Double dimension array read error - c++

Question

I have a double dimension std::string array which I need to pass as argument of function sortString but I get a runtime error when variable student is first read. "0" is output via cout, but not "1". Any idea of where I am wrong?
Here is my code:
#include <iostream>
#include <string>
void sortString(std::string **student, std::string **output, int size)
{
std::string names[5];
std::cout << "0" << std::endl;
for (int i = 0 ; i < size ; i++)
names[i] = student[i][0];
std::cout << "1" << std::endl;
}
int main()
{
std::string student1 [ ] = {"Joe Lime", "15", "2019"};
std::string student2 [ ] = {"Bob Green", "3", "2020"};
std::string student3 [ ] = {"SallyAnne Green" , "1", "2017"};
std::string student4 [ ] = {"Annie Blue", "10", "2020"};
std::string student5 [ ] = {"Jose Lemon", "25", "2016"};
int const size = 5;
std::string student [5][3] = {student1, student2, student3, student4, student5};
std::string sortedByName[5][3];
sortString((std::string**)student, (std::string**)sortedByName, size);
return 0;
}
** ------------ EDIT ------------ **
I wanted to do the same thing as I do for unidimensional arrays, so I don't understand why it doesn't work for 2-dimensional arrays
e.g, this works :
#include <iostream>
int test(int *a)
{
std::cout << a[0] << std::endl;
}
int main()
{
int a[] = {1,2,3,4,5,6,7};
test(a);
}
int test(int *a)
{
std::cout << a[0] << std::endl;
}

Answer 1

You have a big confusion with arrays, pointers and strings. As #Quentin and
#molbdnilo pointed you out, you are doing a C-style conversion from a bidimensional array of std::strings to a pointer to a pointer to a string, and neither arrays are pointers nor pointers are arrays.
My guess is that you want to sort all the students according to their name, while keeping the rest of the student information associated to the corresponding student.
A couple of advices:
Do not use C-style arrays whenever you can use std::array.
To define the constants in your code, create a constant variable, do not write, for example, 5 as size, this can involve multiple changes in different parts of your code when you want to change that constant value, since it can be written in multiple locations.
You don't need to use pointers in your example. They don't make sense in this scenario.
An example of what you are trying to achieve that uses the std::sort function:
#include <string>
#include <array>
#include <iostream>
#include <algorithm>
const unsigned NUMBER_OF_STUDENTS = 5;
const unsigned NUMBER_OF_STUDENT_DATA_FIELDS = 3;
using studentType = std::array<std::string, NUMBER_OF_STUDENT_DATA_FIELDS>;
int main()
{
std::array<std::string, NUMBER_OF_STUDENT_DATA_FIELDS> student1 = {"Joe Lime", "15", "2019"};
std::array<std::string, NUMBER_OF_STUDENT_DATA_FIELDS> student2 = {"Bob Green", "3", "2020"};
std::array<std::string, NUMBER_OF_STUDENT_DATA_FIELDS> student3 = {"SallyAnne Green" , "1", "2017"};
std::array<std::string, NUMBER_OF_STUDENT_DATA_FIELDS> student4 = {"Annie Blue", "10", "2020"};
std::array<std::string, NUMBER_OF_STUDENT_DATA_FIELDS> student5 = {"Jose Lemon", "25", "2016"};
std::array<studentType, NUMBER_OF_STUDENTS> students = {student1, student2, student3, student4, student5};
std::sort(students.begin(), students.end(), [](const studentType& student1, const studentType& student2) {
// first string in the array is the name
const std::string& nameStudent1 = student1.front();
const std::string& nameStudent2 = student2.front();
// we return if the name of student 1 is lexicographically smaller than the name of student 2
return nameStudent1 < nameStudent2;
});
// Let's print the students to see we have order everything correctly
for (const auto& student: students) // for each student
{
for (const auto& studentData : student) // for each field in the student string
{
std::cout << studentData << " ";
}
std::cout << "\n"; // jump to the next line
}
}
Annie Blue 10 2020
Bob Green 3 2020
Joe Lime 15 2019
Jose Lemon 25 2016
SallyAnne Green 1 2017

Answer 2

i fixed your code a bit and got it to work:
#include <iostream>
#include <string>
void sortString(std::string student[][3], std::string output[][3], int size)
{
std::string names[5];
std::cout << "0" << std::endl;
for (int i = 0; i < size; i++)
{
names[i] = student[i][0];
std::cout << names[i] << "\n";
}
std::cout << "1" << std::endl;
}
int main()
{
int const size = 5;
std::string students[5][3] =
{
{ "Joe Lime", "15", "2019" },
{ "Bob Green", "3", "2020" },
{ "SallyAnne Green", "1", "2017" },
{ "Annie Blue", "10", "2020" },
{ "Jose Lemon", "25", "2016" }
};
std::string sortedByName[5][3];
sortString(students, sortedByName, size);
return 0;
}
but i highly recommend you use arrays, vectors and structs/classes. following a made up an example with vector and arrays and vector and structs
#include <iostream>
#include <string>
#include <vector>
#include <array>
#include <algorithm>
void sortString(std::vector<std::array<std::string, 3>>& students)
{
// for example: print all names with range base for loop
for (const auto& s : students)
{
std::cout << s[0] << std::endl;
}
// for example: print all names with "normal" for loop
for (std::size_t i = 0; i < students.size(); ++i)
{
std::cout << students[i][0] << std::endl;
}
// sort by name
std::sort(std::begin(students), std::end(students), [](const std::array<std::string, 3>& a, const std::array<std::string, 3>& b){ return a[0] < b[0]; });
}
int main()
{
int const size = 5;
std::vector<std::array<std::string, 3>> students;
students.push_back({ "Joe Lime", "15", "2019" });
students.push_back({ "Bob Green", "3", "2020" });
students.push_back({ "SallyAnne Green", "1", "2017" });
students.push_back({ "SallyAnne Green", "1", "2017" });
students.push_back({ "Jose Lemon", "25", "2016" });
sortString(students);
return 0;
}
with struct:
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
struct Student
{
std::string name;
std::string dontKnow;
std::string year;
};
void sortString(std::vector<Student>& students)
{
// for example: print all names with range base for loop
for (const auto& s : students)
{
std::cout << s.name << std::endl;
}
// for example: print all names with "normal" for loop
for (std::size_t i = 0; i < students.size(); ++i)
{
std::cout << students[i].name << std::endl;
}
// sort by name
std::sort(std::begin(students), std::end(students), [](const Student& a, const Student& b){ return a.name < b.name; });
}
int main()
{
int const size = 5;
std::vector<Student> students;
students.push_back({ "Joe Lime", "15", "2019" });
students.push_back({ "Bob Green", "3", "2020" });
students.push_back({ "SallyAnne Green", "1", "2017" });
students.push_back({ "SallyAnne Green", "1", "2017" });
students.push_back({ "Jose Lemon", "25", "2016" });
sortString(students);
return 0;
}
i hope you see how much cleaner your code gets