Cannot input a 2D array in c++ using for [duplicate] - c++

Question

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What is error in code for adding 3*3 matrix? [duplicate]
(2 answers)
Closed 1 year ago.
I'm trying to input a 2D array in c++ but my code doesn't even get to calculation lines:
int dim1, dim2;
cout << "Enter first dimension: ";
cin >> dim1;
cout << endl << "Enter second dimension: ";
cin >> dim2;
int arr[dim1][dim2];
for (int i = 1; i <= dim1; i++) {
for (int j = 1; j <= dim2; j++) {
cin >> arr[i][j];
}
}
for (int i = 1; i <= dim1; i++) {
for (int j = 1; j <= dim2; j++) {
cout << arr[i][j] << ' ';
}
cout << endl;
}

Answer 1

It's simple, you should start inputing an array in c++ from 0, so your loop should begin with i = 0 and should be stopped when i == n.
for (int i = 0; i < dim1; i++) {
for (int j = 0; j < dim2; j++) {

Answer 2

Your code has two problems.
First, VLA arrays is not valid C++ code. You have to use std::vector.
Second. C++ array indexes start at 0 and end at size - 1. But you are looping from 1 to n. This creates out-of-bounds.
Your code should look like this:
int dim1, dim2;
std::cout << "Enter first dimension: ";
std::cin >> dim1;
std::cout << endl << "Enter second dimension: ";
std::cin >> dim2;
std::vector<std::vector<int>> arr;
arr.resize(dim1, std::vector<int>(dim2));
for (int i = 0; i < dim1; i++) {
for (int j = 0; j < dim2; j++) {
std::cin >> arr[i][j];
}
}
for (int i = 0; i < dim1; i++) {
for (int j = 0; j < dim2; j++) {
std::cout << arr[i][j] << ' ';
}
std::cout << endl;
}

Answer 3

First in C++, the size of an array must be a compile-time constant.So, take for example the following code snippets:
int n = 10;
int arr[n]; //INCORRECT because n is not a constant expression
The correct way to write the above would be:
const int n = 10;
int arr[n]; //CORRECT
Similarly, the following(which you did in your code example) is incorrect:
int dim1, dim2;
cout << "Enter first dimension: ";
cin >> dim1;
cout << endl << "Enter second dimension: ";
cin >> dim2;
int arr[dim1][dim2];//INCORRECT
You should instead use std::vector<> as shown below:
#include <iostream>
#include <vector>
int main()
{
int dim1, dim2;
std::cout << "Enter first dimension: ";
std::cin >> dim1;
std::cout << std::endl << "Enter second dimension: ";
std::cin >> dim2;
std::vector<std::vector<int>> arr(dim1, std::vector<int>(dim2));
for (int i = 0; i < dim1; i++) {
for (int j = 0; j < dim2; j++) {
std::cin >> arr[i][j];
}
}
for (int i = 0; i < dim1; i++) {
for (int j = 0; j < dim2; j++) {
std::cout << arr[i][j] << ' ';
}
std::cout << std::endl;
}
return 0;
}