c++) i have error when putting 1D array in 2D array

Question

whats the prob

#include <iostream>
using namespace std;
int main() {
    int M; //height
    int N; //length
    cin >> M;
    cin >> N;
    int list[M][N], smaller[N];
    string smaller_str;
    for (int i = 0;i < M; ++i){
        getline(cin, smaller_str);
        for (int j = 0; j < N; i = i++) {
            cin >> smaller_str[j];
        }
        list[i] = smaller;
    }
}

i want to put 1D array "smaller" in to 2D array list however i do get errors in the "list[i] = smaller;" part i need help guys

You cannot assign things to arrays (not elements of arrays). What do you want to do? — MikeCAT, Feb 03 '21 at 14:45
Also be careful fot this: [c++ - Why does std::getline() skip input after a formatted extraction? - Stack Overflow](https://stackoverflow.com/questions/21567291/why-does-stdgetline-skip-input-after-a-formatted-extraction) — MikeCAT, Feb 03 '21 at 14:45
int list[M][N], smaller[N]; // this is not valid c++, M and N must be constants. you need to use "new" to allocate the arrays — Sven Nilsson, Feb 03 '21 at 14:47
[Why aren't variable-length arrays part of the C++ standard?](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) — 463035818_is_not_an_ai, Feb 03 '21 at 14:47
@SvenNilsson no they dont need to use `new`. They can use `new` but shouldnt — 463035818_is_not_an_ai, Feb 03 '21 at 14:47
i want to put array "smaller" as the element of array "list" — hagi, Feb 03 '21 at 14:49
but you can't do that. Arrays do not work this way. You can copy the elements though — 463035818_is_not_an_ai, Feb 03 '21 at 14:50
@SvenNilsson Using [`std::vector`](https://en.cppreference.com/w/cpp/container/vector) is better. — MikeCAT, Feb 03 '21 at 14:51
First you read a line with `getline(cin, smaller_str);`, then you read `N` characters to replace the characters in `smaller_str` (regardless of whether it has enough room or not). I suspect that you meant to use a `std::istringstream` created from `smaller_str` and read from that into `smaller`. — molbdnilo, Feb 03 '21 at 14:53

MikeCAT · Accepted Answer · 2021-02-03T15:00:52.600

Unless you want to check for errors, you won't need to use getline in this case. Just read each elements via >> operator.

Also note that Variable-Length Arrays like int list[M][N]; is not in the standard C++. You should use std::vector instead of that.

Another point is the the i = i++ in the inner loop is wrong. It should be ++j.

#include <iostream>
#include <vector>
using namespace std;
int main() {
    int M; //height
    int N; //length
    cin >> M;
    cin >> N;
    //int list[M][N];
    std::vector<std::vector<int> > list(M, std::vector<int>(N));
    for (int i = 0;i < M; ++i){
        for (int j = 0; j < N; ++j) {
            cin >> list[i][j];
        }
    }
}

score 0 · Answer 2 · answered Feb 03 '21 at 14:53

You cannot assign another arrays to an array, what you can do is copy/ move its content. For this, use std::copy,

...
int list[M][N], smaller[N];
string smaller_str;
for (int i = 0; i < M; ++i) {
    getline(cin, smaller_str);
    for (int j = 0; j < N; i = i++) {
        cin >> smaller_str[j];
    }

    std::copy(smaller, smaller + N, reinterpret_cast<int*>(&list[i]));
}

Note: If you would like to move the content rather than copying, swap the std::copy with std::move.

c++) i have error when putting 1D array in 2D array

2 Answers2