SIMSCRIPT II.5 Programming Language

Programming Language Concepts
2.8 Nested DO Loops
As shown above, a number of for statements, for instance, may be nested to provide complex control
over the repetition of a do...loop. Do...loops may also be nested within one another.
This proves convenient when processing variables with multiple subscripts. Consider a program
fragment that computes the row and column sums of a two-dimensional matrix. The sums are accumulated
in the appropriate elements of two one-dimensional arrays:
for I = 1 to NROWS
do
for J = 1 to NCOLS
do
add MATRIX(I,J) to ROWSUM(I)
add MATRIX(I,J) to COLSUM(J)
loop
loop
The do...loop controlled by the phrase for I = 1 to NROWS is termed the outer loop. Recall
that all the statements between the bounds of the do and the matching loop statements are repeated
for each value of the control variable. Thus, the inner do loop is repeated as each row of the matrix
is indexed. Clearly, the inner loop indexes, for any one row, each matrix element within that row.
When do...loops are nested, each do should be paired with a matching loop as shown. Adopting
a convention of indenting the controlled statements makes this clear. Where nested loops terminate
on successive loop statements, however, a special construct, similar to the then if
construct, may be used. When also is prefixed to a for phrase, SIMSCRIPT II.5 automatically
pairs the do that follows with the loop that matches the do of the preceding for statement. Using
this statement, the above example can be written as:
for I = 1 to NROWS
do
also for J = 1 to NCOLS
do
add MATRIX(I,J) to ROWSUM(I)
add MATRIX(I,J) to COLSUM(J)
loop
The do statements themselves need not be immediately adjacent, but they must terminate on adjacent
loop statements. Statements to be repeated only under the control of the outer loop must appear
after the first do and preceding the also. Care should be taken, when using the also for
construct, not to obscure the logical intent of the repetition. To this end, the example above retains
the indenting of the earlier example.
2.9 The Structure of a SIMSCRIPT II.5 Program
In the previous discussion, a program has been understood to contain a number of instruction statements,
possibly preceded by some variable declaration statements in the preamble section of the
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