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
Arrays
int array1[5]={1,2,4,8,16};
RAM
char name[5]=“Mary”;
int *ptr = &array1[0];
Symbol Table
Identifier Type Location
array1 int* 0xfa18
name char* 0xfa2c
ptr
int* 0xfa18
Note: null terminated
‘\0’ ‘\0’ ‘\0’ ‘\0’
‘y’ ‘a’ ‘r’ ‘M’
0x00000010
0x00000008
0x00000004
0x00000002
0x00000001
Address
0xfa58
0xfa54
0xfa50
0xfa4c
0xfa48
0xfa44
0xfa40
0xfa3c
0xfa38
0xfa34
0xfa30
0xfa2c
0xfa28
0xfa24
0xfa20
0xfa1c
0xfa18
0xfa14
0xfa10
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!!!!
2-D Arrays
char names[3]={“Julian”,
“James”,“John”};
Symbol Table
Identifier Type Location
array1 int* 0xfa18
name char* 0xfa2c
ptr
int* 0xfa18
names char*[] 0xfa34
RAM
? ? ‘\0’ ‘n’
‘h’ ‘o’ ‘J’ ‘\0’
‘s’ ‘e’ ‘m’ ‘a’
‘J’ ‘\0’ ‘n’ ‘a’
‘i’ ‘l’ ‘u’ ‘J’
0x0000fa4d
0x0000fa47
0x00000fa40
‘\0’ ‘\0’ ‘\0’ ‘\0’
‘y’ ‘a’ ‘r’ ‘M’
0x00000010
0x00000008
0x00000004
0x00000002
0x00000001
Address
0xfa58
0xfa54
0xfa50
0xfa4c
0xfa48
0xfa44
0xfa40
0xfa3c
0xfa38
0xfa34
0xfa30
0xfa2c
0xfa28
0xfa24
0xfa20
0xfa1c
0xfa18
0xfa14
0xfa10
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 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 <strings.h>
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 <string>
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<double> dvec; // ivec empty
vector<int> ivec(10, 5); /* 10 ints
all 10 have value 5 */
vector<int> ivec2(ivec); // copy ivec
vector<int> ivec3 = ivec; // also copy
vector<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 given
vector<int> ivec6 = {1,2,4,8,16}
// 5 elements with values given
Vector Initialization
vector<int> ivec1(10);
// 10 ints all value 0
vector<int> ivec2{10};
// one int value 10
vector<int> ivec3(10, 5);
// 10 ints all value 5
vector<int> ivec4{10, 5};
// 2 ints, values 10 & 5
vector<string> sv1{“Al”,“Mo”, “Jo”};
// list initialization
vector<string> sv2(“Al”,“Mo”,“Jo”);
// error
Adding to a Vector
• Vector size may not be known
• Vector may be large
• Use push_back member function
vector<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;