Inserting into dynamic array

Question

I am trying to insert into a dynamic array. Everything seems to be working except for the first insertion.

Here is my insert function:

void account::insert(value_type entry) {

if (numberOfItems == 0) {
    data = new value_type[numberOfItems + 1];
}

data[numberOfItems] = entry;
numberOfItems++;

value_type* temp = new value_type[numberOfItems + 1];
for (size_t i = 0; i < numberOfItems; i++) {
    temp[i] = data[i];
}

delete[] data;
data = temp;
}

This is where I call my insert:

void account::deposit(double deposit) {
balance += deposit;
insert(balance);
}

here are my tests:

 account savings(3.00);
    savings.deposit(1.00);
    cout << savings << endl;


    savings.advanceDay(7);
    cout << savings << endl;


    savings.deposit(450.55);
    cout << savings << endl;


    savings.advanceDay(23);
    cout << savings << endl;


    savings.advanceDay(65);
    cout << savings << endl;

    //Deposit more
    savings.deposit(1000);
    cout << savings << endl;

    savings.advanceDay(65);
    cout << savings << endl;

here is the output I get:

Account Tester
day: 1, balance: $-6277438562204192487878988888393020692503707483087375482269988814848.00
day: 8, balance: $-6277438562204192487878988888393020692503707483087375482269988814848.00
day: 8, balance: $451.55
day: 31, balance: $451.55
day: 96, balance: $451.55
day: 96, balance: $451.55
day: 161, balance: $1451.55
day: 161, balance: $1451.55
day: 161, balance: $1451.55
day: 161, balance: $1451.55
day: 161, balance: $1451.55
day: 191, balance: $11401.55
day: 221, balance: $11401.55

I am confused why the fisrt balance is a garbage address value(or at least thats what I think it is)and not $1 but all the other depostits are correct.

You don't show where and how you use data. Show a minimal reproducible example — Igor Tandetnik
– Igor Tandetnik, Commented Feb 23, 2020 at 18:39
I see a few simple logical bugs here. If only you knew how to use a debugger to run your program one line at a time, inspect the values of all variables, and observe your program's logical execution flow. If you did know how to use your debugger, you would've figured out what the bug is faster than it took you to type this question. Perhaps you want to take this opportunity to learn how to debug your own code, now, so that next time you'll save yourself a lot of time and won't have to depend on other people to find bugs in your code. Does that make sense? — Sam Varshavchik
– Sam Varshavchik, Commented Feb 23, 2020 at 18:41
Your insertion logic is backwards: "first store the new element, then make room for another element". — molbdnilo
– molbdnilo, Commented Feb 23, 2020 at 18:46

Vlad from Moscow · Accepted Answer · 2020-02-23 19:09:53Z

This function

void account::insert(value_type entry) {

if (numberOfItems == 0) {
    data = new value_type[numberOfItems + 1];
}

data[numberOfItems] = entry;
numberOfItems++;

value_type* temp = new value_type[numberOfItems + 1];
for (size_t i = 0; i < numberOfItems; i++) {
    temp[i] = data[i];
}

delete[] data;
data = temp;
}

Has a bug. When numberOfItems us equal to 0 you are allocating an array with one element

if (numberOfItems == 0) {
    data = new value_type[numberOfItems + 1];
}

data[numberOfItems] = entry;
numberOfItems++;

Then you are increasing numberOfItems. And then again you are allocating an array but now with 2 elements.

value_type* temp = new value_type[numberOfItems + 1];

and as a result the array has an element with an indeterminate value.

There is no sense to separate the function into two code snippets that allocate memory.

Initially the data member data shall be equal to nullptr. The function can look like

void account::insert( const value_type &entry ) 
{
    value_type* temp = new value_type[numberOfItems + 1];

    for ( size_t i = 0; i < numberOfItems; i++ ) 
    {
        temp[i] = data[i];
    }

    temp[numberOfItems++] = entry;

    delete [] data;

    data = temp
}

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