modify number of variables in istringstream - c++

Question

I need to get input in a line based on a modifiable number of variables that are determined from an earlier input.
If the input you get in an earlier function is 1, the the code would be like
std::string istring;
std::getline(std::cin, istring);
std::istringstream stream(istring);
while (stream >> a)
{
statement;
}
is it possible to make a condition for while loop that changes depending on what your input is ? so that if the input was for example 5, it would behave like
while (stream >> a >> a >> a >> a >> a)
{
statement;
}
I tried doing while ((stream>>a)*number) but that doesn't work

Answer 1

Specific Solution
First to answer your specific question:
I need to get input in a line based on a modifiable number of variables that are determined from an earlier input.
The solution in C++ to that is a combination of a istringstream, a while loop and a vector:
#include <iostream>
#include <string>
#include <sstream>
#include <vector>
int main() {
unsigned int earlierInput = 5; // this is the earlier input
std::vector<int> numbers; // The type of vector also controls what
// data type you want the user to give
// you. Because the stream will try to
// the user input into that type.
while (numbers.size() != earlierInput) {
// Here we ask the user for input and convert the input
// to a stream, like that we can try to convert the user
// input into the data type we want.
std::string istring;
std::cout << "Please type in " << earlierInput << " numbers: ";
std::getline(std::cin, istring);
std::istringstream reader(istring);
// read as many numbers as possible.
for (int number; reader >> number;) {
numbers.push_back(number);
}
if (numbers.size() != earlierInput) {
// if not enough numbers empty vector and print
// error message
std::cout << "Not the right amount of numbers!";
std::cout << "Try again!" << std::endl;
numbers.clear();
}
}
// If we get here the user typed in the right amount of numbers
std::cout << "You typed the following numbers:" << std::endl;
for (const int& i : numbers) {
std::cout << i << std::endl;
}
}
This is for exactly one user input. If you want to ask the user an arbitrary amount of times to do this (e.g you want to ask the user 10 times for 5 numbers), then you need to apply the rule above again and as such stack up:
#include <iostream>
#include <string>
#include <sstream>
#include <vector>
int main() {
unsigned int amountOfInputs = 2; // this is the amount of times
// you want the user to type
// something in
unsigned int earlierInput = 5; // this is the earlier input
std::vector<std::vector<int>> allNumbers; // Now we need a vector in
// a vector to store the results.
// The type of the inner vector also
// controls what data type you want the
// user to give you.
while (allNumbers.size() != amountOfInputs) {
std::vector<int> numbers; // This is the vector as in the example
// above.
while (numbers.size() != earlierInput) {
// Here we ask the user for input and convert the input
// to a stream, like that we can try to convert the user
// input into the data type we want.
std::string istring;
std::cout << "Please type in " << earlierInput << " numbers: ";
std::getline(std::cin, istring);
std::istringstream reader(istring);
// read as many numbers as possible.
for (int number; reader >> number;) {
numbers.push_back(number);
}
if (numbers.size() != earlierInput) {
// if not enough numbers empty vector and print
// error message
std::cout << "Not the right amount of numbers!";
std::cout << "Try again!" << std::endl;
numbers.clear();
}
}
// If we get here the user typed in the right amount of numbers
// and we can save them and clear the array for using it again.
allNumbers.push_back(numbers);
numbers.clear();
}
std::cout << "You typed the following numbers:" << std::endl;
unsigned int round = 1;
for (auto numbersOfRound : allNumbers) {
std::cout << "For round " << round++ << ": ";
for (auto i: numbersOfRound) {
std::cout << i;
}
std::cout << std::endl;
}
}
Theory
Why Vectors
To save an arbitrary amount of data and be able to access it, you must use a dynamic allocated array or anything alike. That is because at compile time you don't know how many variables you will need at runtime and as such you cannot give all of them a name.
Why streams
In theory a stream is a possibly infinit long string or data (see also here). A user input is therefor a stream as it may theoretically be infinit long. Though this does not apply in praxis.
To extract informations from a stream one has to use the >> operator, also known as the extraction operator. This operator does not support vectors as an (rhs) operand as stated in the documentation here. It supports only basic datatypes (that is why we need a temporary variable int number in the examples above).