Create the largest possible number from input - implementation issue with cout - c++

Question

I am trying to implement an algorithm that will take a set of numbers and output the largest possible number (without breaking up the individual numbers). So in an example like this where I give 4 numbers:
4
43 12 3 91
The output would be
91-43-3-12 or 9143312.
My attempt is below.
#include <algorithm>
#include <sstream>
#include <iostream>
#include <vector>
#include <string>
using std::vector;
using std::string;
bool compare (int x, int y) {
std::cout << "in func \n";
string a = std::to_string(x);
string b = std::to_string(y);
std::cout << a << " " << b << "\n";
std::cout << std::stoi(a.substr(0, 1)) << " " << std::stoi(b.substr(0, 1)) << "\n" ;
if (std::stoi(a.substr(0, 1)) < std::stoi(b.substr(0, 1))) {
std::cout.flush();
std::cout << "if \n";
return true;
}
else {
std::cout.flush();
std::cout <<"else \n";
return false;
}
}
string largest_number(vector<string> a) {
std::stringstream ret;
while (a.size() > 0) {
int maxNumber =-1;
int index = -1;
std::cout << "going into for " << a.size() << "\n";
for (size_t i = 0; i < a.size(); i++) {
if (! compare (stoi(a[i]), maxNumber ) ) { //stoi(a[i]) >= maxNumber) {
maxNumber = stoi(a[i]);
std::cout << maxNumber << " " << i << "\n";
index = i;
}
std::cout << "here \n";
}
ret << maxNumber;
a.erase(a.begin() + index);
}
string result;
ret >> result;
return result;
}
int main() {
int n;
std::cin >> n;
vector<string> a(n);
for (size_t i = 0; i < a.size(); i++) {
std::cin >> a[i];
}
std::cout << largest_number(a);
}
I do not understand what is wrong with my compare function. When I run it, say with this input:
$ g++ -pipe -O2 -std=c++14 largest_number.cpp -lm -o largest1
$ ./largest1.exe
4
4 23 1 45
going into for 4
in func
4 -1
It doesnt print the cout statements in the conditional if or else. How could this be possible? I even tried flushing. However, if I take the entire conditional out, put a cout statement and the return true or something, then it runs the program in entirety (although this is not the expected output).
I do not mind harsh criticism. What am I doing wrong here? Any advice will be appreciated.
Thanks in advance.

Answer 1

In this statement
std::cout << std::stoi(a.substr(0, 1)) << " " << std::stoi(b.substr(0, 1)) << "\n" ;
when b is equal to -1 the expression b.substr(0, 1) is equal to an object of type std::string that contains one character '-' that is the minus sign.
If to apply the standard function std::stoi to such a string then an exception will be thrown.
Consider the following code snippet
std::string s("-");
try
{
std::stoi(s);
}
catch (const std::exception &e)
{
std::cout << e.what() << std::endl;
}
Its output will be
invalid stoi argument
It seems what you need is just to sort the strings. For example
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
int main()
{
std::vector<std::string> v { "4", "23", "1", "45" };
auto cmp = [](const std::string &a, const std::string &b)
{
std::string::size_type i = 0, m = a.size();
std::string::size_type j = 0, n = b.size();
int result;
do
{
if (m < n)
{
result = a.compare(i, m, b, j, m);
j += m;
n -= m;
}
else
{
result = a.compare(i, n, b, j, n);
i += n;
m -= n;
}
} while (result == 0 && m && n);
return 0 < result;
};
std::sort(v.begin(), v.end(), cmp);
for (const auto &s : v) std::cout << s;
std::cout << std::endl;
return 0;
}
The output of the program will be
454231
Or for this set of numbers
std::vector<std::string> v{ "43", "12", "3", "91" };
the output will be
9143312
or for one more set of numbers
std::vector<std::string> v{ "93", "938" };
the output will be
93938