Read binary file and count specific number c++ - c++

Question

Hey everyone I've been looking everywhere for insight on how to do this particular assignment. I saw something similar but it didn't have a clear explanation. I'm trying to read a bin file and count the number of times a specific number appears. I saw examples of this using a .txt file and it seemed very straight forward using getline. I tried to replicate the similar structure but using a binary file.
int main() {
int searching = 3;
int counter = 0;
unsigned char * memblock;
long long int size;
//open bin file
ifstream file;
file.open("threesData.bin", ios:: in | ios::binary | ios::ate);
//read bin file
if (file.is_open()) {
cout << "it opened\n";
size = file.tellg();
memblock = new unsigned char[size];
file.seekg(0, ios::beg);
file.read((char * ) memblock, size);
while (file.read((char * ) memblock, size)) {
for (int i = 0; i < size; i++) {
(int) memblock[i];
if (memblock[i] == searching) {
counter++;
}
}
}
}
file.close();
cout << "The number " << searching << " appears ";
cout << counter << " times!";
return 0;
}
When I run the program it's clear that it opens but it doesn't count the number I'm searching for. What am I doing wrong?

Answer 1

You seem to be thinking this through but here's how I would go about doing it.
Initialize a buffer with a sensible size.
Cast it to integers, so you can do array[size_t] syntax for simpler arithmetic.
Open the stream, and read while the stream is valid.
Convert the number of read bytes to the number of ints you would expect.
Increment the counter for each character you find that is valid.
Code
#include <fstream>
#include <iostream>
bool check_character(int value)
{
return value == 3;
}
int main(void)
{
// choose the size, cast a pointer as an int type, and initialize
// our counter
static constexpr size_t size = 4096;
char* buffer = new char[size];
int* ints = (int*) buffer;
size_t counter = 0;
// create our stream,
std::ifstream stream("file.bin", std::ios_base::binary);
while (stream) {
// keep reading while the stream is valid
stream.read(buffer, size);
auto count = stream.gcount();
// we only want to go to the last valid integer
// if we expect the file to be only integers,
// we could do `assert(count % sizeof(int) == 0);
// otherwise, we may have trailing characters
// if we have trailing characters, we may want to move them
// to the front of the buffer....
auto chars = count / sizeof(int); // floor division
for (size_t i = 0; i < chars; ++i) {
// false == 0, true == 1, so we can just add
// if the value is 3
counter += check_character(ints[i]);
}
}
std::cout << "Counter is: " << counter << std::endl;
delete[] buffer;
return 0;
}
As NeilButterworth points out, you could also use a vector. I don't really like this, but "meh".
#include <fstream>
#include <iostream>
#include <vector>
/* ellipsed lines */
int main(void)
{
/* ellipsed lines */
static constexpr size_t size = 4096;
std::vector<int> ints;
ints.resize(size / sizeof(int));
char* buffer = (char*) ints.data();
/* ellipsed lines */
/* ellipsed lines */
std::cout << "Counter is: " << counter << std::endl;
// no delete[]
return 0;
}