Read file and create integer array dynamically - c++

Question

This seems like such an easy task, but everything I've tried hasn't worked so far.
I have a file foo.txt:
3
3 4 2
Now I want to read this file, read the first line and instantiate an int array with the size of the number it read on the first line.
Then it should populate that array with the elements in the second line, which has the exact same amount of elements and noted in line one.

Answer 1

If we're going to give you example code, might as well show you the best way to do it:
std::ifstream datafile("foo.txt");
if (!datafile) {
std::cerr << "Could not open \'foo.txt\', make sure it is in the correct directory." << std::endl;
exit(-1);
}
int num_entries;
// this tests whether the number was gotten successfully
if (!(datafile >> num_entries)) {
std::cerr << "The first item in the file must be the number of entries." << std::endl;
exit(-1);
}
// here we range check the input... never trust that information from the user is reasonable!
if (num_entries < 0) {
std::cerr << "Number of entries cannot be negative." << std::endl;
exit(-2);
}
// here we allocate an array of the requested size.
// vector will take care of freeing the memory when we're done with it (the vector goes out of scope)
std::vector<int> ints(num_entries);
for( int i = 0; i < num_entries; ++i )
// again, we'll check if there was any problem reading the numbers
if (!(datafile >> ints[i])) {
std::cerr << "Error reading entry #" << i << std::endl;
exit(-3);
}
}
Demo (with small changes because I can't provide a file with the right name on ideone): http://ideone.com/0vzPPN

Answer 2

You need to use ifstream object just like you use cin
ifstream fin("foo.txt"); //open the file
if(!fin.fail()){
int count;
fin>>count; //read the count
int *Arr = new int[count];
for(int i=0;i<count;i++){ //read numbers
fin>>Arr[i];
}
//... do what you need ...
//... and finally ...
delete [] Arr;
}

Answer 3

If you open a file using input filestream you can simply do that:
std::ifstream file_txt("file.txt");
int number_count = 0;
file_txt >> number_count; // read '3' from first line
for (int number, i = 0; i < number_count; ++i) {
file_txt >> number; // read other numbers
// process number
}
Filestreams just like other standard streams (std::cin, std::cout) can apply formatting depending on type supplied to operator>> (in this case an int).
This apply to both input and output.

Answer 4

Alternatively, you could avoid the entire need to read in the size beforehand by simply loading it into a std::vector:
std::ifstream fin("myfile.txt");
std::vector<int> vec{std::istream_iterator<int>(fin), std::istream_iterator<int>()};
fin.close();
or, if you cannot use C++11 syntax:
std::ifstream fin("myfile.txt");
std::vector<int> vec;
std::copy(std::istream_iterator<int>(fin), std::istream_iterator<int>(), std::back_inserter(vec));
fin.close();