Contiguous chunks of memory Arrays/strings: pointers and memory - PowerPoint PPT Presentation

Contiguous chunks of memory Arrays/strings: pointers and memory allocation int * a = new int[100]; ● Why not rely solely on string and vector classes? In C++ allocate using array form ● of new ➤ how are string and vector implemented? 0 1 32 33 98 99 int * a = new int[100]; ➤ lower level access can be more efficient (but be leery of double * b = new double[300]; a is a pointer claims that C-style arrays/strings required for efficiency) *a is an int ● new [] returns a pointer to a ➤ real understanding comes when more levels of abstraction a[0] is an int (same as *a) block o f memory are understood a[1] is an int ➤ how big? where? a+1 is a pointer size of chunk can be set at ● ● string and vector classes insulate programmers from a+32 is a pointer runtime, not the case with inadvertent attempts to access memory that’s not accessible *(a+1) is an int (same as a[1]) int a[100]; *(a+99) is an int cin >> howBig; ➤ what is the value of a pointer? int a[howBig]; *(a+100) is trouble ➤ what is a segmentation violation? a+100 is valid for comparison delete [] a; // storage returned ● Duke CPS 108 7.1 Duke CPS 108 7.2 C-style contiguous chunks of memory Address calculations: sizeof(…)? int * a = new int[100]; int * a = (int *) x is a pointer, what is x+33? ● In C, malloc is used to allocate ● malloc(100*sizeof(int)); ➤ a pointer, but where? memory int * a = (int *) 0 1 32 33 98 99 ➤ what does calculation 0 1 32 33 98 99 malloc(100 * sizeof(int)); a[33] is the same as *(a+33) depend on? double * d = (double *) if a is 0x00a0, then a+1 is malloc(200 * sizeof(double)); a is a pointer 0x00a4, a+2 is 0x00a8 result of adding an int to a ● *a is an int (think 160, 164, 168) pointer depends on size of ● malloc must be cast, is NOT type- a[0] is an int (same as *a) object pointed to safe (returns void *) double * d = new double[200]; a[1] is an int a+1 is a pointer ➤ void * is ‘generic’ type, can be ● result of subtracting two cast to any pointer type a+32 is a pointer 0 1 33 199 pointers is an int: *(a+1) is an int (same as a[1]) *(d+33) is the same as d[33] *(a+99) is an int free(d); // return storage ● if d is 0x00b0, then d+1 is (d + 3) - d == _______ *(a+100) is trouble 0x00b8, d+2 is 0x00c0 a+100 is valid for comparison (think 176, 184, 192) 7.3 7.4 Duke CPS 108 Duke CPS 108

More pointer arithmetic What is a C-style string? char * a = new int[44]; address one past the end of an ● array of char terminated by sentinel ‘\0’ char ● char * begin = a; array is ok for pointer char * end = a + 44; ➤ sentinel char facilitates string functions comparison only ➤ ‘\0’ is nul char, unfortunate terminology ➤ how big an array is needed for string “hello”? ● what about *(begin+44)? 0 1 15 16 42 43 while (begin < end) ● a string is a pointer to the first character just as an array is a what does begin++ mean? ● { pointer to the first element *begin = ‘z’; ➤ char * s = new char[6]; begin++; // *begin++ = ‘z’ ● how are pointers compared } using < and using == ? ➤ what is the value of s? of s[0]? ● char * string functions in <cstring> (or <string.h> ) what is value of end - begin? ● Duke CPS 108 7.5 Duke CPS 108 7.6 C style strings/string functions More string functions (from < cstring>) what about relational operators ● ● strlen is the # of characters in a what’s “wrong” with this code? strcpy copies strings ● ● <, ==, etc.? string ➤ who supplies storage? int countQs(char * s) ➤ same as # elements in char can’t overload operators for ● // pre: ‘\0’ terminated ➤ what’s wrong with s = t? array? pointers, no overloaded // post: returns # q’s char s[5]; { operators in C int strlen(char * s) char t[6]; int count=0; // pre: ‘\0’ terminated char * h = “hello”; ● strcmp (also strncmp) // post: returns # chars for(k=0;k < strlen(s);k++) strcpy(s,h); // trouble! { if (s[k]==‘q’) count++; ➤ return 0 if equal strcpy(t,h); // ok int count=0; return count; ➤ return neg if lhs < rhs while (*s++) count++; } char * strcpy(char* t,char* s) return count; //pre: t, target, has space ➤ return pos if lhs > rhs } //post: copies s to t,returns t { if (strcmp(s,t)==0) // equal ● Are these less cryptic? ● how many chars examined for int k=0; if (strcmp(s,t) < 0)// less 10 character string? while (t[k] = s[k]) k++; while (s[count]) count++; if (strcmp(s,t) > 0)// ???? // OR, is this right? return t; solution? ● } char * t = s; while (*t++); strncpy copies n chars (safer?) ● return t-s; 7.7 7.8 Duke CPS 108 Duke CPS 108

Arrays and pointers Microsoft question ● These definitions are related, but not the same ● Write atoi, write itoa, which is harder? int a[100]; int * ap = new int[10]; ● Questions? Issues? Problems? ● both a and ap represent ‘arrays’, but ap is an lvalue int atoi(const char * sp); char * itoa(int num); ● arrays converted to pointers for function calls: char s[] = “hello”; // prototype: int strlen(char * sp); ● Difference between const char * p and char * const p cout << strlen(s) << endl; ➤ one is a pointer to a constant character ● multidimensional arrays and arrays of arrays int a[20][5]; ➤ one is a constant pointer to a character int * b[10]; for(k=0; k < 10; k++) b[k] = new int[30]; Duke CPS 108 7.9 Duke CPS 108 7.10