Lesson #7 Sales Trend - Radio Shack TRS-80 Model 100 Basic Manual

Lesson #7 Sales Trend
In this Lesson, you will learn how to create a program loop with a predetermined
number of repetitions. This is a useful technique when combined with subscripted
variables, which will also be introduced in this lesson.
Experiment #1 Sales Trend
The program below is a "Sales Trend" program. Its purpose is to help predict future
sales based upon the trend of previous sales. The concept underlining this program is
to find a straight line which best fits the historical data and then to project this line
into the future.
Clear working memory with the NEW command and enter the following program:
10
CLS
20
INPUT "NUMBER OF PERIODS"; N
30
FOR X =
1
TO N
110
PRINT "SALES FOR PERIOO";X;
50
INPUT Y
60
SX = SX
+
X:XX
=
XX
+
X*X
70
SY
=
SY
+
Y:XY
=
XY
+
X*Y
80
NEXT X
80
B
;;;
(N*XY
-
SX*SY)
/
(N*XX
-
SX*SX)
100
A
;;;
(SY -
B*SX)
/
N
110
PRINT "FORECAST FOR PERIOD X IS"
120
PRINT A; "
+
II ;
B; ..
* X"
Execute this program.
The program begins by asking you to enter the number of periods of historical sales
data. Type 6 and press (ENTER).
The program then prompts you for the sales data for each of the six time periods.
Enter the following sales data:
SALES FOR PERIOD
17 103
SALES FOR PERIOD
27 110
SALES FOR PERIOD
37 108
SALES FOR PERIOD
117
120
SALES FOR PERIOD
57 118
SALES FOR PERIOD
6? 133
The formula for a straight line is:
Y
=
A
+
B*X
where Y stands for Sales and X for the time period number. A is called the
"intercept" and B the "slope" of the line. The program uses the sales data which
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