Parsing .txt file for double variable input in C++

Question

Currently working on a snippet to input variables in which a user changes a text file to be used later. Storing these in an array, and later referencing them for some openGL.

The input text file looks something like this.

something = 18.0;

something else = 23.4;

... 6 lines total

//the variable of type ifstream:
ifstream patientInput(".../Patient1.txt");
double n[6]= {0.0,0.0,0.0,0.0,0.0,0.0};
register int i=0;
string line;
//check to see if the file is opened:
 if (patientInput) printf("Patient File Successfully Opened.\n");

else printf("Unable to open patient file\n");

 while(!patientInput.eof())
 {
    getline(patientInput,line);
    char *ptr, *buf;
    buf = new char[line.size() + 1];
    strcpy(buf, line.c_str());
    n[i]=strtod(strtok(buf, ";"), NULL);
    printf("%f\n",n[i]);
    i++;
 }
//close the stream:
patientInput.close();

Right now it is saving all the values in the array as initialized but not overwriting them later, as it should when I am breaking the lines into the tokens. Any help is appreciated.

This doesn't fix your problem, but why are you mixing C++ and C-style strings? For a start, you've engineered yourself a memory leak (where does the memory pointed to `buf` go?). — Oliver Charlesworth, Apr 29 '11 at 18:33
I see that however to use strtok I have to have create buf to convert to a char from a string. Not sure of any other way to fix this.. Also any other ideas for fixing the problem? — Kyle, Apr 29 '11 at 18:50

score 1 · Answer 1 · edited May 23 '17 at 10:24

It looks to me like the bug is here:

n[i]=strtod(strtok(buf, ";"), NULL);

On the first run through the while loop, strtok() will return a C string like "something = 18.0".

And then strtod() will try to convert that to a double, but the string "something = 18.0" is not so easily converted to a double. You'll want to tokenize the initial "something =" first, and throw that data out if necessary (or do something with it, if you want to).

You may want to refer to this thread to get ideas for some more C++-style ways to tokenize your string, instead of C-style like you're currently using:

How do I tokenize a string in C++?

Good luck!

score 0 · Accepted Answer · answered Apr 29 '11 at 20:12

To apply what NattyBumppo had to say just change:

n[i]=strtod(strtok(buf, ";"), NULL);

to:

strtok(buf," =");
n[i] = strtod(strtok(NULL, " ;"), NULL);
delete buf;

Of course, there are a lot of other ways to do it without strtok.

Here's one example:

ifstream input;
input.open("temp.txt", ios::in);
if( !input.good() )
    cout << "input not opened successfully";

while( !input.eof() )
{
    double n = -1;
    while( input.get() != '=' && !input.eof() );

    input >> n;
    if( input.good() )
        cout << n << endl;
    else
        input.clear();

    while( input.get() != '\n' && !input.eof() );
}

Parsing .txt file for double variable input in C++

2 Answers2