Rewriting data to disk - c++

Question

I ask user for input and write this data to disk. However I want the user to be able to update some of this data. It writes the data the first pass of the loop. But the second loop is not updating the description. Here is the code:
#include <iostream>
struct LabData {
int recordNumber;
char desc[16];
};
int main(int argc, const char * argv[]) {
// insert code here...
FILE *file = fopen("myfile.txt", "wb+");
if(file == 0)
std::cout << "ERROR FILE IS NULL!";
for(int i = 0; i < 10; i++)
{
LabData lb;
lb.recordNumber = i;
std::cout << "Enter a description: ";
std::cin.getline(lb.desc, 15);
if(fwrite(&lb, sizeof(lb), 1, file) != 1)
std::cout <<" ERROR IN WRITE!";
}
LabData lb;
for(int i = 9; i >= 0; i--)
{
fseek(file, i * sizeof(LabData), SEEK_SET);
fread(&lb, sizeof(LabData), 1, file);
if(lb.recordNumber != i)
std::cout << "Wrong record!";
else
{
std::cout << "Record number: " << lb.recordNumber << " Description: " << lb.desc << std::endl;
std::cout << "Enter new record description: ";
std::cin.getline(lb.desc, 15);
if(fwrite(&lb, sizeof(lb), 1, file) != 1)
std::cout <<" ERROR IN WRITE!";
}
}
for(int i = 0; i < 10; i++)
{
LabData lb;
fseek(file, i * sizeof(LabData), SEEK_SET);
fread(&lb, sizeof(LabData), 1, file);
std::cout << "Record number: " << lb.recordNumber << " Description: " << lb.desc << std::endl;
}
fclose(file);
return 0;
}

Answer 1

you need to put fseek(file, i * sizeof(LabData), SEEK_SET); before writing into the file, in the second loop,
std::cout << "Record number: " << lb.recordNumber << " Description: " << lb.desc << std::endl;
std::cout << "Enter new record description: ";
std::cin.getline(lb.desc, 15);
fseek(file, i * sizeof(LabData), SEEK_SET);
if(fwrite(&lb, sizeof(lb), 1, file) != 1)
std::cout <<" ERROR IN WRITE!";
Once you read from the file, you have to seek back to rewrite the data in same place.