Adding values in filestream - c++

Question

For example i have a notepad for numbers.
1 2 3 4 5
Then i want to add 4 in the third line which is 3 so that its new value will be
1 2 7 4 5
Question is how can i do that?
Please help me thank you!
string add;
cout<<"Enter value to be added: ";
cin>>add;
fstream file;
file.open("quantity.txt");

Answer 1

You have not been accurate enough with your question, but I think this is something like what you expect:
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
int main(int argc, const char *argv[]) {
std::string filename;
std::cout << "Enter name of file to modify: ";
std::cin >> filename;
std::ifstream inputFile;
inputFile.open(filename);
if(inputFile.fail()) {
std::cout << "Unable to open file \"" << filename << "\" for reading\n";
return 1;
}
unsigned offset;
std::cout << "Enter offset to modify: ";
std::cin >> offset;
int toAdd;
std::cout << "Enter value to be added to line #" << lineNumber << ": ";
std::cin >> toAdd;
std::vector<int> nums;
for(;;) {
int num;
inputFile >> num;
if(inputFile.eof())
break;
nums.push_back(num);
}
inputFile.close();
if(offset >= nums.length()) {
std::cout << "Offset " << offset << " out of bounds!\n";
return 1;
}
nums[offset] += toAdd;
std::ofstream outputFile;
outputFile.open(filename);
if(outputFile.fail()) {
std::cout << "Unable to open file \"" << filename << "\" for writing\n";
return 1;
}
for(int num : nums) // Warning: C++11
outputFile << num << ' ';
outputFile.close();
}