A vector or an array of unknown size

Dear All,
Can anyoneone help me to declare and initialize an array or a vector which size is to be determined in the course of the programm?
Thank you

Below is an attached program that takes an undefined length string from the user and reverses it. Make not of the use of realloc().

Code:

/* Passing addresses demo
   This program takes a string of undeterminate length and reverses it displaying both to the user */

#include <stdio.h>
#include <stdlib.h> //used for realloc()
#include <string.h> //used for strlen() function

int reverseString(char*, char*); //function prototype

int main(void) {

	char* str = NULL; //set up and 
	char* rstr = NULL;// initialize pointers
	int ch, len = 0;

	printf("Enter a string to reverse: ");
	fflush(stdout);

	//this takes the string types character by character and adds it to the string str
	while((ch = getchar()) != '\n' && ch != EOF) {
		if(!(str = (char*)realloc(str, len+1))) { //<-----------------Increases array size by 1 char
			return(1);
		}
		str[len++] = ch;
	}
	//-------------------------------------------------------------------------------
	str[len] = '\0'; //adds flag to signal string is finished
	
	rstr = (char*)realloc(rstr, len);//<------------------------------Makes rstr same size as len = size of str
	
	reverseString(str, rstr);

	printf("You entered: %s \n", str);
	printf("The reverse is: %s \n", rstr);
	
	return(0);
}
//this function starts at the end of input and writes to the beginning of revInput
int reverseString(char* input, char* revInput) {

	int curChar = 0;

	for(int i = (strlen(input)-1); i>=0; i--) {
		revInput[curChar] = input[i];
		curChar++;
	}
	revInput[curChar] = '\0';

	return(0);
}

This should answer your question of I understand it.

Happy Coding!!

as we are programming with C++, it is easy to make an array with variable size .

a dynamic list:

Code:

//class to store info of books and  basic method
#include<string>
#include<sstream>
#include<cstdlib>
using namespace std;

const long MAX_C =100000;
const int MAX_Q =10000;

class BMSBook
{
public:

BMSBook()
{
    isbn="";
    title="";
    press="";
    instruction="";
    type="";
    quantity=0;
    next=NULL;
}
BMSBook(string _isbn,string _title,string _press,string _instruction,string _type,int _quantity,BMSBook *_next)
{
    isbn=_isbn;
    title=_title;
    press=_press;
    instruction=_instruction;
    type=_type;
    quantity=_quantity;
    next=_next;
}
BMSBook(BMSBook *_book)
{
    isbn=_book->isbn;
    title=_book->title;
    press=_book->press;
    instruction=_book->instruction;
    type=_book->type;
    quantity=_book->quantity;
    next=_book->next;
}



protected:
string isbn;
string title;
string press;
string instruction;
string type;
int quantity;
BMSBook *next;
friend class BMSBooks;
};



class BMSBooks
{
public:
BMSBooks()
{
    thebook=NULL;
    current;
    count=0;
}
~BMSBooks()
{
    while(thebook!=NULL)
    {
        current=thebook;
        thebook=current->next;
        delete current;
    }
}


bool getdata(string &_isbn,string &_title,string &_press,string &_instruction,string &_type,int &_quantity)
{
  if(current!=NULL)
  {
    _isbn=current->isbn;
    _title=current->title;
    _press=current->press;
    _instruction=current->instruction;
    _type=current->type;
    _quantity=current->quantity;
    return true;
  }
return false;
}


bool gettitle(string &_title)
{
  if(current!=NULL)
  {
    _title=current->title;
    return true;
  }
return false;
}


void moveforward(int index)
{
    while(index&&(current->next!=NULL))
    {
        current=current->next;
        index--;
    }
}


bool match(string title,string press,string instruction,string type,int quantity)
{
      if(isempty())return false;
            if(title!="")
            {
            if(title!=current->title)return false;
            }
            if(press!="")
            {
            if(press!=current->press)return false;
            }
            if(instruction!="")
            {
            if(instruction!=current->instruction)return false;
            }
            if(type!="")
            {
            if(type!=current->type)return false;
            }
            if(quantity!=0)
            {
            if(quantity!=current->quantity)return false;
            }
      return true;
}

bool moveonestep()
{
    if(current->next!=NULL)
    {
        current=current->next;
        return true;
    }
return false;
}


long searchbyisbn(int &cmp,const string &isbn)
{
BMSBook *temp;
temp=thebook;
movetotop();
long tempindex=1;
long top=count;
long bottom=1;
        moveforward((top+bottom)/2-1);
        cmp=strcmp(isbn.c_str(),current->isbn.c_str());
    do
    {
        if(!cmp)
          {
            return (top+bottom)/2;
          }
        else if(cmp>0)
          {
            tempindex=(top+bottom)/2;
            temp=current;
            bottom=tempindex+1;
            moveforward((top+bottom)/2-tempindex);
          }
        else if(cmp<0)
          {
            current=temp;
            top=(top+bottom)/2;
            moveforward((top+bottom)/2-tempindex);
          }
         cmp=strcmp(isbn.c_str(),current->isbn.c_str());
    }while(top>bottom);
return -1;
}



short add(string isbn,string title,string press,string instruction,string type,int quantity)
{
if((count>=MAX_C)||(quantity>=MAX_Q))return 2;
if(checkisbn(isbn))
    {
        BMSBook *temp;
        int cmp;
        if(count)
            {
                searchbyisbn(cmp,isbn);
                if(!cmp)
                {
                    if((current->quantity+quantity)>MAX_Q)return 2;
                    current->quantity+=quantity;
                    return 0;
                }
                else if(cmp>0)
                {
                    temp=current->next;
                    current->next=new BMSBook(isbn, title, press, instruction, type,quantity,temp);
                    if(current->next==NULL)return 1;
                    count++;
                    return 0;
                }
                else if(cmp<0)
                {
                    temp=new BMSBook(current->isbn,current->title,current->press,current->instruction,current->type,current->quantity,current->next);
                    if(temp==NULL)return 1;
                    current->isbn=isbn;
                    current->title=title;
                    current->press=press;
                    current->type=type;
                    current->instruction=instruction;
                    current->quantity=quantity;
                    current->next=temp;
                    count++;
                    return 0;
                }
            }
        else
        {
            thebook=new BMSBook(isbn,title,press,instruction,type,quantity,NULL);
            if(thebook==NULL)return 1;
            count=1;
            return 0;
        }
    }
return -1;
}


bool remove()
{
    if(count)
    {
        BMSBook *temp;            
        if(current==thebook)
        {
            thebook=current->next;
            delete current;
        }
        else if(current->next==NULL)
        {
            delete current;
        }
        else
        {
            temp=current->next;
            current->isbn=temp->isbn;
            current->title=temp->title;
            current->press=temp->press;
            current->instruction=temp->instruction;
            current->type=temp->type;
            current->quantity=temp->quantity;
            current->next=temp->next;
            delete temp;
        }
        count--;
        return true;            
    }
return false;
}


bool checkisbn(const string &isbn)
{
int sum=0,num=0,num1=0;
if(isbn.size()!=13)return false;
for(int loops=0;loops<12;loops++)
  {
  if(isdigit(isbn[loops]))
  {
  istringstream temp(isbn.substr(loops,1));
  temp>>num;
  sum+=(num*(++num1));
  }
  }
num=sum%11;
if(num>=10)
{
if((isbn.substr(12,1)!="x")&&(isbn.substr(12,1)!="X"))return false;
}
else
{
if(!isdigit(isbn[12]))return false;
istringstream temp(isbn.substr(12,1));
temp>>sum;
if(num!=sum)return false;
}
return true;
}

bool update(string isbn,string title,string press,string instruction,string type,int quantity)
{
    if(count&&(current!=NULL))
    {
                    current->isbn=isbn;
                    current->title=title;
                    current->press=press;
                    current->type=type;
                    current->instruction=instruction;
                    current->quantity=quantity;
                    return true;
    }
    return false;
}

bool isempty()
{
if(count)return false;
return true;
}

void movetotop()
{
current=thebook;
}

void getcount(long &num)
{
num=count;
}

private:
long count;
BMSBook *thebook;
BMSBook *current;
};

Hermitsky, andyhunter...you guys are awesome, *copies* *pastes* *compiles*

A linked-list isn't exactly appropriate since the OP mentioned arrays and vectors. It is a simple matter to initialize an STL vector.

Code:

int main()
{
	vector<int> a(20); //declares a vector of 20 elements
	vector<int> b(20,5); //declares a vector of 20 elements initialized to 5
	//do stuff
	return 0;
}

My STL implementation initializes everything to 0 (or empty, in the case of strings), but I'm not sure if that's standard. You can also initialize a vector with another vector or two iterators from a vector that denote a range of elements.