ] ; REXX has an unusual form of arithmetic. Most programming languages use integer and floating point arithmetic, where numbers are coded as bits in the computers native memory words. However, REXX uses floating point arithmetic of arbitrary precision, that operates on strings representing the numbers. Although much slower, this approach gives lots of interesting functionality. Unless number-crunching is your task, the extra time spent by the interpreter is generally quite acceptable and often almost unnoticeable. The NUMERIC statement is used to control most aspects of arithmetic operations. It has three distinct forms: DIGITS, FORM and FUZZ; which to choose is given by the second token in the instruction: DIGITS Is used to set the number of significant digits in arithmetic operations. The initial value is 9, which is also the default value if expr is not specified. Large values for DIGITS tend to slow down some arithmetic operations considerably. If specified, expr must be a positive integer. FUZZ Is used in numeric comparisons, and its initial and default value is 0. Normally, two numbers must have identical numeric values for a number of their most significant digits in order to be considered equal. How many digit are considered is determined by DIGITS. If DIGITS is 4, then 12345 and 12346 are equal, but not 12345 and 12356. However, when FUZZ is non-zero, then only the DIGITS minus FUZZ most significant digits are checked. E.g. if DIGITS is 4 and FUZZ are 2, then 1234 and 1245 are equal, but not 1234 and 1345. The value for FUZZ must be a non-negative integer, and less than the value of DIGITS. FUZZ is seldom used, but is useful when you want to make comparisons less influenced by inaccuracies. Note that using with values of FUZZ that is close to DIGITS may give highly surprising results. FORM Is used to set the form in which exponential numbers are written. It can be set to either SCIENTIFIC or ENGINEERING. The former uses a mantissa in the range 1.000... to 9.999..., and an exponent which can be any integer; while the latter uses a mantissa in the range 1.000... to 999.999..., and an exponent which is dividable by 3. The initial and default setting is SCIENTIFIC. Following the subkeyword FORM may be the subkeywords SCIENTIFIC and ENGINEERING, or the subkeyword VALUE. In the latter case, the rest of the statement is considered an expression, which will evaluate to either SCIENTIFIC or ENGINEERING. However, if the first token of the expression following VALUE is neither a symbol nor literal string, then the VALUE subkeyword can be omitted. The setting of FORM never affects the decision about whether to choose exponential form or normal floating point form; it only affects the appearance of the exponential form once that form has been selected. Many things can be said about the usefulness of FUZZ. My impression is that it is seldom used in REXX programs. One problem is that it only addresses relative inaccuracy: i.e. that the smaller value must be within a certain range, that is determined by a percentage of the larger value. Often one needs absolute inaccuracy, e.g. two measurements are equal if their difference are less than a certain absolute threshold. Example: Simulating relative accuracy with absolute accuracy As explained above, REXX arithmetic has only relative accuracy, in order to obtain absolute accuracy, one can use the following trick: numeric fuzz 3 if a=b then say ‘relative accuracy’ if abs(a-b)<=500 then say ‘absolute accuracy’ In the first IF instruction, if A is 100,000, then the range of values for B which makes the expression true is 99,500—100,499, i.e. an inaccuracy of about +-500. If A has the value 10,000,000, then B must be within the range 9,950,000—10,049,999; i.e. an inaccuracy of about +-50,000. However, in the second IF instruction, assuming A is 100,000, the expression becomes true for values of B in the range 99,500—100,500. Assuming that A is 10,000,000, the expression becomes true for values of B in the range 9,999,500—10,000,500. The effect is largely to force an absolute accuracy for the second example, no matter what the values of A and B are. This transformation has taken place since an arithmetic subtraction is not affected by the NUMERIC FUZZ, only numeric comparison operations. Thus, the effect of NUMERIC FUZZ on the implicit subtraction in the operation = in the first IF has been removed by making the subtraction explicit. Note that there are some minor differences in how numbers are rounded, but this can be fixed by transforming the expression into something more complex. To retrieve the values set for NUMERIC, you can use the built-in functions DIGITS(), FORM(), and FUZZ(). These values are saved across subroutine calls and restored upon return.

PREV NEXT