Instructor: Ioannis A. Vetsikas E-mail: vetsikas@cs.cornell.edu CS113 Introduction to C Instructor: Ioannis A. Vetsikas E-mail: vetsikas@cs.cornell.edu Lecture 3 : August 30 webpage: http://www.cs.cornell.edu/Courses/cs113/2000FA/cs113.htm

Logical Operators (additions) Unary negation operator ! Ex: if (!var) same as if (var == 0) What is if (!!var) equivalent to? Lazy evaluation (logical AND/OR): if ((x!=0) && (1/x>1)) … If x equals 0 then the whole boolean expression is false and thus (1/x>1) does not get evaluated (good since otherwise it would give a divide by zero type error) The evaluation order for && and || is guaranteed to be from left to right

Logical Operators (examples) a==1 && b!=2 || !c !(a==1 || b>=3) && c a>b == b>c

A little bit about Functions Should perform a well-defined task Why? Adds no functionality. Breaking tasks into smaller ones make them easier to think about Hiding details tends to make code less complicated, rendering it more readable Easier to debug the code as well Code can be re-used, not just within one program but in others. Recursion easier to do more on that later…

More on functions Syntax: Call as: Can also declare them [return type] <name> ([type param_name]*) e.g. int factorial(int n) e.g. void execute_loop(char c, float f) Call as: i=factorial(3); execute_loop(townInitial, distance); Can also declare them int factorial(int n); or int factorial(int); Return (w/ or w/o value): return [expr];

Example: A simple function #include <stdio.h> int max( int a, int b ); void main() { int i = 8, j = 17; printf( “Maximum of %d and %d is %d\n”, i, j, max(i, j)); } int max( int a, int b ) if( a > b ) return a; else return b;

Example 2 (Call by value) What does it print? A parameter of the function can be a constant, expression, variable etc. (anything that has a value!) Only the value is passed (not variable!) #include <stdio.h> void printDouble( int x ) { printf(“Double of %d”, x); x *= 2; printf(“is %d\n”, x); } void main() int i = 13; printDouble(i); printf(“i=%d\n”, i);

One-Dimensional Arrays Often, programs use homogeneous data. As an example, if we want to manipulate some grades, we might declare int grade0, grade1, grade2; If we have a large number of grades, it becomes cumbersome to represent/manipulate the data using unique identifiers. Arrays allow us to refer to a large number of the same data type using a single name. For instance, int grade[3];

One-Dimensional Arrays (continued) Makes available the use of integer variables grade[0], grade[1], grade[2] in a program. Declaration syntax: Type array_name[number_of_elements] WARNING: arrays are zero-indexed (numbering always starts at 0). Now, to access elements of this array, we can write grade[expr], where expr is an integral expression. Example: for( i = 0; i < 3; i++ ) sum += grade[i];

Pointers A variable in a program is stored in a certain number of bytes at a particular memory location, or address, in the machine. Pointers allow us to manipulate these addresses explicitly. Two unary operators: (“inverses”) & operator – “address of”. Can be applied to any variable. “Adds a star to type”. * operator – “information at”. Can be applied only to pointers. “Removes a star from type” Pointer when declared points to an invalid location usually; so you must make it point to a valid one.

Pointers (continued) int a = 1, b = 2, *p; void *void_p; char *char_p; p = &a; b = *p; An assignment like char_p = &a; is illegal, as the types do not match. void * is a generic pointer type; can make assignments such as void_p = char_p; or void_p = &b; void * is also the type of pointer returned by memory allocation functions (more later…)

Constructs not to be pointed at Do not point at constants: int *ptr; *ptr = 3; /* OK */ ptr = &3; /* illegal */ Do not point at arrays; an array name is a constant. int a[77]; void *ptr; ptr = a; /* OK */ ptr = &a; /* illegal */ Do not point at expressions that are not variables. int k = 1, *ptr; *ptr = k + 99; /* OK */ ptr = &(k + 99); /* illegal */ Do not point at register variables (not presented yet!) register int k=1; int *ptr; ptr = &k;

“Call by reference” (not really) Pointers allow us to perform something similar to call-by-reference (we are passing pointers/references by value) “call-by-reference” allows a function to make changes to a variable that persist void set_int_to_3( int *p ) { *p = 3; } void swap( int *p, int *q ) { int temp; temp = *p; *p = *q; *q = temp; }

Arrays and Pointers Assume int i, a[10], *p; Correspondingly, In fact, The type of a is “int *”. a is equivalent to &a[0] a + i is equivalent to &a[i] Correspondingly, a[i] is equivalent to *(a + i) In fact, p[i] is equivalent to *(p + i) for( p = a; p < &a[10]; p++ ) sum += *p; for( i = 0; i < 10; i++ ) sum += *(a + i); p = a; for( i = 0; i < 10; i++ ) sum += p[i];

Example: Arrays as Function Arguments (Array passed by reference, so changes to array persist) int change_and_sum( int a[], int size ) { int i, sum = 0; a[0] = 100; for( i = 0; i < size; i++ ) sum += a[i]; return sum; } void main() int a[5] = {0, 1, 2, 3, 4}; printf( “sum of a: %d\n”, change_and_sum( a, 5 )); printf( “value of a[0]: %d\n”, a[0] );

Arrays and Pointers (a difference) An array is in essence a pointer However: int i, a[10], *p; p=a; /* equiv. to p=&a[0]; */ p++; /* valid */ a++; /* error! */ The name of an array is not a variable, so the only operator you can apply to it is [] E.g. a[i+3]