Treat a space(or tab) separated file “like” and array - C++

Question

I have a short tab/space separated file (I can create it accordingly) with the structure

[data00] <space> [data01] <space> [data02] <space> [data03] <newline>
[data10] <space> [data11] <space> [data12] <space> [data13] <newline>
...

The first column representing a numerical ID. I create this file to feed it to another executable, so the format is fixed. After feeding it, the executable outputs another file with the similar structure:

[data00] <space> [data01]<newline>
[data10] <space> [data11]<newline>
...

Given an ID, I need to read the corresponding [dataX1] , perform operations on [dataX3] in the first file, feed it back to the executable, and iterate.

I think of two way of doing this:

• Operate on the two textfile "as if" they were array, given that their structure is fixed, but I am lost on what function/syntax to use. This should be a small function that would allow me to read the interesting bit by passing it the relevant numeric ID hiding all the pesky I/O code, as I probably need to repeat this operation a lot in different context
• Keep the first file in arrays and trick the executable by feeding it a stream (is this possible? the executable expects a file as argument).

I could easily read the files into arrays and write the files anew each time, but I want to avoid useless read and write operation, when what I need to read/write is just one cell each time. What I don't now how to do is how to stop/identify the interest bit when I read a whole line from the text file by using,say, getline .

Answer 1

First we will write a function that will split an inputted string based upon a given separator. (In this case we will use space.)

int split(const std::string& line, const std::string& seperator, std::vector<std::string> * values){
    std::string tString = "";
    unsigned counter = 0;
    for(unsigned l = 0; l < line.size(); ++l){
        for(unsigned i = 0; i < seperator.size(); ++i){
            if(line[l+i]==seperator[i]){
                if(i==seperator.size()-1){
                    values->push_back(tString);
                    tString = "";
                    ++counter;
                }else continue;
            }else{
                tString.push_back(line[l]);
                break;
            }
        }
    }
    return counter;
}

Now we will write ourselves a simple main to read a file, use split to break it up, and then output the data based upon its location within the file.

int main(){
    std::vector<std::vector<std::string> > lines;
    std::string tString = "";
    std::vector<std::string> tVector;
    std::ifstream fileToLoad;

    fileToLoad.open(FILE_NAME);

    if(fileToLoad.is_open()){
        while(std::getline(fileToLoad,tString)){
            split(tString, " ", &tVector);
            lines.push_back(tVector);
            tVector.clear();
        }

        //Now print our output.
        for(unsigned i1 = 0; i1 < lines.size(); ++i1){
            for(unsigned i2 = 0; i2 < lines[i1].size(); ++i2){
                std::cout<<"["<<i1<<","<<i2<<"] = "<<lines[i1][i2]<<std::endl;
            }
        }
    }else{
        std::cerr<<"FAILED TO OPEN FILE: "<<FILE_NAME<<std::endl;
        return 1;
    }
    return 0;
}

The input file I used has the data:

450 105 10 10.5 -10.56001 23
10 478 1290 384 1289 3489234 1 2 3 4 5
1 2 3 4 5 6.1 19 -1.5

And the output gives:

[0,0] = 450
[0,1] = 105
[0,2] = 10
[0,3] = 10.5
[0,4] = -10.56001
[1,0] = 10
[1,1] = 478
[1,2] = 1290
[1,3] = 384
[1,4] = 1289
[1,5] = 3489234
[1,6] = 1
[1,7] = 2
[1,8] = 3
[1,9] = 4
[2,0] = 1
[2,1] = 2
[2,2] = 3
[2,3] = 4
[2,4] = 5
[2,5] = 6.1
[2,6] = 19

Now all that you need to do is use your favorite parsing algorithm to change each string into a double. (strtod, atof, etc) Depending how important optimization is you may also want to modify the container from vector, depending upon your use cases.