Reading from file in C++, dealing with blanks - c++

Question

I am tying to read from a text file in C++98. It has a pattern, but sometimes a field is empty:
ID Name Grade level
1 a 80 A
2 b B
3 c 90 A
How can I read from file such that I can ignore the blanks?
( I wish I could simply use Regex: \d*)
Is there any simple way of doing that?

Answer 1

You need to use what knowledge you have about the input to make assumptions about what is missing. You can use std::stringstream to parse individual terms from a text line. In other words std::stringstream deals with blanks by ignoring spaces and getting a complete term only, for example std::stringstream("aaa bbb") >> a >> b will load strings a with "aaa" and b with "bbb".
Here is an example program that parses the input, making a robust parser from scratch can be difficult, but if your input is strict and you know exactly what to expect then you can get away with some simple code:
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
//-----------------------------------------------------------------------------
// holds a data entry
struct Entry {
int id;
std::string name;
int grade;
std::string level;
Entry() {
// default values, if they are missing.
id = 0;
name = "Unknown";
grade = 0;
level = "?";
}
void ParseFromStream( std::stringstream &line ) {
std::string s;
line >> s;
if( s[0] >= '0' && s[0] <= '9' ) {
// a number, this is the ID.
id = atoi( s.c_str() );
// get next term
if( line.eof() ) return;
line >> s;
}
if( s[0] >= 'a' && s[0] <= 'z' || s[0] >= 'A' && s[0] <= 'Z' ) {
// a letter, this is the name
name = s;
// get next term
if( line.eof() ) return;
line >> s;
}
if( s[0] >= '0' && s[0] <= '9' ) {
// a number, this is the grade
grade = atoi( s.c_str() );
// get next term
if( line.eof() ) return;
line >> s;
}
// last term, must be level
level = s;
}
};
//-----------------------------------------------------------------------------
int main(void)
{
std::ifstream input( "test.txt" );
std::string line;
std::getline( input, line ); // (ignore text header)
while( !input.eof() ) {
Entry entry;
std::getline( input, line ); // skip header
if( line == "" ) continue; // skip empty lines.
entry.ParseFromStream( std::stringstream( line ));
std::cout << entry.id << ' ' << entry.name << ' ' <<
entry.grade << ' ' << entry.level << std::endl;
}
return 0;
}