Expressions and Assignment Statements Chapter 7 Expressions and Assignment Statements

Chapter 7 Topics Introduction Arithmetic Expressions Overloaded Operators Type Conversions Relational and Boolean Expressions Short-Circuit Evaluation Assignment Statements Mixed-Mode Assignment Copyright © 2006 Addison-Wesley. All rights reserved. 2

Introduction Expressions are the fundamental means of specifying computations in a programming language Expressions are motivation for early language dominant role: assignment imperative language must understand: order of operators operand evaluation Copyright © 2006 Addison-Wesley. All rights reserved.

Arithmetic Expressions: Design Issues Design issues for arithmetic expressions operator precedence rules operator associativity rules order of operand evaluation operand evaluation side effects operator overloading mode mixing expressions more interesting Copyright © 2006 Addison-Wesley. All rights reserved.

Arithmetic Expressions: Operators infix - most prefix - Scheme unary (1 operand) binary (2 operands) ternary (3 operands) operators Typical associativity rules Left to right, except assignment and **, which are right to left Sometimes unary operators associate right to left (e.g., in FORTRAN and C-based, ++ and --, also =) typical precedence: parentheses unary operators ** (if the language supports it, Fortran, Ruby, VB and Ada do) *, /, % +, - APL is different; all operators have equal precedence and all operators associate right to left, precedence and associativity rules can be overridden with parentheses would this be more or less confusing? Copyright © 2006 Addison-Wesley. All rights reserved.

Arithmetic Expressions: Conditional Expressions C-based languages (e.g., C, C++) Also in Perl, JavaScript and Ruby An example: average = (count == 0)? 0 : sum / count Evaluates as if written like if (count == 0) average = 0 else average = sum /count Copyright © 2006 Addison-Wesley. All rights reserved.

Arithmetic Expressions: Potentials for Side Effects Functional side effects: when a function changes a two-way parameter or a non-local variable Problem with functional side effects: When a function referenced in an expression alters another operand of the same expression; e.g., for a parameter change: a = 10; b = a + fun(a); void fun(int& a) { a = 15; } What value will b have after the above statements are executed? https://www.securecoding.cert.org/ The order in which operands in an expression are evaluated is unspecified in C++. The only guarantee is that they will all be completely evaluated at the next sequence point. Copyright © 2006 Addison-Wesley. All rights reserved.

Functional Side Effects Two possible solutions to the problem Write the language definition to disallow functional side effects No two-way parameters in functions No non-local references in functions Advantage: it works! Disadvantage: inflexibility Write the language definition to demand that operand evaluation order be fixed Disadvantage: limits some compiler optimizations* * compilers draw graphs to detect when values are created/modified Copyright © 2006 Addison-Wesley. All rights reserved.

Referential Transparency A program has the property of referential transparency if any two expressions in the program with same value can be substituted for one another anywhere, without affecting the action of the program. result1 = (fun(a) + b / (fun(a) – c); temp = fun(a); result2 = (temp + b) / (temp – c); If fun has no side effects, result1 and result2 are equal. If fun modifies b or c, then not equal, violates referential transparency. Functional languages are referentially transparent, easier to reason about, more like math functions. Copyright © 2006 Addison-Wesley. All rights reserved.

Overloaded Operators & as address vs bitwise AND ptr = &y; if (x & y == 0) … - unary vs binary z = -x + y; z = x – -y; Integer vs floating point division avg = sum/count; Pascal avoided: avg = sum div count; would this be a good idea in C++/Java? Copyright © 2006 Addison-Wesley. All rights reserved.

Overloaded Operators (continued) C++, Ruby, Ada and C# allow user-defined overloaded operators Overloading was not included in Java Potential problems: Users can define nonsense operations Readability may suffer, even when the operators make sense Should it have been? Copyright © 2006 Addison-Wesley. All rights reserved.

Type Conversions Widening Conversions: can include at least approximations to all of the values of the original type byte to short, int, long, float or double short to int, long, float or double char to int, long, float or double int to long, float or double long to float or double float to double Narrowing conversions: cannot include all of the values of the original type short to byte or char char to byte or short int to byte, short, or char long to byte, short, char or int float to byte, short, char, int or long double to byte, short, char, int, long or float Copyright © 2006 Addison-Wesley. All rights reserved.

Type conversions – dangerous? Even widening conversions may lose accuracy. Example: integers stored in 32 bits, 9 digits of precision. Floating point values also stored in 32 bits, only about 7 digits of precision (because of space used for exponent). Conversions should be used with care! Warnings should not just be ignored… Copyright © 2006 Addison-Wesley. All rights reserved.

Type Conversions: Mixed Mode A mixed-mode expression is one that has operands of different types A coercion is an implicit type conversion Disadvantage of coercions: They decrease the type error detection ability of the compiler In most languages, all numeric types are coerced in expressions using widening conversions In Ada, there are virtually no coercions in expressions Ada is strongly typed Copyright © 2006 Addison-Wesley. All rights reserved.

Explicit Type Conversions Called casting in C-based language Examples C: (int) angle Ada: Float (sum) Note that Ada’s syntax is similar to function calls – may be better because casting does have associated code (convert bit patterns) Copyright © 2006 Addison-Wesley. All rights reserved.

Relational and Boolean Expressions Relational Expressions Use relational operators and operands of various types Evaluate to some Boolean representation .NE. /= How do you say “not equal”? # != <> Do you want to coerce that? !== === eql? Leave me like you find me! (JavaScript/Perl) Ruby Copyright © 2006 Addison-Wesley. All rights reserved.

Relational and Boolean Expressions Operands are Boolean and the result is Boolean AND and OR have equal precedence in some languages, not others (AND higher in C++) FORTRAN 77 FORTRAN 90 C Ada .AND. and && and .OR. or || or .NOT. not ! not xor Copyright © 2006 Addison-Wesley. All rights reserved.

Relational and Boolean Expressions - review C has no Boolean type (0=false, >0 = true) SO: a < b < c is a legal expression but is the result what you want???? (a < b) => produces 0 or 1 (0 < c) OR (1 < c) Who cares whether 0 < c ??? Some error detection is lost if no Boolean type How is a < b < c evaluated by compiler in Java? Copyright © 2006 Addison-Wesley. All rights reserved.

Short Circuit Evaluation - review An expression in which the result is determined without evaluating all of the operands and/or operators (a >= 0) && (b < 10) Problem with non-short-circuit evaluation: index = 0; while ((index < length) && (LIST[index] != value)) index++; When index=length, LIST [index] will cause an indexing problem (assuming last index of LIST is length - 1) Copyright © 2006 Addison-Wesley. All rights reserved.

Short Circuit Evaluation (continued) 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 |) Ruby, Perl and Python are short-circuit evaluated Short-circuit is a choice in Ada Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3) will b be incremented or not? Copyright © 2006 Addison-Wesley. All rights reserved.

Assignment Statements The general syntax <target_var> <assign_operator> <expression> The assignment operator = FORTRAN, BASIC, PL/I, C, C++, Java := ALGOLs, Pascal, Ada = can be bad when it is overloaded for the relational operator for equality confusing for beginners even when it’s not overloaded Copyright © 2006 Addison-Wesley. All rights reserved.

Assignment Statements: Conditional Targets Conditional targets (C, C++, and Java) (flag)? total : subtotal = 0 Which is equivalent to if (flag) total = 0 else subtotal = 0 Copyright © 2006 Addison-Wesley. All rights reserved.

Assignment Statements: Shorthand Syntax Adding two values: a = a + b is written as a += b Introduced in ALGOL; adopted by C, Perl, JavaScript, Python and Ruby Incrementing or decrementing: int count=4; int sum=++count; // increment then use int count2=4; int sum2=count2++; // use then increment cout << sum << " " << sum2 << endl; // 5 4 count++ (count incremented) -count++ (count incremented then negated) Included in C-based languages, Perl and JavaScript Copyright © 2006 Addison-Wesley. All rights reserved.

Assignment as an Expression In C, C++, Java, Perl and JavaScript the assignment statement produces a result and can be used as operand An example: while ((ch = getchar())!= EOF){…} ch = getchar() is carried out; the result (assigned to ch) is used as a conditional value for the while statement; must be in (); can be harder to read Copyright © 2006 Addison-Wesley. All rights reserved.

List Assignments Some recent languages like Perl and Ruby provide multiple-target, multiple-source assignments In Perl: ($first, $second, $third) = (20, 30, 30); ($first, $second) = ($second, $first); excess values on right ignored excess values on left set to undef How useful is this feature? Copyright © 2006 Addison-Wesley. All rights reserved.

Mixed-Mode Assignment Assignment statements can also be mixed-mode, for example int a, b; float c; c = a / b; Coercion takes place after rhs evaluated Fortran, C, C++ and Perl use coercion rules for mixed mode assignment In Java, only widening assignment coercions are done (increases reliability) In Ada, there is no assignment coercion Copyright © 2006 Addison-Wesley. All rights reserved.