Stop reading at format mismatch

Question

I have a file such as:

 1.0000000e+01   8.0123000e+01   1.0000000e+01   1.0000000e+01   1.0000000e+01
-1.0000000e+01   1.0000000e+01   1.0001110e+01   1.0000000e+01   1.0000000e+01
 1.0000000e+01   1.0000000e+01  -5.0000000e+01   1.0000000e+01   1.0000000e+01
 //... (repeated scientific numbers)
 1 2 3 4
 2 4 5 60
 100 3 5 63
 //... (repeated integer numbers)

I would like to read these numbers from a file in C++, but only the numbers which are in scientific format, so I need the code to stop when the number format changes. I also have this advantage that the float numbers come in 5 columns, whereas integers come in 4 columns.

So, what's the best way to do that in C++?

Answer 1

You could use strstr to search for "e+" in each line.

http://www.cplusplus.com/reference/clibrary/cstring/strstr/

If you wanted to be more fancy you use a regular expression library (such as boost::regex) which would also help you extract the strings from each line.

Answer 2

I am afraid there is no direct way to do this. That is you can't stream in ( >> ) a float number in a specific format. So if you need that functionality, you must read the lines as strings, then manually parse them. Of course, this doesn't mean you have to build a float number digit by digit. Once you've established the boundaries of the input file from which you want to read float, use stringstreams to read them.

Answer 3

You can use a regex to match only the ones you care : -?\d+\.\d+e[+-]\d+

I'm sure that this is not the best way but if performance is not a big issue it's an easy way out

Warning : Auto generated code from RegexBuddy

pcre *myregexp;
const char *error;
int erroroffset;
int offsetcount;
int offsets[(0+1)*3]; // (max_capturing_groups+1)*3
myregexp = pcre_compile("-?\\d+\\.\\d+e[+-]\\d+", 0, &error, &erroroffset, NULL);
if (myregexp != NULL) {
    offsetcount = pcre_exec(myregexp, NULL, subject, strlen(subject), 0, 0, offsets, (0+1)*3);
    while (offsetcount > 0) {
        // match offset = offsets[0];
        // match length = offsets[1] - offsets[0];
        if (pcre_get_substring(subject, &offsets, offsetcount, 0, &result) >= 0) {
            // Do something with match we just stored into result
        }
        offsetcount = pcre_exec(myregexp, NULL, subject, strlen(subject), 0, offsets[1], offsets, (0+1)*3);
    } 
} else {
    // Syntax error in the regular expression at erroroffset
}

Answer 4

Ignoring EOL (continues reading integers):

typedef double d[5] Datum;
Datum d;
vector<Datum> data;
while (true) {
  Datum t;
  istr >> t[0] >> t[1] >> t[2] >> t[3] >> t[4];
  if (!istr) break;
  data.push_back(t);
}

Using column count and EOL:

while (istr) {
  string line;
  getline(istr, line);
  Datum t;
  istringstream temp(line);
  temp >> t[0] >> t[1] >> t[2] >> t[3] >> t[4];
  if (temp.fail()) break;
  data.push_back(t);
}

Answer 5

regex is best way to do that here is alternatively you can try with fscanf()