How to print long int to the screen in c++? - c++

Question

I am trying to return long int from a function but it is not working at all, then I tried to print a long int number to the screen and still not working.
#include<iostream>
using namespace std;
long int maximum_product(long int *input_array, int n){
long int a,b;
a = *input_array;
b = *(input_array + 1);
if (b > a){
long int temp = a;
a = b;
b = temp;
}
for (int i = 2; i < n; i++){
if (input_array[i] > b){
if(input_array[i] > a){
b = a;
a = input_array[i];
} else
b = input_array[i];
}
}
return a * b;
}
int main(){
int n;
cin >>n;
long int *input_array = new long int[n];
for (int i = 0; i < n; i++)
cin >> input_array[i];
cout << maximum_product(input_array, n);
return 0;
}
Here is what I mean by "not working":
#include<iostream>
using namespace std;
int main(){
long int y;
cin >>y;
cout<<y;
return 0;
}
Result of the second program

Answer 1

If you want to make std::cin read
2,147,483,646
as a long you need to do a little bit extra, because without you std::cin will only read the first 2 and ignore the rest.
Lets start simple....std::cin has an overload of its operator>> for long but we want it to do something else. How do we make it pick a different overload? We pass a different type. However, as we dont actually want anything else than a long we use a thin wrapper:
struct read_long_with_comma_simple_ref {
long& long;
read_long_with_comma_simple_ref(long& value) : value(value) {}
};
Once we supply our own operator>> overload we will be able to write something like this:
long x;
std::cin >> read_long_with_comma_simple_ref(x);
So far so simple. How can we implement that operator>>? The most simple I can think of is to simply ignore the commas:
std::istream& operator>>(std::istream& in,read_long_with_comma_simple_ref& ref) {
std::string temp;
// read till white space or new-line
in >> temp;
// remove all commas
temp.erase(std::remove(temp.begin(), temp.end(), ','), temp.end());
// use a stringstream to convert to number
std::stringstream ss(temp);
ss >> rwcs.value;
return in;
}
However, this will accept non-sense such as 12,,,34 as input and interpret it as 1234. This we of course want to avoid and add a bit of error checking:
#include <iostream>
#include <limits>
#include <algorithm>
#include <sstream>
#include <ios>
// same as before, just different name for a different type
struct read_long_with_comma_ref {
long& long;
read_long_with_comma_ref(long& value) : value(value) {}
};
std::istream& operator>>(std::istream& in,read_long_with_comma_ref&& rwc){
std::string temp;
in >> temp;
std::reverse(temp.begin(),temp.end());
std::stringstream ss(temp);
rwc.value = 0;
long factor = 1;
for (int i=0;i<temp.size();++i){
char digit;
ss >> digit;
if (((i+1)%4)==0) {
if (digit != ',') {
in.setstate(std::ios::failbit);
rwc.value = 0;
return in;
}
} else {
int dig = digit - '0';
if (dig < 0 || dig > 9) {
in.setstate(std::ios::failbit);
rwc.value = 0;
return in;
}
rwc.value += factor* (digit-'0');
factor*=10;
}
}
return in;
}
And we can use it like this:
long x;
std::cin >> read_long_with_comma_ref(x);
if (std::cin.fail()) std::cout << "reading number failed \n";
std::cout << x;
TL;DR if you want to make std::cin read the value 2147483646 then simply type 2147483646 and dont use the , as seperator, or read it as string and remove the commas before converting it to a number.