why is this code not giving the desired output? - c++

Question

#include <sstream>
#include <vector>
#include <iostream>
using namespace std;
vector<int> parseInts(string str) {
istringstream ss(str);
vector<int> integ;
int val;
while(ss){
if(ss>>val){
integ.push_back(val);
}
}
return integ;
}
vector<int> parseInts2(string str)
{
vector<int> vec;
stringstream ss(str);
char ch;
int temp;
while(ss)
{
ss>>temp>>ch; >> operator
vec.push_back(temp);
}
return vec;
}
int main() {
string str;
cin >> str;
vector<int> integers = parseInts(str);
for(int i = 0; i < integers.size(); i++) {
cout << integers[i] << "\n";
}
return 0;
}
i want to create a stream,to a string,read integers to the stream from the string and insert it in a vector and display its elements while the output is displaying nothing.what is wrong with the code?
EDIT
basically the question ask inputs in the form of integers that are separated by commas and asks us to print the integers after parsing it. i find no significant difference between the 2 functions but parseInt2 still works(while calling the function in main,of course instead of parseInt). Why?

Answer 1

I fear that your question will be closed by people on SO.
But let me give you the answer.
Basically everything set already in the comments. Why not in an answer? I do not know.
Before you can read something from an std::istringstream, you need to put something in it. You need to initialize it. That is usually done by using its constructor:
istringstream ss(str);
In main, you have the problem, that you read only one value from std::cin with cin >> str;. You want to use std::getline instead, which reads a complete line. And not only "something" up to the next space. So
getline(cin, str);
will help you further.
In modern C++, with keeping the std::istringstream approach, you would probably write
#include <iostream>
#include <string>
#include <vector>
#include <iterator>
#include <sstream>
int main() {
// Read a line and check, if that worked
if (std::string str; std::getline(std::cin, str)) {
// Create and initialize a std::istringstream
std::istringstream iss(str);
// Define a variable integers, use its range constructor with iterators
std::vector integers(std::istream_iterator<int>(iss), {});
// Range based for loop
for (const int& i : integers) {
std::cout << i << "\n";
}
}
return 0;
}
That will save the subfunction.
EDIT:
OK, you want to read csv and you must use ">>".
If you want to read data separated by comma from a stream, then you need to extract:
an integer value from the stream
then a comma
then a integer
then a comma
then a integer
. . .
The extractor operator, or the functionality behind it, will always extract characters from a stream and convert it to a requested type (e.g. an integer), until it reaches a space or the conversion can not be continued any longer (for example, a "," is a separator).
That is the reason, why your 2nd function works.
It is important that you alwys check the status of the extraction operation. In the below example you will see that, at the end of the string, we try to read a comma, where there is none. The extraction fails, but we do not care. We ignore it by intent. To understand the functionality better, please see.
#include <iostream>
#include <string>
#include <sstream>
#include <vector>
int main() {
// Source or test data. We put it directly into the stream;
std::istringstream ss{ "1,2,3, 4 , 5,6" };
std::vector<int> integers{};
char comma{};
int integer{};
while (ss) {
// Read integer and check, if it could be read
if (ss >> integer) {
integers.push_back(integer);
std::cout << "Read Integer " << integer << "\n";
}
else
std::cerr << "Error: Could not read integer\n";
// Now read the comma
if (ss && (ss >> comma))
std::cout << "Read Comma: " << comma << "\n";
else
std::cerr << "Error: Could not read comma\n";
}
// SHow all values
for (const int i : integers) std::cout << i << "\n";
return 0;
}
If you have questions, I am happy to answer.