C++ read csv file; get two strings per line

Question

I have a csv file of the following type (more than only three line, this is just that you get the idea):

0000000000005791;Output_0000000000005791_RectImgLeft.bmp;Output_0000000000005791_RectImgRight.bmp
0000000000072517;Output_0000000000072517_RectImgLeft.bmp;Output_0000000000072517_RectImgRight.bmp
0000000000137939;Output_0000000000137939_RectImgLeft.bmp;Output_0000000000137939_RectImgRight.bmp

Note: There is no ";" at the end of each line. I'd like to store the second and third string after the ";" in a string img1 and string img2 and iterate over each row of the csv file, so something like this:

ifstream read_file ( "file.csv" )

while ( read_file.good() ){
      string img1 = get_string_after_first_semicolon;
      string img2 = get_string_after_second_semicolon;
      do_stuff(img1, img1)
}

In the first iteration the strings stored in img1 and img2 shall be

img1 = "Output_0000000000005791_RectImgLeft.bmp"
img2 = "Output_0000000000005791_RectImgRight.bmp"

in the second iteration

img1 = "Output_0000000000072517_RectImgLeft.bmp"
img2 = "Output_0000000000072517_RectImgRight.bmp"

and so forth...

As I've never worked with csv files I don't know how to evaluate each line and each string after a ";".

Just read the file line by line and parse each line. Do you know how to read a file line by line? Do you know how to split a string at a character? — David Schwartz, Aug 13 '15 at 10:44
How do I read a file line by line and take care of the correct splitting of the strings? That's exaclty my question ;) — SemtexB, Aug 13 '15 at 10:45
How to read a file line by line? Or how to split a string at a delimiter character? These are both basic operations that have nothing to do with CSV files. — David Schwartz, Aug 13 '15 at 10:46
Yeah just real line by line (like David said) using std::getline() and use regex to extract the required data from each string. There are a lot of tutorials on the web for regex in C++ 11. If you dont want to use C++ 11, you could use boost libraries or simply make a parser yourself (it's not very difficult). — malhotraprateek, Aug 13 '15 at 10:48
This is a duplicate of http://stackoverflow.com/questions/236129/split-a-string-in-c — aho, Aug 13 '15 at 11:04

Christophe · Answer 1 · 2015-08-13T11:25:01.243

getline() shall be your friend for such parsing:

you could either use getline with delimiter (except for the last string of the line as you have no terminating ';')
or you could simply read the line with it, and iterate through the string with find()

and there are certainly many more ways.

Examples:

I just picked these two, so that you have the basics to read lines and parse characaters in strings.

Illustration of the first approach:

ifstream read_file ( "file.csv" )
string s1,s2,s3; 
while ( getline(read_file,s1,';') &&  getline(read_file,s2,';') &&  getline(read_file,s3) ){
      string img1 = s2;
      string img2 = s3;
      do_stuff(img1, img1)
}

Inconvenience of this approach: as you don't read full lines, you can't ignore wrong input; at the first error you have to stop pasring the file.

The second approach would look like:

string line; 
while ( getline(read_file, line) ){
      int pos1 = line.find(';');  // search from beginning
      if (pos1 != std::string::npos) { // if first found
         int pos2 = line.find (';',pos1+1); 
         if (pos2 != std::string::npos) {
            string img1 = line.substr(pos1+1, pos2-pos1-1); 
            string img2 = line.substr(pos2+1);
            do_stuff(img1, img2);
         }
         else cout << "wrong line, missing 3rd item"<<endl;
      }
      else cout << "wrong line, missing 2nd and 3rd item"<<endl;           
}

Here, it's easier to process errors, count lines, etc.

C++ read csv file; get two strings per line

1 Answers1