How to delete/update specific data in txt file using C++? - c++

Question

I'm writing a library system using struct as seen
struct Book
{
char name[50];
int ID;
char author[50];
double price;
int copies;
};
and the file is organised as seen.
ID Name Author Price Copies
1 HarryPotter Lol 30 5
2 EnglishMan English 30 5
3 Spiderman Marvel 30 5
4 Avengers Marvel 30 5
Let's say I want to use the program to update book no. 2 (EnglishMan) and change its name to IronMan, how can I do that using files?

Answer 1

If you use plain text files as data storage you just have to follow this inconvenient workflow:
Read the complete file into your data structures.
Alter the data.
Truncate or remove the file.
Write all the data into the file.
There are ugly hacks to edit parts of the file, but they don't make things better.
For managing tabular data, as in your example, relational databases have been invented a long time ago. Start to learn SQLite, and your life will be much easier in the long run.

Answer 2

What you are doing is, essentially, trying to create your own database which is counterproductive at best. But if it's for learning file I/O and string streams, following code can help you understand the concepts although It's probably not the most efficient way of doing things.
In general, as #Murphy said, you need to read a file, copy it to a buffer, adjust the buffer to your liking, truncate the file and write your own buffer to the file.
int searchbyID(string filename, string ID, string newName);
int main()
{
searchbyID("d:\\test.txt", "2", "silmarillion");
}
int searchbyID(string filename, string ID, string newName)
{
// open an input file stream
ifstream inputfile(filename);
if (!inputfile)
return -1; // couldn't open the file
string line,word;
string buffer;
// read the file line by line
while (getline(inputfile, line))
{
std::stringstream record(line);
//read the id from the file and check if it's the asked one
record >> word;
if (word == ID)
{
// append the ID first
buffer += ID + "\t";
//append the new name instead of the old one
buffer += newName + "\t";
//pass the old name
record >> word;
//copy the rest of the line just as it is
while (record >> word)
buffer += "\t" + word + "\t";
buffer += "\n";
}
else
{
//if not, just pass the line as it is
buffer += line + "\n";
}
}
// close input file stream
inputfile.close();
// open an output file stream
ofstream outputfile(filename);
// write new buffer to the file
outputfile << buffer;
//close the output file stream
outputfile.close();
return 0;
}