Question

Noobie Alert.
Ugh. I’m having some real trouble getting some basic file I/O stuff done using <stdio.h> or <fstream>. They both seem so clunky and non-intuitive to use. I mean, why couldn’t C++ just provide a way to get a char* pointer to the first char in the file? That’s all I’d ever want.

I’m doing Project Euler Question 13 and need to play with 50-digit numbers. I have the 150 numbers stored in the file 13.txt and I’m trying to create a 150×50 array so I can play with the digits of each number directly. But I’m having tons of trouble. I’ve tried using the C++ <fstream> library and recently straight <stdio.h> to get it done, but something must not be clicking for me. Here’s what I have;

#include <iostream>
#include <stdio.h>
int main() {

const unsigned N = 100;
const unsigned D = 50; 

unsigned short nums[N][D];

FILE* f = fopen("13.txt", "r");
//error-checking for NULL return

unsigned short *d_ptr = &nums[0][0];
int c = 0;
while ((c = fgetc(f)) != EOF) {
    if (c == '\n' || c == '\t' || c == ' ') {
        continue;
    }   
    *d_ptr = (short)(c-0x30);
    ++d_ptr;
}   
fclose(f);
//do stuff
return 0;
}

Can someone offer some advice? Perhaps a C++ guy on which I/O library they prefer?

Answer 1

Here’s a nice efficient solution (but doesn’t work with pipes):

std::vector<char> content;
FILE* f = fopen("13.txt", "r");
// error-checking goes here
fseek(f, 0, SEEK_END);
content.resize(ftell(f));
fseek(f, 0, SEEK_BEGIN);
fread(&content[0], 1, content.size(), f);
fclose(f);

Here’s another:

std::vector<char> content;
struct stat fileinfo;
stat("13.txt", &fileinfo);
// error-checking goes here
content.resize(fileinfo.st_size);
FILE* f = fopen("13.txt", "r");
// error-checking goes here
fread(&content[0], 1, content.size(), f);
// error-checking goes here
fclose(f);