IBM 1620 User Manual

FORTRAN Arithmetic
Functions
If an arithmetic statement is in mixed mode, the mode of the variable on the
left side of the equal sign determines the mode of the result.
If the variable on the left is in fixed point and the expression on the right is
in floating point, the expression will first be evaluated in floating point, the portion
following the decimal point will be dropped, and the remainder will be converted
to fixed
point by retaining only the four digits immediately to the left of the decimal.
If a result is
123456.78
the fixed point quantity stored is 3456.
If the variable on the left is in floating point and the expression on the right
is in fixed point, the expression will be evaluated in fixed point and the result con-
verted to floating point. For example,
Arithmetic
Statement
A = 5/3
A = 5./3.
I = 5/3
I = 3./2.
I = 123456.78/4.
Result
A=1.
A = 1.6666666
I = 1
I
= 1
I = 864 (was computed as 30864.195)
If your problems are programmed in floating point rather than in fixed point, you
will find
it is easier to process fractions because you will not have the problem of
locating decimal points.
If a particular problem that you are programming requires
the use of fixed point quantities, you must understand exactly how fixed point
arithmetic is accomplished.
In fixed point calculations, if the result is not an integer (whole number) the
result is truncated to a whole number. That is, the fractional portion of the result
is discarded,
and no rounding takes place.
The fixed point division 5/3 is 1, not 2. Therefore, if you write an expression
with a series of operations that includes a division, you must be careful when
grouping. For example,
A = 5./3. ~< 4.
In floating point, 5 divided by .3 equals 1.6666666, and this value multiplied
by 4 equals 6.6666664.
If this arithmetic statement is written in fixed point,
I = 5/3 * 4
then 5 divided by .3 equals 1.6, which is truncated to 1. The 1 is multiplied by 4
and the answer is 4.
If you had reversed the grouping in the statement,
I = 4 *5/3
the result would be 6. Remember, in a statement with a series of multiplications
and divisions where the parentheses have been omitted the operations are per-
formed from left to right.
Assume
that you are writing a FORTRAN program that requires taking the square
root of a number at different locations in the program. The statements to perform
the sql1are root would be identical, except for the different arguments used each
time.
Writing the 1620 FORTRAN Program 21

IBM 1620 Specifications

General

Brand	IBM
Model	1620
Category	Desktop
Language	English