Simple read from text file algorithm doesn't work

Question

I have a text file that contains keys and values like this:

keyOne=1
keyTwo=734
keyThree=22.3
keyFour=5

The keys are just lower-case and upper-case letters like in my example. The values are either integers or floats. Each key and value is separated by an equals sign (=). Now I want to read the values into variables I have in my program.

This is the code I have tried to read the values: (I omitted the part where I store the values in my program's variables, and just print them out now for demonstration.)

std::fstream file(optionsFile, std::fstream::in);

if (file.good()) {
  int begin;
  int end;
  std::string line;

  while(std::getline(file, line)) {

    // find the position of the value in the line
    for (unsigned int i = 0; i < line.length(); i++) {
      if (line.at(i) == '=') {
        begin = i + 1;
        end = line.length();
        break;
      }
    }

    // build the string... it starts at <begin> and ends at <end>
    const char *string = "";
    for (int i = begin; i < end; i++) {
      string += line.at(i);
    }

    // only gibberish is printed in the following line :(
    std::cout << "string=" << string << std::endl;
  }
}

I don't understand why it won't print the value.. instead only weird stuff or even nothing is printed

Please help this broke my spirit so hard :(

Please write a self-contained snippet with a main function that people can take an experiment with. — László Papp
– László Papp, Commented Aug 24, 2013 at 15:44
What do you mean by: "Check if the key is a certain key (to check if the user messed with it)"? — jrd1
– jrd1, Commented Aug 24, 2013 at 15:49

Nemanja Boric · Accepted Answer · 2013-08-24 15:51:32Z

1

You are using C-style strings (char arrays) without properly allocated memory, and you are just manipulating with the pointer, so you are not appending characters into your string:

   // build the string... it starts at <begin> and ends at <end>
const char *string = "";
for (int i = begin; i < end; i++) {
  string += line.at(i);
}

Use std::string instead:

/// build the string... it starts at <begin> and ends at <end>
std::string str;
for (int i = begin; i < end; i++) {
  str += line.at(i);
}

Or allocate memory by hand, use the proper indexing, terminate the string with '\0' character and don't forget to delete the string after you don't need it anymore:

char *string = new char[end - begin + 1];
int j = 0;
for (int i = begin;  i < end; i++) {
  string[j++] = line.at(i);
}

// Don't forget to end the string!
string[j] = '\0';

// Don't forget to delete string afterwards!
delete [] string;

So, just use std::string.

Edit Why did you mix C strings and std::string in the first place?

edited Aug 24, 2013 at 15:51

answered Aug 24, 2013 at 15:46

Nemanja Boric

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1 Comment

A. D. Over a year ago

Thank you my friend that did the trick.. I don't know why I did such a stupid thing ^^ I used std::string at the very beginning but the compiler said I had to use const char or something I don't remember so I changed it. But I did something wrong but now it's all good

Alex K · Accepted Answer · 2013-08-24 16:31:45Z

As was already mentioned, native string types in c/c++ do not support straightforward concatenation since they are essentially pointers to some preallocated memory. You should always use std::string when a string is supposed to be mutable.

Btw, think about the following refactoring:

void process_option (const std::string& a_key, const std::string& a_value)
{
    std::cout << a_key << " <-- " << a_value << std::endl;
}

void read_options (std::istream& a_in, const char* a_source)
{
    int line_n = 0;
    std::string line;
    while (std::getline(a_in, line))
    {
        ++ line_n;
        std::string::size_type p = line. find('=');
        if (p == line. npos)
        {
        //  invalid_entry(a_source, line_n);
            continue;
        }

        process_option(
            line. substr(0, p), // key
            line. substr(p + 1, line. find_first_of("\t\r\n", p + 1)) // value
        );
    }
}

void read_options (const char* a_filename)
{
    std::ifstream file(a_filename);
    if (! file)
    {
    //  read_error(a_filename);
        return;
    }
    read_options(file, a_filename);
    file. close();
}

void read_options (const std::string& a_filename)
{
    read_options(a_filename. c_str());
}

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