Arrays, Vectors, and Strings Allocation and referencing

Array Type Arrays are a compound type Base_type name[size]; Array dimension specifies the number of elements Dimension must be a positive integer literal OR a constant expression (i.e., known at compile time)

Array Examples cnt = 100;// not a const expr constexpr max = 100;// a const expr int array1[100];// good int array2[max];// good int array3[cnt];// error – not const int array4[getsize(something)]; // good iff getsize() is constexpr

Arrays Array indices run from 0 to Max-1 Always check your index!! Can reference element by index or by pointer dereference A[i] or *(A + i) Type allows compiler to compute correct offset into memory

0xfa10 0xfa58 0xfa54 0xfa50 0xfa4c 0xfa48 0xfa44 0xfa40 0xfa3c 0xfa38 0xfa34 0xfa30 0xfa2c 0xfa28 0xfa24 0xfa20 0xfa1c 0xfa18 0xfa14 AddressRAM 0x int array1[5]={1,2,4,8,16}; char name[5]=“Mary”; int *ptr = &array1[0]; 0x x x x ‘y’ ‘a’ ‘r’ ‘M’ ‘\0’ ‘\0’ Symbol Table IdentifierTypeLocation array1int*0xfa18 namechar*0xfa2c ptrint*0xfa18 Note: null terminated Arrays

2-D Arrays C++ does not really have multi-D arrays Looks kind of like it: int A[M][N]; Arrays are really pointers to the first element in a consecutively stored sequence of elements of same type 2-D array is really pointer to a 1-D array of pointers to first row elements

C-Style Strings C++ has a String class Can be referenced by index like array But it is a true object C strings are not a primitive type, nor are they a struct A C-style string is just a 1-D array of char, with NULL termination NOTA BENE: always a '\0' at end!!!!

0xfa10 0xfa58 0xfa54 0xfa50 0xfa4c 0xfa48 0xfa44 0xfa40 0xfa3c 0xfa38 0xfa34 0xfa30 0xfa2c 0xfa28 0xfa24 0xfa20 0xfa1c 0xfa18 0xfa14 AddressRAM 0x char names[3]={“Julian”, “James”,“John”}; 0x x x x ‘y’ ‘a’ ‘r’ ‘M’ ‘\0’ ‘\0’ Symbol Table IdentifierTypeLocation array1int*0xfa18 namechar*0xfa2c ptrint*0xfa18 2-D Arrays names char*[] 0xfa34 0x00000fa40 0x0000fa47 0x0000fa4d ‘i’ ‘l’ ‘u’ ‘J’ ‘J’ ‘\0’ ‘n’ ‘a’ ‘s’ ‘e’ ‘m’ ‘a’ ‘h’ ‘o’ ‘J’ ‘\0’ ? ? ‘\0’ ‘n’

C-Style Strings and Chars Remember, char and string are not the same 'a' is a char literal – uses one byte of RAM “a” is a string literal – uses two bytes of RAM Name[] = “Joe”; - uses... 4 bytes for the 3 characters plus null character Char *name – allocate pointer ONLY!! library – many functions

C String Library #include i=strlen(p);// string length less null i=strcmp(p1,p2);// neg if p1<p2, etc. p1=strcat(p1,p2);// appends p2 to p1 p1=strcpy(p1,p2);// copies p2 to p1 /*** WARNING – NO BOUNDS CHECKING! ***/ /* use these safe versions below! */ i=strncmp(p1,p2,n);// … only up to n p1=strncat(p1,p2,n);// … only up to n p1=strncpy(p1,p2,n);// … only up to n

C String Library #include size_t strlen(char s[])// not int! int atoi(char s[])// int value of s[] double atof(char s[])// double value long atol(char s[])// long value void itoa(int val, char s[], int radix) // converts integer value to string

C++ Strings C++ has string type #include using std::string; string s1;// default to empty string s2=s1;// s2 is a *copy* of s1 string s3=“Joe”;// s3 is copy of literal string s4=(10,’c’);// s4 is cccccccccc while (cin >> word) …// whitespace delim while (getline(cin, line)) … // \n delim

Strings & C-Strings string s(“Hello World”); char *str=s;// error! const char *str = s.c_str();// OK /* Achtung! Contents of *str may change * Copy into local version if need * continued access to contents */ Use s.insert(), s.erase(), s.assign(), s.append(), and s.replace() on strings

Vectors Vectors very similar to arrays Except they are class templates, not a type – must include type in declaration Take care of memory management Use push_back() to expand

Vector Initialization vector dvec;// ivec empty vector ivec(10, 5); /* 10 ints all 10 have value 5 */ vector ivec2(ivec); // copy ivec vector ivec3 = ivec;// also copy vector vec4(10);/* 10 item vector of type T objs default init */ vector ivec5{2,3,5,7,11,13} // 6 elements with values given vector ivec6 = {1,2,4,8,16} // 5 elements with values given

Vector Initialization vector ivec1(10); vector ivec2{10}; vector ivec3(10, 5); vector ivec4{10, 5}; vector sv1{“Al”,“Mo”, “Jo”}; vector sv2(“Al”,“Mo”,“Jo”); // 10 ints all value 0 // one int value 10 // 10 ints all value 5 // 2 ints, values 10 & 5 // list initialization // error

Adding to a Vector Vector size may not be known Vector may be large Use push_back member function vector iv; for (int i=0; i != MAX; ++i) iv.push_back(i*i); vector text; while (cin >> word) // word is string text.push_back(word); Subscripting does not add elements!

Range For Auto type definition if unknown Range for – like “foreach” vector iv2{1,2,3,4,5,6,7,8,9}; for (auto &i=0 : iv2) // reference! i *= i;// square elements for (auto i : iv2)// non-reference cout << i << endl;