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NUMBERS AND ARITHMETIC FUNCTIONS
13.0 General Comments
There are three different types of numbers in INTERLISP: small integers,
large integers, and floating point numbers. Since a large integer or
floating point number can be (in value) any full word quantity (and vice
versa), it is necessary to distinguish between those full word
quantities that represent large integers or floating point numbers, and
other INTERLISP pointers. We do this by "boxing" the number, which is
sort of like a special "cons": when a large integer or floating point
number is created (via an arithmetic operation or by read), INTERLISP
gets a new word from "number storage" and puts the large integer or
floating point number into that word. INTERLISP then passes around the
pointer to that word, i.e., the "boxed number", rather than the actual
quantity itself. Then when a numeric function needs the actual numeric
quantity, it performs the extra level of addressing to obtain the
"value" of the number. This latter process is called "unboxing". Note
that unboxing does not use any storage, but that each boxing operation
uses one new word of number storage. Thus, if a computation creates
many large integers or floating point numbers, i.e., does lots of boxes,
it may cause a garbage collection of large integer space, GC: 18, or of
floating point number space, GC: 16.
13.1 Integer Arithmetic
Small integers are those integers for which smallp is true. In
Floating point numbers are created by the read program when a . or
an E appears in a number, e.g., 1000 is an integer, 1000. a floating
point number, as are 1E3 and 1.E3. Note that 1000D, 1000F, and 1E3D
are perfectly legal literal atoms.
Different implementations of INTERLISP-10 may use different boxing
strategies. Thus, while lots of arithmetic operations may lead to
garbage collections, this is not necessarily always the case.
INTERLISP-10, these are integers whose absolute value is less than 1536.
Small integers are boxed by offsetting them by a constant so that they
overlay an area of INTERLISP's address space that does not correspond to
any INTERLISP data type. Thus boxing small numbers does not use any
storage, and furthermore, each small number has a unique representation,
so that eq may be used to check equality. Note that eq should not be
used for large integers or floating point numbers, e.g.,
eq[2000;add1] is NIL! eqp or equal must be used instead.
All of the functions described below work on integers. Unless specified
otherwise, if given a floating point number, they first convert the
number to an integer by truncating the fractional bits, e.g.,
iplus[2.3;3.8]=5; if given a non-numeric argument, they generate an
error, NON-NUMERIC ARG.
It is important to use the integer arithmetic functions, whenever
possible, in place of the more general arithmetic functions which allow
mixed floating point and integer arithmetic, e.g., iplus vs plus,
igreaterp vs greaterp, because the integer functions compile open, and
therefore run faster than the general arithmetic functions, and because
the compiler is "smart" about eliminating unnecessary boxing and
unboxing. Thus, the expression
(IPLUS (IQUOTIENT (ITIMES N 100) M) (ITIMES X Y)) will compile to
perform only one box, the outer one, and the expression
(IGREATERP (IPLUS X Y) (IDIFFERENCE A B)) will compile to do no boxing
Note that the PDP-10 is a 36 bit machine, so that in INTERLISP-10 all
integers are between -2^35 and 2^35-1. Adding two integers which
produce a result outside this range causes overflow, e.g., 2^34 + 2^34.
The procedure on overflow is to return the largest possible integer,
i.e., in INTERLISP-10 2^35 - 1.
iplus[x1;x2;...;xn] x1 + x2 + ... + xn
iminus[x] - x
idifference[x;y] x - y
Approximately 34 billion
If the overflow occurs by trying to create a negative number of too
large a magnitude, -2^35+1 is used instead of 2^35-1.
add1[x] x + 1
sub1[x] x - 1
itimes[x1;x2;...;xn] the product of x1,x2,...xn
iquotient[x;y] x/y truncated, e.g., iquotient[3;2]=1,
iremainder[x;y] the remainder when x is divided by y, e.g.,
igreaterp[x;y] T, if x > y; NIL otherwise
ilessp[x;y] T, if x < y; NIL otherwise
ieqp[n;m] T, if n and m are eq, or equal integers, NIL
otherwise. Note that eq may be used if n and m
are known to be small integers. ieqp converts n
and m to integers, e.g., ieqp[2000;2000.3]=T.
ieqp compiles open.
zerop[x] defined as eq[x;0].
Note that zerop should not be used for floating point numbers because it
uses eq. Use eqp[x;0] instead.
minusp[x] T if x is negative; NIL otherwise. Does not
convert x to an integer, but simply checks sign
eqp[n;m] T, if n and m are eq, or equal numbers, NIL
otherwise. Note that eq may be used if n and m
are known to be small integers. eqp does not
convert n and m to integers, e.g.,
eqp[2000;2000.3]=NIL, but it can be used to
compare an integer and a floating point number,
e.g., eqp[2000;2000.0]=T. eqp does not generate
an error if n or m are not numbers.
eqp is also T if n and m are both stack descriptors that refer to
the same frame extension (see Section 12).
smallp[n] n, if n is a small integer, else NIL. smallp
does not generate an error if n is not a number.
fixp[x] x, if x is an integer, else NIL. Does not
generate an error if x is not a number.
fix[x] Converts x to an integer by truncating
fractional bits, e.g., fix[2.3] = 2, fix[-
1.7] = -1. If x is already an integer, fix[x]=x
and doesn't use any storage.
logand[x1;x2;...;xn] lambda no-spread, value is logical and of all
its arguments, as an integer, e.g.,
logor[x1;x2;...;xn] lambda no-spread, value is the logical or of all
its arguments, as an integer, e.g.,
logxor[x1;x2;...;xn] lambda no-spread, value is the logical exclusive
or of its arguments, as an integer, e.g.,
logxor[11;5] = 14,
logxor[11;5;9] = logxor[14;9] = 7.
lsh[n;m] (arithmetic) left shift, value is n*2^m,i.e., n
is shifted left m places. n can be positive or
negative. If m is negative, n is shifted
right -m places.
rsh[n;m] (arithmetic) right shift, value is n*2^-m, i.e.,
n is shifted right m places. n can be positive
or negative. If m is negative, n is left -m
llsh[n;m] logical left shift. On PDP-10, llsh is
equivalent to lsh.
lrsh[n;m] logical right shift.
The difference between a logical and arithmetic right shift lies in the
Since FIX is also a lispx command (Section 22), typing FIX directly
to lispx will not cause the function fix to be called.
treatment of the sign bit for negative numbers. For arithmetic right
shifting of negative numbers, the sign bit is propagated, i.e., the
value is a negative number. For logical right shift, zeroes are
propagated. Note that shifting (arithmetic) a negative number "all the
way" to the right yields -1, not 0.
gcd[x;y] value is the greatest common divisor of x and y,
13.2 Floating Point Arithmetic
All of the functions described below work on floating point numbers.
Unless specified otherwise, if given an integer, they first convert the
number to a floating point number, e.g.,
fplus[1;2.3] = fplus[1.0;2.3] = 3.3; if given a non-numeric argument,
they generate an error, NON-NUMERIC ARG.
The largest floating point number (in INTERLISP-10) is 1.7014118E38, the
smallest positive (non-zero) floating point number is 1.4693679E-39.
The procedure on overflow is the same as for integer arithmetic. For
underflow, i.e., trying to create a number of too small a magnitude, the
value will be 0.
fplus[x1;x2;...xn] x1 + x2 + ... + xn
fminus[x] - x
fdifference[x;y] x - y
ftimes[x1;x2;...;xn] x1 * x2 * ... * xn
fremainder[x;y] the remainder when x is divided by y, e.g.,
minusp[x] T, if x is negative; NIL otherwise. Works for
both integers and floating point numbers.
eqp[x;y] T, if x and y are eq, or equal numbers. See
discussion page 13.3.
fgtp[x;y] T, if x > y, NIL otherwise.
floatp[x] is x if x is a floating point number; NIL
otherwise. Does not give an error if x is not a
Note that if numberp[x] is true, then either fixp[x] or floatp[x] is
float[x] Converts x to a floating point number, e.g.,
float = 0.0.
13.3 Mixed Arithmetic
The functions in this section are "contagious floating point arithmetic"
functions, i.e., if any of the arguments are floating point numbers,
they act exactly like floating point functions, and float all arguments,
and return a floating point number as their value. Otherwise, they act
like the integer functions. If given a non-numeric argument, they
generate an error, NON-NUMERIC ARG.
plus[x1;x2;...;xn] x1 + x2 + ... + xn
minus[x] - x
difference[x;y] x - y
times[x1;x2;...;xn] x1 * x2 * ... * xn
quotient[x;y] if x and y are both integers, value is
iquotient[x;y], otherwise fquotient[x;y].
remainder[x;y] if x and y are both integers, value is
iremainder[x;y], otherwise fremainder[x;y].
greaterp[x;y] T, if x > y, NIL otherwise.
lessp[x;y] T if x < y, NIL otherwise.
abs[x] x if x > 0, otherwise -x. abs uses greaterp and
minus, (not igreaterp and iminus).
13.4 Special Functions
In INTERLISP-10, these functions were implemented by J. W. Goodwin
by "borrowing" the corresponding routines from the FORTRAN library,
and hand coding them in INTERLISP-10 via ASSEMBLE.
They utilize a power series expansion and their values are (supposed to
be) 27 bits accurate, e.g., sin=.5 exactly.
expt[m;n] value is m^n. If m is an integer and n is a
positive integer, value is an integer, e.g,
expt[3;4]=81, otherwise the value is a floating
point number. If m is negative and n
fractional, an error is generated.
sqrt[n] value is a square root of n as a floating point
number. n may be fixed or floating point.
Generates an error if n is negative. sqrt[n] is
about twice as fast as expt[n;.5]
log[x] value is natural logarithm of x as a floating
point number. x can be integer or floating
antilog[x] value is floating point number whose logarithm
is x. x can be integer or floating point, e.g.,
antilog = e = 2.71828...
sin[x;radiansflg] x in degrees unless radiansflg=T. Value is sine
of x as a floating point number.
cos[x;radiansflg] Similar to sin.
tan[x;radiansflg] Similar to sin.
arcsin[x;radiansflg] x is a number between -1 and 1 (or an error is
generated). The value of arcsin is a floating
point number, and is in degrees unless
radiansflg=T. In other words, if
arcsin[x;radiansflg]=z then sin[z;radiansflg]=x.
The range of the value of arcsin is -90 to +90
for degrees, -A/2 to A/2 for radians.
arccos[x;radiansflg] Similar to arcsin. Range is 0 to 180, 0 to A.
arctan[x;radiansflg] Similar to arcsin. Range is 0 to 180, 0 to A.
rand[lower;upper] Value is a pseudo-random number between lower
and upper inclusive, i.e., rand can be used to
generate a sequence of random numbers. If both
limits are integers, the value of rand is an
integer, otherwise it is a floating point
number. The algorithm is completely
deterministic, i.e., given the same initial
state, rand produces the same sequence of
values. The internal state of rand is
initialized using the function randset described
below, and is stored on the free variable
randset[x] Value is internal state of rand after randset
has finished operating, as a dotted pair of two
integers. If x=NIL, value is current state. If
x=T, randstate is initialized using the clocks.
Otherwise, x is interpreted as a previous
internal state, i.e., a value of randset, and is
used to reset randstate. For example,
1. (SETQ OLDSTATE (RANDSET))
2. Use rand to generate some random numbers.
3. (RANDSET OLDSTATE)
4. rand will generate same sequence as in 2.
13.5 Reusing Boxed Numbers in INTERLISP-10 - SETN
rplaca and rplacd provide a way of cannibalizing list structure for
reuse in order to avoid making new structure and causing garbage
collections. This section describes an analogous function in
INTERLISP-10 for large integers and floating point numbers, setn. setn
is used like setq, i.e., its first argument is considered as quoted, its
second is evaluated. If the current value of the variable being set is
a large integer or floating point number, the new value is deposited
into that word in number storage, i.e., no new storage is used. If the
current value is not a large integer or floating point number, e.g., it
can be NIL, setn operates exactly like setq, i.e., the large integer or
floating point number is boxed, and the variable is set. This
eliminates initialization of the variable.
setn will work interpretively, i.e., reuse a word in number storage, but
will not yield any savings of storage because the boxing of the second
argument will still take place, when it is evaluated. The elimination
of a box is achieved only when the call to setn is compiled, since setn
compiles open, and does not perform the box if the old value of the
variable can be reused.
This technique is frowned upon except in well-defined, localized
situations where efficiency is paramount.
The second argument to setn must always be a number or a NON-NUMERIC
ARG error is generated.
Caveats concerning use of SETN
There are three situations to watch out for when using setn. The first
occurs when the same variable is being used for floating point numbers
and large integers. If the current value of the variable is a floating
point number, and it is reset to a large integer, via setn, the large
integer is simply deposited into a word in floating point number
storage, and hence will be interpreted as a floating point number.
_(SETQ FOO 2.3)
_(SETN FOO 10000)
Similarly, if the current value is a large integer, and the new value is
a floating point number, equally strange results occur.
The second situation occurs when a setn variable is reset from a large
integer to a small integer. In this case, the small integer is simply
deposited into large integer storage. It will then print correctly, and
function arithmetically correctly, but it is not a small integer, and
hence will not be eq to another integer of the same value, e.g.,
_(SETQ FOO 10000)
_(SETN FOO 1)
_(IPLUS FOO 5)
_(EQ FOO 1)
In particular, note that zerop will return NIL even if the variable is
equal to 0. Thus a program which begins with FOO set to a large integer
and counts it down by (SETN FOO (SUB1 FOO)) must terminate with
(EQP FOO 0), not (ZEROP FOO).
Finally, the third situation to watch out for occurs when you want to
save the current value of a setn variable for later use. For example,
if FOO is being used by setn, and the user wants to save its current
value on FIE, (SETQ FOO FIE) is not sufficent, since the next setn on
FOO will also change FIE, because its changes the word in number storage
pointed to by FOO, and hence pointed to by FIE. The number must be
copied, e.g., (SETQ FIE (IPLUS FOO)), which sets FIE to a new word in
setn[var;x] nlambda function like setq. var is quoted, x is
evaluated, and its value must be a number. var
will be set to this number. If the current
value of var is a large integer or floating
point number, that word in number storage is
cannibalized. The value of setn is the (new)
value of var.
13.6 Box and Unbox in INTERLISP-10
Some applications may require that a user program explicitly perform the
boxing and unboxing operations that are usually implicit (and invisible)
to most programs. The functions that perform these operations are loc
and vag respectively. For example, if a user program executes a TENEX
JSYS using the ASSEMBLE directive, the value of the ASSEMBLE expression
will have to be boxed to be used arithmetically, e.g.,
(IPLUS X (LOC (ASSEMBLE --))). It must be emphasized that
Arbitrary unboxed numbers should not be passed around as ordinary values
because they can cause trouble for the garbage collector.
For example, suppose the value of x were 150000, and you created
(VAG X), and this just happened to be an address on the free storage
list. The next garbage collection could be disastrous. For this
reason, the function vag must be used with extreme caution when its
argument's range is not known.
loc is the inverse of vag. It takes an address, i.e., a 36 bit
quantity, and treats it as a number and boxes it. For example, loc of
an atom, e.g., (LOC (QUOTE FOO)), treats the atom as a 36 bit quantity,
and makes a number out of it. If the address of the atom FOO were
125000, (LOC (QUOTE FOO)) would be 125000, i.e., the location of FOO.
It is for this reason that the box operation is called loc, which is
short for location.
Note that FOO does not print as #364110 (125000 in octal) because the
print routine recognizes that it is an atom, and therefore prints it in
a special way, i.e., by printing the individual characters that comprise
it. Thus (VAG 125000) would print as FOO, and would in fact be FOO.
loc[x] Makes a number out of x, i.e., returns the
location of x.
vag[x] The inverse of loc. x must be a number; the
value of vag is the unbox of x.
The compiler eliminates extra vag's and loc's for example
(IPLUS X (LOC (ASSEMBLE --))) will not box the value of the ASSEMBLE,
and then unbox it for the addition.
vag is an abbreviation of value get.