How to access data in a struct? C++ - c++

Question

Just quick one, how should I go about printing a value from a struct? the 'winningnums' contains a string from another function (edited down for minimal example)
I've tried the below but the program doesnt output anything at all, is my syntax incorrect?
struct past_results {
std::string date;
std::string winningnums;
};
int main(){
past_results results;
std::cout << results.winningnums;
return 0;
}
EDIT:
just to give some more insight, here's the function that populates my struct members. Is it something here im doing wrong?
//function to read csv
void csv_reader(){
std::string line;
past_results results;
past_results res[104];
int linenum = 0;
//open csv file for reading
std::ifstream file("Hi S.O!/path/to/csv", std::ios::in);
if(file.is_open())
{
while (getline(file, line))
{
std::istringstream linestream(line);
std::string item, item1;
//gets up to first comma
getline(linestream, item, ',');
results.date = item;
//convert to a string stream and put into winningnums
getline(linestream, item1);
results.winningnums = item1;
//add data to struct
res[linenum] = results;
linenum++;
}
}
//display data from struct
for(int i = 0; i < linenum; i++) {
std::cout << "Date: " << res[i].date << " \\\\ Winning numbers: " << res[i].winningnums << std::endl;
}
}

Answer 1

is my syntax incorrect?
No, it's just fine and if you add the inclusion of the necessary header files
#include <iostream>
#include <string>
then your whole program is ok and will print the value of the default constructed std::string winningnums in the results instance of past_results. A default constructed std::string is empty, so your program will not produce any output.
Your edited question shows another problem. You never call csv_reader() and even if you did, the result would not be visible in main() since all the variables in csv_reader() are local. Given a file with the content:
today,123
tomorrow,456
and if you call csv_reader() from main(), it would produce the output:
Date: today \\ Winning numbers: 123
Date: tomorrow \\ Winning numbers: 456
but as I mentioned, this would not be available in main().
Here's an example of how you could read from the file and make the result available in main(). I've used a std::vector to store all the past_results in. It's very practical since it grows dynamically, so you don't have to declare a fixed size array.
#include <fstream>
#include <iostream>
#include <sstream> // istringstream
#include <string> // string
#include <utility> // move
#include <vector> // vector
struct past_results {
std::string date;
std::string winningnums;
};
// added operator to read one `past_results` from any istream
std::istream& operator>>(std::istream& is, past_results& pr) {
std::string line;
if(std::getline(is, line)) {
std::istringstream linestream(line);
if(!(std::getline(linestream, pr.date, ',') &&
std::getline(linestream, pr.winningnums)))
{ // if reading both fields failed, set the failbit on the stream
is.setstate(std::ios::failbit);
}
}
return is;
}
std::vector<past_results> csv_reader() { // not `void` but returns the result
std::vector<past_results> result; // to store all past_results read in
// open csv file for reading
std::ifstream file("csv"); // std::ios::in is default for an ifstream
if(file) {
// loop and read records from the file until that fails:
past_results tmp;
while(file >> tmp) { // this uses the `operator>>` we added above
// and save them in the `result` vector:
result.push_back(std::move(tmp));
}
}
return result; // return the vector with all the records in
}
int main() {
// get the result from the function:
std::vector<past_results> results = csv_reader();
// display data from all the structs
for(past_results& pr : results) {
std::cout << "Date: " << pr.date
<< " \\\\ Winning numbers: " << pr.winningnums << '\n';
}
}

Answer 2

Your example doesn't initialize the struct members. There is no data to print, so why would you expect it to output anything?