How to dynamically increase the array size?

Question

I've been trying to make a program that adds 2 arrays of different size. But I would like to know to to dynamically increase the array size capacity? Ex: array[4] then upgrade the size to 2 to make array[6];? EDIT: WIthout using vectors

I tried creating a new ptr but it does not work. I get the error: Read only variable is not assignable.

int *ptr2 = new int[a2.size]; // new ptr2 copies ptr1 for (int i=0; i<(a1.size); i++) { ptr2[i] = a1.ptr[i]; } // we want ptr1 to point to ptr2 for (int i=0; i<(a2.size); i++) { ptr2[i] += a2.ptr[i]; } delete [] a1.ptr; a1.ptr=ptr2;

I don't want to use vectors. Where can I allocate the new memory? And why do you downvote so quickly? — EEstud
– EEstud, Commented Aug 20, 2012 at 4:29
@EEstud - You can allocate memory in the constructor. And I dint downvote this question.. yet. — vidit
– vidit, Commented Aug 20, 2012 at 4:53
Possible duplicate of Can you resize a C++ array after initialization? — Archmede
– Archmede, Commented Jul 8, 2017 at 17:17

David Schwartz · Accepted Answer · 2012-08-20 04:31:56Z

23

You can't change the size of the array, but you don't need to. You can just allocate a new array that's larger, copy the values you want to keep, delete the original array, and change the member variable to point to the new array.

Allocate a new[] array and store it in a temporary pointer.
Copy over the previous values that you want to keep.
Delete[] the old array.
Change the member variables, ptr and size to point to the new array and hold the new size.

edited Aug 20, 2012 at 4:31

answered Aug 20, 2012 at 4:19

David Schwartz

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7 Comments

David Schwartz Over a year ago

You can't use realloc on a block allocated with new[].

Ed Swangren Over a year ago

That's a good point, I didn't actually look at the code and assumed C... my mistake.

Dennis Meng Over a year ago

Also as a general rule, don't mix new/delete with *alloc/free.

David Schwartz Over a year ago

He could switch to malloc/free given that his type is POD, or better yet, just use vector. Eventually, you need to learn how to use all these things properly.

HAL9000 Over a year ago

Don't use new/delete, except as a learning experience. c++ has smart pointers and container classes.

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Sanjay Agarwal · Accepted Answer · 2014-01-19 06:31:21Z

14

 int* newArr = new int[new_size]; std::copy(oldArr, oldArr + std::min(old_size, new_size), newArr); delete[] oldArr; oldArr = newArr;

answered Jan 19, 2014 at 6:31

Sanjay Agarwal

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1 Comment

Silidrone Over a year ago

Considering std::vector already exists and the OP is specifically looking for a low level code dealing with pointers and trying to implement what already exists in stl, why would you prefer std::copy over a for loop in your answer?

BoP · Accepted Answer · 2021-12-16 06:23:48Z

#include<bits/stdc++.h> using namespace std; main(){ int *p = new int[5]; // locate memory in heap int *q = new int[10];// locate memory in heap for(int j=0; j<5;j++) p[j] = j; for(int i=0; i<5;i++) q[i] = p[i]; delete []p;//Delete the old array 'p' p = q; // Assign the pointer of 'q' to 'p' q = NULL; // delete the location of pointer 'q' return 0; }

Sourabh Semalty · Accepted Answer · 2021-12-25 02:47:31Z

It may be late too answer but i'll explain some things to you..

Array Size cannot be increase because of contiguous memory allocation in the memory. For Example => The address of the every location is

arr[5] => [2001,2002,2003,2004,2005]

Now, The main problem is if you allocate it to arr[10] so, as we don't know the next location 2006 will be free . because array need to be in contiguous so, we won't be able to allocate memory

From my Suggestions use Vector or use dynamic Memory allocation in Cpp

int *old = new int[5]; int *nw = new int[10]; for (size_t i = 0; i < sizeof(old); i++) nw[i] = old[i]; delete []old; old = nw; nw = NULL;

tech dreams · Accepted Answer · 2019-09-07 06:48:39Z

#include <stdio.h> #include <stdlib.h> int main() { int *p,*q; int i; p=(int *)malloc(5*sizeof(int)); p[0]=3;p[1]=5;p[2]=7;p[3]=9;p[4]=11; q=(int *)malloc(10*sizeof(int)); for(i=0;i<5;i++) q[i]=p[i]; free(p); p=q; q=NULL; for(i=0;i<5;i++) printf("%d \n",p[i]); return 0; }

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