C++ Need help sorting a 2D string array - c++

Question

I'm a little stuck with sorting a string Table[X][Y]. As tagged, Im using C++ and have to use standard libraries and make it for all C++ (not only C++ 11).
The size of the Table is fixed (i get the X reading how many lines a file has and the Y is fixed because thats the different "attributes" has each line).
When i create the Table, each part of it is obtained as Table[X][Y] = stringX.data(); from things previously read from a file and stored in strings. I have numbers in the first column (the one im going to use as sorting criteria), names, address, etc in the others.
The part where the Table is created is:
Table[i][0] = string1.data();
Table[i][1] = string2.data();
Table[i][2] = string3.data();
Table[i][3] = string4.data();
Table[i][4] = string5.data();
Where "i" is the current "iteration" of a while(fgets) that reads one line at a time from a file, does some operations and stores in those strings the "final values" of each part of the line read.
I have to sort that Table using the first column as criteria in decreasing order.
Lets imagine the Table is like this: Table[4][3]
20 | Jhon | 14th July
2 | Mary | 9th June
44 | Mark | 10th December
1 | Chris | 4th Feb
And i need the output to be this:
44 | Mark | 10th December
20 | Jhon | 14th July
2 | Mary | 9th June
1 | Chris | 4th Feb
I have been reading several questions and pages and they sort int/chars arrays or convert the array into vector and then work with them. Im trying to sort the string Table i have without converting anything (dunno if possible).
I dont know if i managed to explain the issue and the situation i have clear enough. Im not putting all the code i have because apart from the declaration of the string Table and the strings that are then placed as string.data in the Table, the rest of the code has nothing to do with the Table and the sorting process. The code opens the file, reads line by line, filters the info i need from some separators and special characters and places each of the "rankings criteria" to a string, then assigns a "ranking" after evaluating each of the criterias and giving a total score (which then is stored in "string1").
After all this is done, i create the string Table[x][y] and place the filtered and processed information in that Table one row at a time (because i assing this while reading each line from the file).
The only thing that remains is the sorting of the table from the best scored to the last and then create a file with the top 10.
I appreciate and thank in advance the time you took reading this and any tip, information, code or source from where i can read this that you could provide.

Answer 1

First, as mentioned in the comments, a variable length array is accomplished in C++ by using std::vector. The current syntax you're using
std::string Table[X][Y]
where either X or Y are runtime variables, is not legal C++. Given your example, a standard C++ declaration would be this:
std::vector<std::array<std::string, 3>> Table;
So let's assume that this is what you are going to use.
The next step is to sort the data based on the first column. That can be accomplished by utilizing the std::sort algorithm function, along with the appropriate predicate indicating that you are using the first column as the sorting criteria.
Here is a short example, using your data, of how this is all accomplished:
#include <vector>
#include <array>
#include <iostream>
#include <algorithm>
#include <string>
int main()
{
std::vector<std::array<std::string, 3>> Table;
// Set up the test data
Table.push_back({"20", "Jhon", "14th July"});
Table.push_back({"2", "Mary", "9th June"});
Table.push_back({"44", "Mark", "10th December"});
Table.push_back({"1", "Chris", "4th Feb"});
std::cout << "Before sort:\n\n";
for (auto& s : Table)
std::cout << s[0] << " | " << s[1] << " | " << s[2] << "\n";
std::cout << "\n\nAfter sort:\n\n";
// Sort the data using the first column of each `std::array` as the criteria
std::sort(Table.begin(), Table.end(), [&](auto& a1, auto& a2)
{ return std::stoi(a1[0]) > std::stoi(a2[0]); });
// Output the results:
for (auto& s : Table)
std::cout << s[0] << " | " << s[1] << " | " << s[2] << "\n";
}
Here is the final output:
Before sort:
20 | Jhon | 14th July
2 | Mary | 9th June
44 | Mark | 10th December
1 | Chris | 4th Feb
After sort:
44 | Mark | 10th December
20 | Jhon | 14th July
2 | Mary | 9th June
1 | Chris | 4th Feb
The output needs a little bit of formatting, but that's not important.
Remember, it is not important as to where the data comes from, whether it is from a file or hardcoded as the example above shows. However you populate the Table, that's up to you. The goal is to show you once populated, how to sort the data.
The first thing we did was create the Table and fill it in with the test data. Note that the vector has a push_back function to add entries to the vector.
Then the call to std::sort has a predicate function (the lambda), where the predicate is given two items, in this case it would be two std::array's by reference. Then the goal is to return if the first std::array (in this case, a1) should be placed before the second std::array (a2).
Note that we only care about the first column, so we only need to consider array[0] of each of those arrays, and compare them.
Also note that since array[0] is a std::string, we simply can't compare it lexicographically -- we need to convert the string to an int and compare the int value. That's the reason for the std::stoi call to convert to an integer.
The final thing about the sort predicate is that we want to have a descending sort. Thus the comparing operator to use is > instead of the "traditional" < (which would have sorted in a ascending manner).
Hopefully this explains what the code is doing.
Edit:
Since you are attempting to get this code to work in C++98, the easiest way to do that is
Change to std::vector<std::vector<std::string>> instead of std::vector<std::array<std::string, 3>>
Not use the brace-initialization that C++11 offers
Use a comparison function instead of a lambda.
Given that, here is the code for C++98:
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
bool SortFirstColumn(const std::vector<std::string>& a1,
const std::vector<std::string>& a2)
{
return atoi(a1[0].c_str()) > atoi(a2[0].c_str());
}
int main()
{
std::vector<std::vector<std::string>> Table;
// Set up the test data
std::vector<std::string> vect(3);
vect[0] = "20";
vect[1] = "Jhon";
vect[2] = "14th July";
Table.push_back(vect);
vect[0] = "2";
vect[1] = "Mary";
vect[2] = "9th June";
Table.push_back(vect);
vect[0] = "44";
vect[1] = "Mark";
vect[2] = "10th December";
Table.push_back(vect);
vect[0] = "1";
vect[1] = "Chris";
vect[2] = "10th December";
Table.push_back(vect);
std::cout << "Before sort:\n\n";
for (size_t i = 0; i < Table.size(); ++i)
std::cout << Table[i][0] << " | " << Table[i][1] << " | " << Table[i][2] << "\n";
std::cout << "\n\nAfter sort:\n\n";
// Sort the data using the first column of each `std::vector<std::string>` as the criteria
std::sort(Table.begin(), Table.end(), SortFirstColumn);
// Output the results:
for (size_t i = 0; i < Table.size(); ++i)
std::cout << Table[i][0] << " | " << Table[i][1] << " | " << Table[i][2] << "\n";
}