Exhibit 8.12. Lazy evaluation of OR. Value of L T T F F Value of R T F T F Value of L OR R T T T F in .NET Generation datamatrix 2d barcode in .NET Exhibit 8.12. Lazy evaluation of OR. Value of L T T F F Value of R T F T F Value of L OR R T T T F

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Exhibit 8.12. Lazy evaluation of OR. Value of L T T F F Value of R T F T F Value of L OR R T T T F using visual studio .net toattach gs1 datamatrix barcode in web,windows application VS.NET We see that if L is TRUE, the .NET Data Matrix value of the expression is always TRUE, and R does not need to be evaluated..


13. Extra code needed with inside-out evaluation. TYPE array_type = array[0.

.100] of char; FUNCTION search (a:array_type, key:char):integer; VAR done_flag: boolean; scan: -1 ..

101; (* An integer in the range -1 to 101. *) *). BEGIN done := FALSE; (* Flag VS .NET Data Matrix barcode to control loop exit. scan := 0; WHILE ( scan < 101) and not done DO IF a[scan] = key THEN done := TRUE (* Leave loop next time.

*) ELSE scan := scan + 1; IF scan = 101 THEN search := -1 ELSE search := scan; END. performing the WHILE test. To code this function in Pascal, the two tests must be written separately. An extra boolean variable is then introduced to e ect exit from the loop when the second condition becomes true.

We see that the Pascal version requires an extra control statement and makes exit from a simple loop awkward. Compare this to the parallel form in Exhibit 8.14 written in C.

C uses lazy evaluation for Boolean expressions, with arguments evaluated in a left-to-right order. Further evaluation is aborted as soon as the outcome of the expression is determined. The expression that controls the while statement can, therefore, encompass both the subscript-out-of-range test and the search test.

The error condition is tested rst. If the subscript is too large, the rest of the expression (including the part that would malfunction) is not evaluated. This greatly simpli es construction of the loop.

. Exhibit 8.14. Simpler control visual .

net Data Matrix barcode structures with outside-in evaluation. int search (char a[], char key); { int scan; scan = 0; while ( scan < 101 && a[scan] != key ) ++scan; if (scan == 101) return -1; else return scan; }. CHAPTER 8. EXPRESSIONS AND EVALUATION Exhibit 8.15. Expressions who .

NET ECC200 se values are indeterminate in C. int x, z, a[10]; int throwaway (int b, int c) { return c }; x = 3; x = 3; x = 3; z = x * ++x; z = throwaway(++x, x); a[x] = x++; /* z could be 9 or 12. */ /* z could be 3 or 4.

*/ /* 3 is stored in either a[3] or a[4].. Order of Evaluation of Arguments One nal design decision is w DataMatrix for .NET hether to specify that arguments are evaluated left-to-right or rightto-left, or leave the order unspeci ed. This makes no di erence when inside-out evaluation is used and the subexpressions have no side e ects.

But when expressions can produce output or modify memory, the order of evaluation can determine the outcome of the program. In the modern functional languages, this order is unspeci ed. These languages do not implement destructive assignment, and, therefore, there is no problem with side e ects except where output is concerned.

For these situations, the languages provide a way, called strict evaluation, to specify an ordered evaluation of all subexpressions. In languages such as Pascal where functions and operators cannot have side e ects, there is also no problem. All side e ects (assignment, input, output) are restricted to statements, whose order is clearly speci ed.

It is impossible to tell whether a Pascal compiler evaluates expressions left-to-right or right-to-left. Finally, there are languages such as C and APL where expressions can have side e ects. Assignment, input, and output are all expressions in these languages.

In addition, C has an increment operator. The language de nition in these cases must clearly specify what the evaluation order is. APL does specify right-to-left.

In C, though, the decision was left to the language implementor, and both right-to-left and left-to-right evaluation are permitted and considered to conform to the standard. It is, therefore, incumbent upon a C programmer to avoid using any variable in an expression whose value is changed by a preceding or following subexpression with a side e ect [Exhibit 8.15].

The result of such an expression is called indeterminate; that is, it may vary from compiler to compiler, even on the same hardware..
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