Question

Possible Duplicate:
c++ – printf on strings prints gibberish

I would like to write several strings to file . The strings are

37 1 0 0 0 0
15 1 0 0 0 0
33 1 0 0 0 0
29 1 0 0 0 0
18 1 0 0 0 0
25 1 0 0 0 0

I first would like to store each line as elements of a string array, then call the same string array and write its element to file.

#include <stdio.h>
#include <vector>
#include <string>
using namespace std;

int writeFile() {

  char line[100];
  char* fname_r = "someFile_r.txt"
  char* fname_w = "someFile_w.txt"; 
  vector<string> vec;

  FILE fp_r = fopen(fname_r, "r");
  if(fgets(line, 256,fp_r) != NULL)   {
     vec.push_back(line);
  }

  FILE fp_w = fopen(fname_w, "w");
  for(int j = 0; j< vec.size(); j++) {
    fprintf(fp_w, "%s", vec[j]); // What did I miss? I get funny symbols here. I am expecting an ASCII
  }

  fclose(fp_w);
  fclose(fp_r);
  return 0;
}

Answer 1

The format specifier "%s" expects a C-style null terminated string, not a std::string. Change to:

fprintf(fp_w, "%s", vec[j].c_str());

As this is C++, you should consider using ofstream instead which are type-safe and accept std::string as input:

std::ofstream out(fname_w);
if (out.is_open())
{
    // There are several other ways to code this loop.
    for(int j = 0; j< vec.size(); j++)
        out << vec[j];
}

Equally, use ifstream for input. The posted code has a potential buffer overrun:

char line[100];
...
if(fgets(line, 256,fp_r) != NULL)

line can store a maximum of 100 characters but the fgets() is stating that it can hold 256. Using std::getline() removes this potential hazard as it populates a std::string:

std::ifstream in(fname_r);
std::string line;
while (std::getline(in, line)) vec.push_back(line);