Dart - Matrix Spiral - dart

Question

I'm trying to solve this common interview question but I'm getting a RangeError: Valid value range is empty in the body of the first for loop (results[startColumn][i] = counter;)
The problem is Given an integer n, generate a square matrix filled with elements in spiral order.
For example, Given n = 3,
You should return the following matrix: [ [ 1, 2, 3 ], [ 8, 9, 4 ], [ 7, 6, 5 ] ]
This is my solution:
void matrix(int n) {
List<List> results = [];
for (var i = 0; i < n; i++) {
results.add([]);
}
int counter = 1;
int startColumn = 0;
int endColumn = n - 1;
int startRow = 0;
int endRow = n - 1;
while (startColumn <= endColumn && startRow <= endRow) {
// Top row
for (var i = startColumn; i <= endColumn; i++) {
results[startColumn][i] = counter;
counter++;
}
startRow++;
// Right column
for (var i = startRow; i <= endRow; i++) {
results[endColumn][i] = counter;
counter++;
}
endColumn--;
// Bottom row
for (var i = endColumn; i >= startColumn; i--) {
results[endRow][i] = counter;
counter++;
}
endRow--;
// Start column
for (var i = endRow; i >= startRow; i--) {
results[startColumn][i] = counter;
counter++;
}
startColumn++;
}
print(results);
}
Any ideas why its going out of bounds? 0,0 should really be the first index of the first inner List so I'm not sure why its out of bounds?

Answer 1

Well, the range error is about the results list is not initialized completely. You are inserting empty lists to the list but these lists are empty so you cannot ask for a specific element like
(not even for inserting an element to the list) results[startColumn][i].
So you need to specify a length to the lists you are adding to your results list:
final results = <List<int>>[];
for (var i = 0; i < n; i++) {
results.add(List<int>(n));
}
By making that change the code now "works" and gives the following results:
[[1, 8, 3], [null, 9, null], [7, 6, 5]]
Since your question was about the "range error" I guess fixing your matrix method is out of scope. But please tell me if you also want me to take a look at it. :)
Updated with answer to the Matrix problem
You were really close with the solution but with using a debugger it becomes clear that you have a problem when updating vertical rows. This can be seen when you makes the update for Right column and Start column.
The full solution looks like the following (I have added comments the two places I made some changes):
void matrix(int n) {
final results = <List<int>>[];
for (var i = 0; i < n; i++) {
results.add(List<int>(n));
}
int counter = 1;
int startColumn = 0;
int endColumn = n - 1;
int startRow = 0;
int endRow = n - 1;
while (startColumn <= endColumn && startRow <= endRow) {
// Top row
for (var i = startColumn; i <= endColumn; i++) {
results[startColumn][i] = counter;
counter++;
}
startRow++;
// Right column
for (var i = startRow; i <= endRow; i++) {
results[i][endColumn] = counter; // Switched i and endColumn
counter++;
}
endColumn--;
// Bottom row
for (var i = endColumn; i >= startColumn; i--) {
results[endRow][i] = counter;
counter++;
}
endRow--;
// Start column
for (var i = endRow; i >= startRow; i--) {
results[i][startColumn] = counter; // Switched i and startColumn
counter++;
}
startColumn++;
}
print(results);
}
And when running the code it should give the following results:
matrix(2); // [[1, 2], [4, 3]]
matrix(3); // [[1, 2, 3], [8, 9, 4], [7, 6, 5]]
matrix(4); // [[1, 2, 3, 4], [12, 13, 14, 5], [11, 16, 15, 6], [10, 9, 8, 7]]