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

0xfa580xfa540xfa500xfa4c0xfa480xfa440xfa400xfa3c0xfa380xfa340xfa300xfa2c0xfa280xfa240xfa200xfa1c0xfa180xfa14

AddressRAM

0x00000004

int array1[5]={1,2,4,8,16};

char name[5]=“Mary”;

int *ptr = &array1[0];

0x000000020x00000001

0x000000080x00000010‘y’ ‘a’ ‘r’ ‘M’‘\0’ ‘\0’ ‘\0’ ‘\0’

Symbol Table

Identifier Type Locationarray1 int* 0xfa18name char* 0xfa2c

ptr int* 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

0xfa580xfa540xfa500xfa4c0xfa480xfa440xfa400xfa3c0xfa380xfa340xfa300xfa2c0xfa280xfa240xfa200xfa1c0xfa180xfa14

AddressRAM

0x00000004

char names[3]={“Julian”,

“James”,“John”};

0x000000020x00000001

0x000000080x00000010‘y’ ‘a’ ‘r’ ‘M’‘\0’ ‘\0’ ‘\0’ ‘\0’

Symbol Table

Identifier Type Locationarray1 int* 0xfa18name char* 0xfa2c

ptr int* 0xfa18

2-D Arrays

names char*[] 0xfa34

0x00000fa400x0000fa470x0000fa4d‘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!!

• <strings.h> library – many functions

C String Library#include <strings.h>

i=strlen(p); // string length less nulli=strcmp(p1,p2); // neg if p1<p2, etc.p1=strcat(p1,p2); // appends p2 to p1p1=strcpy(p1,p2); // copies p2 to p1/*** WARNING – NO BOUNDS CHECKING! ***//* use these safe versions below! */i=strncmp(p1,p2,n); // … only up to np1=strncat(p1,p2,n); // … only up to np1=strncpy(p1,p2,n); // … only up to n

C String Library#include <strings.h>

size_t strlen(char s[])// not int!

int atoi(char s[]) // int value of s[]double atof(char s[]) // double valuelong atol(char s[]) // long valuevoid itoa(int val, char s[], int radix)

// converts integer value to string

C++ Strings

• C++ has string type

#include <string>using std::string;string s1; // default to emptystring s2=s1; // s2 is a *copy* of s1string s3=“Joe”; // s3 is copy of literalstring s4=(10,’c’); // s4 is cccccccccc

while (cin >> word) …// whitespace delimwhile (getline(cin, line)) … // \n delim

Strings & C-Stringsstring 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 Initializationvector<double> dvec; // ivec emptyvector<int> ivec(10, 5); /* 10 ints

all 10 have value 5 */vector<int> ivec2(ivec); // copy ivecvector<int> ivec3 = ivec; // also copyvector<T> vec4(10);/* 10 item vector

of type T objs default init */vector<int> ivec5{2,3,5,7,11,13}

// 6 elements with values givenvector<int> ivec6 = {1,2,4,8,16}

// 5 elements with values given

Vector Initializationvector<int> ivec1(10);

vector<int> ivec2{10};

vector<int> ivec3(10, 5);

vector<int> ivec4{10, 5};

vector<string> sv1{“Al”,“Mo”, “Jo”};

vector<string> 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 functionvector<int> iv;for (int i=0; i != MAX; ++i) iv.push_back(i*i);vector<string> 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<int> 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;