Arrays, Strings, and Pointers CSE 2451 Rong Shi

Arrays Store many values of the same type in adjacent memory locations Declaration [ ] Examples: – int rgb[3]; – double percents[10]; – char name[20]; – int s = 4; int trythis[s]; // not valid for most compilers elements with an index/subscript number from 0 to -1

Indexing Dijkstra’s argument for zero index Address of an array is the same as its first element – min = address of min[0]

Initializing Arrays Initialize entire array using values enclosed in braces – Ex: int myArray[5] = {1,2,3,4,5}; int myArray[5] = {1,2};// valid int myArray[5] = {1,2,3,4,5,6};// invalid Initialize individual array locations – Ex: int myArray[2]; myArray[0]=14; myArray[1]=4; Array location acts like a single variable – Ex: x = myArray[0]; myArray[1]=x+y;

Array vs. Pointer Array name is a pointer constant inta[10]; intb[10]; int*c; … c = &a[0];// c points to a[0] c = a;// equivalent b = a;// invalid a = c;// invalid

Copy an array #include int main() { int iMarks[4] = {78, 64, 66, 74}; int newMarks[4]; int i,j; for(i=0; i<4; i++) newMarks[i]=iMarks[i]; for(j=0; j<4; j++) printf("%d\n", newMarks[j]); return 0; }

Shallow and Deep copying

More syntax examples intarray[10]; int*ap = array + 2; // ap points at &array[2] 1. ap 2.*ap 3.ap[0] 4.ap *ap *(ap + 6) 7.ap[6] 8.&ap 9.ap[-1] 10.ap[9]

Index checking Index access is not checked by the compiler – Check for valid range manually – Especially important for user entered indices int array[2] = {0,0}; int input; scanf(“%d”, &input); array[input];// what could possibly go wrong? Attempt to access memory outside of range allocated by OS – Segmentation fault – Access OS memory

Character arrays – strings Character array terminated with the null byte Declaration – char arr[] = {'c','o','d','e','\0'}; – char arr[] = "code"; Array size is not specified in [] Logical size of the array is one more than the number of characters in the string literal – one extra for NUL byte (i.e. the \0 character) Static allocation – size of the array is determined at compile-time

Pointers and Strings In C our string type is an array of characters Array names are pointer constants Use the same syntax char *ptr; char str[40]; ptr = str;

Arrays as function parameters Prototypes: intstrlen( char *string ); intstrlen( char string[] ); Logically, a pointer to the first element is passed copy-by-value (See array operations examples)

Multi-dimensional arrays Declarations – [row][col] subscript order int values [3] [4] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 } – Values is an array of 3 elements, each an array of 4 elements Stored in row order

Backup

Another function topic – default arguments int f1(int arg1, double arg2, char* name, char *opt); int f2(int arg1, double arg2, char* name) { return f1(arg1, arg2, name, "Some option"); }