An Algol 68 program manipulates values. A value is characterised by its mode. A mode is indicated by a mode indicant. Plain values can be denoted. Values occur in contexts, and can sometimes be coerced into values of different modes. Identifiers can be linked to values using identity declarations. The values manipulated by a program are called internal values. External values are data used by, or produced by, a program. Comments describe a program, but add nothing to its elaboration.
Finally, here are some exercises which test you on concepts you have met in this chapter.
1.15 Give denotations of the following
values: Ans
1.16 Is there anything wrong with the following
mode indicants?
Ans
C H A R
INT.CHAR
THISISANEXTREMELYLONGMODEINDICANT
2CHAR
1.17 Write suitable identity declarations for
the following identifiers:
Ans
fifty five
(b)
three times two point seven
colon
1.18 Is there anything wrong with the following identity declarations?
REAL x = 1.234, y = x;
1.19 What is the difference in meaning
between 0
and 0.0
?
Ans
Write a program containing print
phrases to print the
following values on your screen, separated by one space between each
value:
0.5 "G" 1 ":" 34000000