File input/output C++

Question

I am wanting to read a file of the following: Where lines starting with c stand for comments, p stands for graph information (no of nodes, no of edges) and e stands for edges.

c   //Comments
c   //Comments
c   //Comments
p edge  50  654
e 4 1
e 5 3
e 5 4
e 6 2
e 6 3
... // 654 edges

My idea of how this would work is:

1. read line by line until the first index of a line == 'p';
2. initialize my matrix to size of nodes, which would be 50 here.
3. read amount_of_edges -number of lines, saving them into my data structure (which would be 654 lines in the example).

I know how I could do this simply in Python, however I just don't know where to get started with c++.

Answer 1

You can try to do it like this:

ifstream file("myfile.txt");
string line;
while(true){
    getline(file, line);
    if(line[0]=='p') break;
 }
//now line contains a line starting with 'p' which contains the numbers
//that you need
stringstream data_from_line_with_p;
data_from_line_with_p << line;
//we have send the line to a stream, from which we can read to the variables
string p, edges;
int size_of_nodes, amount_of_edges;
//let's now assign these variables with data from the stream:
data_from_line_with_p >> p >> edges >> size_of_nodes >> amount_of_edges;

//now size_of_nodes and _amount_of_edges store the values read from this
//line and you can use them to build the structure that you want
//You also store here the word after 'p' in the variable 'edge' and you 
//can use it also if you need.

Remember to add a header for using stringstreams: #include<sstream> .