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Chapter 7 - Control I: Expressions and Statements

Chapter 7 - Control I: Expressions and Statements. "Control" is the general study of the semantics of execution paths through code: what gets executed, when, and in what order. Expressions.

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Chapter 7 - Control I: Expressions and Statements

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  1. Chapter 7 - Control I: Expressions and Statements "Control" is the general study of the semantics of execution paths through code: what gets executed, when, and in what order. Louden, Programming Languages

  2. Expressions • In their purest form, expressions do not involve control issues: subexpressions can be evaluated in arbitrary order, and the order does not affect the result. Functional programming tries to achieve this goal for whole programs. • If expressions could have arbitrary evaluation order, programs would become non-deterministic: any of a number of different outcomes might be possible. Louden, Programming Languages

  3. Side Effects • A side effect is any observable change to memory, input or output. • A program without any side effect is useless. • Side effects expose evaluation order:class Order{ static int x = 1; public static intgetX() { return x++; } public static void main(String[] args) { System.out.println( x+getX() ); }} • This prints 2, but the corresponding C program will usually print 3! • Why? Louden, Programming Languages

  4. Strictness • An evaluation order for expressions is strict (eager) if all subexpressions of an expression are evaluated, whether or not they are needed to determine the value of the result, non-strict (lazy) otherwise. • Arithmetic is almost always strict. • Every language has at least a few non-strict expressions (?:, &&, ||). • A form of strict evaluation called applicative-order is common: "bottom-up" or "inside-out". Expressions are evaluated in the order they are encountered in evaluating the expression tree. Louden, Programming Languages

  5. Applicative Order Consider: (a+b)/(c+d)+((e-f)-g)/(h+j) – which operation is evaluated first? • Still leaves open whether left-to-right or not. Louden, Programming Languages

  6. Short Circuit Evaluation • Suppose Java did not use short-circuit evaluation • Problem: table look-up i= 1; while(i<=length)&&(LIST[i]!= value) i++; Louden, Programming Languages

  7. Short Circuit Evaluation • C, C++, and Java: use short-circuit evaluation for the usual Boolean operators (&& and ||), but also provide bitwise Boolean operators that are not short circuit (& and |) • Ada: programmer can specify either (short-circuit is specified with and then and or else) • Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3) Louden, Programming Languages

  8. Function calls • Obey evaluation rules like other expressions. • With side effects, order makes a difference. Louden, Programming Languages

  9. Case Statements • Rules • cases can be constants, constant expressions, constant ranges • no overlapping cases • error if unspecified case occurs (or may decide to do nothing if unspecfied case) • Implemented via jump table: vector stored sequentially in memory - each of whose components is an unconditional jump • Need one location in jump table for every value between range of possible values Louden, Programming Languages

  10. Switch Statements typedef enum {ADD, MULT, MINUS, DIV, MOD, BAD} op_type; char unparse_symbol(op_type op) { switch (op) { case ADD : return '+'; case MULT: return '*'; case MINUS: return '-'; case DIV: return '/'; case MOD: return '%'; case BAD: return '?'; } } • Implementation Options • Series of conditionals • Good if few cases • Slow if many • Jump Table • Lookup branch target • Avoids conditionals • Possible when cases are small integer constants • Flexible • Picks one based on case structure Louden, Programming Languages

  11. Targ0: Code Block 0 jtab: Targ0 Targ1 Targ2 Targ1: Code Block 1 • • • Targn-1 Targ2: Code Block 2 • • • Targn-1: Code Block n–1 Jump Table Structure Jump Targets Switch Form Jump Table switch(op) { case 0: Block 0 case 1: Block 1 • • • case n-1: Blockn–1 } Code Generated target = JTab[op]; goto *target; Louden, Programming Languages

  12. A jump table is an array of code addresses: • Tbl[ i ] is the address of the code to execute if the expression evaluates to i. • if the set of case labels have “holes”, the correspond jump table entries point to the default case. • Bounds checks: • Before indexing into a jump table, we must check that the expression value is within the proper bounds (if not, jump to the default case). • The check lower_bound  exp_value  upper bound can be implemented using a single unsigned comparison. Louden, Programming Languages

  13. Jump Table Space Costs • jump tables • best for large no. of case labels ( 8) • may take a large amount of space if the labels are not well-clustered. • A jump table with max. and min. case labels cmax and cmin needs cmax – cmin entries. This can be wasteful if the entries aren’t “dense enough”, e.g.: switch (x) { case 1: … case 1000: … case 1000000: … } • Define the density of a set of case labels as density = number of case labels/(cmax – cmin ) • Compilers will not generate a jump table if density below some threshold (typically, 0.5). Louden, Programming Languages

  14. Use of Switch Statements • if number of case labels is small ( ~ 8), use linear or binary search. • use number of case labels to decide between the two. • if density  threshold (~ 0.5) : • generate a jump table; else : • divide the set of case labels into sub-ranges such that each sub-range has density  threshold; • generate code to use binary search to choose amongst the sub-ranges; • handle each sub-range recursively. Louden, Programming Languages

  15. Guarded Commands Selection: if <boolean> -> <statement> []<boolean> -> <statement> ... [] <boolean> -> <statement> fi • Semantics: when this construct is reached, • Evaluate all boolean expressions • If more than one are true, choose one nondeterministically • If none are true, it is a runtime error Louden, Programming Languages

  16. Guarded Commands Idea: if the order of evaluation is not important, the program should not specify one In Haskell, first one that matches is used. Louden, Programming Languages

  17. Summary • Every language has three major program components: expressions, statements, and declarations. • Expressions are executed for their values (but may have side effects), and may or may not be sequenced. • Statements are executed solely for their side effects, and they must be sequenced. • Declarations define names; they can also give values to those names. They may or may not be viewed by a language as expressions or statements. Louden, Programming Languages

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