File opens, but won't overwrite

Ok, thanks to zen and everyone else helping with this code. I added zen's code to read the file, and it works ok. Reading the file isn't actually part of the assignment, but I use it to check that it's being done correctly. I've been puzzling over this for a few days, don't know why it's so hard. I can finish the assignment, but want to know why it's doing what it's doing.
Even when opening the file in binary, I can read it in Notepad, w/o using memset.
Anyway, in Notepad the file still contains the remnant "gned" in the lastName field (for a 5 character name). Why doesn't the personOut.write line with blank overwrite that?
Tha age variable is a char[4] in the assignment, probably because the fields need to be a set length for random access. Using an int instead gives some binary type character when displaying it.
Thanks again.

Code:

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ofstream personOut("nameAge.dat", ios::binary);
        if (!personOut) {cout << "Error!";}
        person id = {"unassigned", "", 0};
        person blank = {"            ", "", 0};
        for (int i = 0; i < 100; i++)
                personOut.write((char *)&id, sizeof(person));

        // 14.11.b        Enter records sequentially
        int numOfRec;
        cout << "\nEnter number of records to enter: ";
        cin >> numOfRec;
        for (int j = 0; j < numOfRec; j++)
        {
                personOut.seekp(j * sizeof(person));
                personOut.write((char *)&blank, sizeof(person));
                cout << "\nEnter last name, first name and age: ";
                cin >> id.lastName >> id.firstName >> id.age;
                personOut.seekp(j * sizeof(person));
                personOut.write((char *)&id, sizeof(person));
        }
       
        personOut.close();

    ifstream personIn("nameAge.dat", ios::binary);
    for(int k =0;k<numOfRec;k++)
    {
                personIn.seekg(k * sizeof(person));
                personIn.read((char *)&id, sizeof(person));
        cout << id.lastName  <<  " "<< id.firstName << " "<< id.age << endl;       
    }

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