Matrix of characters in c++ [closed] - c++

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I need matrix of characters to save 'n' subject (some subjects have 3-4 words). The user will enter 'n' number of subjects and names of subjects. I can only use CHAR, not STRING or VECTOR.
#include <iostream>
using namespace std;
int main() {
int n;
cin>>n;
char matrix[n][100];
for(int i=0; i<n; i++) {
cin.getline(matrix[i][100]);
}
}

Answer 1

First problem with your code is that you are trying to make a variable size array with static allocation which C++ won't like (your n varies and you are trying to make char matrix[n][100]). You will need to use dynamic allocation for that.
Second problem is that getline requires an additional parameter to work (max number of characters per word you can take in, which is in your case 100).
And the third problem is that you are trying to put a whole word into a place for a single character in cin.getline(matrix[i][100]);. I'm not sure you understand why are you exactly making a matrix[n][100] so I will briefly explain.
One char stores a single character, like 'a'.
char oneCharacter = 'a';
To make a word out of those you would want few characters, so you make an array of characters. In the example I made an array of character which can take up to 5 of them.
char multipleCharacters[5];
And now you want to store a few words, so you make an array of array of characters. In the example I made an array of array of characters which can take up to 10 words of 5 characters. To make an array which size you are going to input, look up dynamic allocation from the first part of my answer.
char multipleWords[10][5];
Knowing this, can you figure out whats wrong with cin.getline(matrix[i][100]);?

Answer 2

As I said in a comment earlier, this is a terrible assignment. You are being asked to do several things in "odd" ways, not using and learning best practices for writing code.
My advice in general: start by separating the input, output, and "work".
Let's suppose that the code for the real lesson here is all in the "work" part. There are indeed many reasons for learning to manipulate memory this way, and C++ for writing low-level, system, or efficient code is a major part of the craft.
So, you want to arrange n words into a 2-D array of characters. Focus on (just) that, by passing in the data to be so-arranged using normal C++ code. Abstract it as a class to hold the data. Maybe this is the binary image to send to a dumb LCD on a keosk, or label printer? There may be a real reason to do something like this.
Putting in a class you can make sure memory is freed correctly; normally you want no naked new's in your code, and will use existing container types rather than low-level memory. Here, we'll use a unique_ptr of rows.
class project {
static constexpr size_t rowlen = 100;
using row = char[rowlen];
std::unique_ptr<row[]> matrix;
public:
void layout (const vector<string>& inputs);
// todo.. provide a good way to access the data
};
Now don't worry about reading from the user, especially reading directly into an odd format! That just makes this extra challenging for no reason. Take the data (presented in a normal easy-to-use way) and concentrate on doing the low-level memory manipulation chore: Allocate input.size() rows, and for each input string clamp to the maximum length and copy the characters into one row (what about the rest of the chars? Assume should be space or what?)
Clamp the length using std::max. Copy the actual characters using std::copy.
Something like this (untested; just off the top of my head):
void project::layout (const vector<string>& inputs)
{
constexpr auto N = input.size();
matrix = new row[N];
for (const size_t i= 0; i < N; ++i) {
const auto len = std::max(input[i].size(), rowlen);
auto& dest = matrix[i]; // alias the whole row
char* p= &dest[0]; // pointer to first char in the row
std::copy_n (input[i].data(), len, p);
std::fill_n (p+len, rowlen-len, ' ');
}
}
See https://en.cppreference.com/w/cpp/algorithm for details on the std algorithms. Keep that site bookmarked; it is a great way to look things up while you write code.
To do this, you need to understand how pointers work in C (and thus in C++). Also, how the same "place" can be different types: A pointer to the whole row is the same address as the first character of that row, but they are different types. So do things a little piece at a time, just as with the earlier definition of matrix.