Loose endsCS-2301, B-Term “Loose Ends” CS-2301, System Programming for Non-Majors (Slides include materials from The C Programming Language, 2 nd edition, by Kernighan and Ritchie and from C: How to Program, 5 th and 6 th editions, by Deitel and Deitel)

Loose endsCS-2301, B-Term Reminder – Relational Operators Relational operators –, >=, ==, != –Return 0 if false, 1 if true Let double pi = ; Then pi < 3 returns 0 pi <= 4 returns 1 pi == 3 returns 0

Loose endsCS-2301, B-Term Logical AND and OR operators A > B && C < D is true (i.e., has value 1 ) if and only if both A > B and C < D are both true A > B || C < D is true (i.e., has value 1 ) if either of A > B and C < D is true

Loose endsCS-2301, B-Term So what is the Difference … … between '|' and '||' ? … between '&' and '&&' ? Answer:– –'|' and '&' are bitwise operators Will evaluate both operands before doing bitwise arithmetic –'||' and '&&' are logical operators Will only evaluate one operand, if answer is known after first operand!

Loose endsCS-2301, B-Term Logical AND and OR operators (continued) expr 1 && expr 2 has value 1 if and only if both expr 1 and expr 2 are non-zero Otherwise, it has the value 0 expr 1 || expr 2 has value 1 if either expr 1 and expr 2 is non-zero

Loose endsCS-2301, B-Term Special Property of Logical Operators Logical operators are evaluated left-to-right Including side effects When the result is known, evaluation stops! E.g.:– in expr 1 && expr 2 –expr 1 is evaluated first –If result is zero, the && expression returns 0 expr 2 is not evaluated! –If result is non-zero, then expr 2 is evaluated If expr 2 is zero, && expression returns 0 Otherwise, && expression returns 1

Loose endsCS-2301, B-Term Special Property (continued) In expr 1 || expr 2 –expr 1 is evaluated first –If result is non zero, || expression returns 1 expr 2 is not evaluated! –If result is zero, then expr 2 is evaluated If expr 2 is non-zero, || expression returns 1 Otherwise, || expression returns 0

Loose endsCS-2301, B-Term Why this Special Property? if (n != 0 && x/n > y) {…} –Don’t want to attempt x/n if n is zero  zero- divide fault! Many similar situations in C, both && and || –E.g., following pointers, indexing arrays, etc. –expr 1 checks a limit situation, and then expr 2 tests what you really want to know.

Loose endsCS-2301, B-Term Questions?

Loose endsCS-2301, B-Term Scope Definition:– Scope of an identifier –The region of a program where an identifier is known –Compiler reports error on use of an identifier outside of its scope –An identifier may be used in different ways in different scopes of the same program –An identifier may be redefined for a different purpose in an inner scope

Loose endsCS-2301, B-Term Possible Scopes of Identifiers The entire program (i.e.,.c file) –Starts at point of declaration and continues to end A function –Starts with function name, continues to end of function (i.e., '}' ), includes parameters A compound statement –Starting at point of declaration and continuing to '}' for loop –Expressions and statement (C99) A function prototype –Limited to the prototype itself

Loose endsCS-2301, B-Term Scope – Examples int n; void func(int m){ int k; { double j; …; } }//func Visible throughout program (from this point on) m visible throughout function k visible throughout function (from this point on) j visible only within '{}' (from this point on)

Loose endsCS-2301, B-Term Scope – Name Spaces Different kinds of identifiers have separate name spaces Do not conflict with identifiers of other spaces E.g., Function and variable can share same name in same scope! Stylistically, this is a really, really bad idea!

Loose endsCS-2301, B-Term Scope – Re-using Identifiers Any identifier can be freely re-used in a separate scope Very common E.g., for (int i; …; …) — separate i variables! An identifier can be re-defined in an inner scope Happens often enough – e.g., int i;... for (int i; …; …) { … i … }; Refers to i of inner scope i of outer scope becomes inaccessible in inner scope

Loose endsCS-2301, B-Term Questions?

Loose endsCS-2301, B-Term Order of Evaluation Operator Precedence rules specify the order of operations … … but –don’t specify which operand is evaluated first z = f(x) + g(y) – side effects make a difference! –don’t specify which argument is evaluated first h(f(x), g(y)) – side effects make a difference! –Note: ',' specifies order in expressions, not argument lists!

Loose endsCS-2301, B-Term Questions?