HELP! Arrays in c++

Question

when we work with an anrray, its size to be constant. is it possible to get an integer number from the user and fix it as a size of array?
int i,
cin>>i;
int arr[i];
I want to use like above

Answer 1

Yes it is possible by allocating memory to arrays dynamically at runtime.

There are two keywords that help in doing so.

1. new - allocates memory
2. delete - releases allocated memory.

Following example may help you to understand the concept of dynamic arrays

#include <iostream>
#include <stdlib.h>
using namespace std;
int main (void){
// a pointer to int type is declared to which will point to the allocation for n elements
int *ptr;
int n,big;

//read no of element
cout<<"Enter no of elements: ";cin>>n;

// Tries to allocated memory for n elements of type int
// if succeeded, address of first element is assigned to the
// pointer variable otherwise NULL is assigned
ptr = new int [n];

//validate allocation - NULL value means allocation failed
if (ptr==NULL){
cerr<<"Failed to allocate memory";
return -1;
}

/*
Since allocation has been successful. Now the pointer ptr can be accessed
as normal array
*/
int x;
for (x=0;x<n;x++){
cout<<"Enter " <<(x+1)<<" value: ";
cin>>ptr[x];
}

big=ptr[0];
for (x=1;x<n;x++){
if (big<ptr[x]){
big = ptr[x];
}
}

cout<<"Biggest value is "<<big;

//Memory allocated has to be released delete statement does the same
delete ptr;
}

Answer 2

Good answer @Devender! Just one small bug at the end - it must be (since you allocated an array):

delete[] ptr;

Answer 3

@Ettienne thanks for debugging out the error.