problem converting string-char-int

[mickey]

Hi,
I'm trying to read from a file a char for time (that are float eg line of file: 1.1 1.0 3);

vector <vector <float> > example;
	char ch;
	string s;
	int i=0,j=0;
	while (f.readChar(ch)) { 	//this is my function return a char  on ch			
		cout.put(ch); 
		if (ch == '\n') lines++;				
		else if (ch != ' ') {
			s += ch;
		} else { 
			example[i][j] = atof(s.c_str()); //but here a runtime error uccurs
			s="";  
			j++;
		}
 
		i++;
	}

vector <vector <float> > example;
	char ch;
	string s;
	int i=0,j=0;
	while (f.readChar(ch)) { 	//this is my function return a char  on ch			
		cout.put(ch); 
		if (ch == '\n') lines++;				
		else if (ch != ' ') {
			s += ch;
		} else { 
			example[i][j] = atof(s.c_str()); //but here a runtime error uccurs
			s="";  
			j++;
		}
		
		i++;
	}

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How to do that conversion? And then: is this the easier way to reach my aim? (I suspect it can be more simple).
thanks