Radio Shack TRS-80 Model 100 Basic Manual page 97

The first time through the loop, therefore, SX will be replaced with the sum of zero,
the initial value stored in the variable SX, and one (the first period number).
Similarly, XX will be replaced with the sum of zero and one times one. The variable
XX now has the value one. The variable SY is replaced with the sum of zero and the
first
period'~
sales (103 in the example above), and the variable XY is replaced with
the sum of zero and one times the first period's sales (again, 103 in the example).
Table 7-1 below illustrates how the four variables SX, XX, SY and XY sum up the
appropriate quantities on each repetition of the loop:
Loop cycle X
Y X*X
X*Y
SX XX SY XY
0 0 0
0 0 0 0 0 0
1
I 103 1 103 1 1 103 103
2 2
110 4 220 3 5 213 323
3 3 109 9 327 6 14 322 650
4 4 120
16 480 10 30 442 1130
5 5 119 25
595
15
55 561 1725
6 6 133 36 798 21 91
694 2523
Table 7-1. Sales Trend Line Calculations
Line 80 The NEXT X statement determines the end of the loop of statements which
are to be repeated with successive values for the index variable X. So long as the
value of X is less than the upper limit specified in the matching FOR X statement, the
loop will start over again with the next value for X.
When X reaches its upper limit (N in this case), execution transfers to the statement
immediately following the NEXT X statement (line 90 in this case).
There must be a NEXT statement to match every FOR statement in a program; without
a NEXT :statement, there would be no way to determine the end of the FOR loop. and
therefore no way to know when to repeat the loop.
Lines 90 . 100 The assignment statements compute the slope, B, and intercept, A,
which define the straight line which best fits the sales data. The use of parentheses
was necessary in the expression to properly compute the ratios:
B=
N *XY
SX
*
SY
N<XX
SX
*
SX
and
A=
SY
B
*
SX
N
Using the sample data entered above, A and B are computed as:
and
B
6
*
2523 21
*
694
21
*
21
91
5.37143
) '