diff --git a/2024/src/main/kotlin/CharGrid.kt b/2024/src/main/kotlin/CharGrid.kt
index 3a4b0b9..8a32f66 100644
--- a/2024/src/main/kotlin/CharGrid.kt
+++ b/2024/src/main/kotlin/CharGrid.kt
@@ -1,7 +1,11 @@
-class CharGrid(val rows: List<String>) : Grid<Char> {
+class CharGrid(rows: List<String>) : Grid<Char> {
+    val rows = rows.toMutableList()
     override val width = rows.firstOrNull()?.count() ?: 0
     override val height get() = rows.count()
 
+    constructor(width: Int, height: Int, char: Char)
+            : this((1..height).map { char.toString().repeat(width) })
+
     init {
         assert(rows.all { it.length == width })
     }
@@ -75,4 +79,8 @@ class CharGrid(val rows: List<String>) : Grid<Char> {
         val data = rows.flatMap { it.map(fn) }
         return ListGrid(width, height, data)
     }
+
+    override fun set(x: Int, y: Int, value: Char) {
+        rows[y] = rows[y].replaceRange(x, x + 1, value.toString())
+    }
 }
diff --git a/2024/src/main/kotlin/Grid.kt b/2024/src/main/kotlin/Grid.kt
index fa72d3b..d668d76 100644
--- a/2024/src/main/kotlin/Grid.kt
+++ b/2024/src/main/kotlin/Grid.kt
@@ -4,22 +4,29 @@ interface Grid<T> {
 
     fun <U> map(fn: (T) -> U): Grid<U>
 
-    fun get(x: Int, y: Int): T
+    operator fun get(x: Int, y: Int): T
+    operator fun set(x: Int, y: Int, value: T)
 
     data class Coordinate(val x: Int, val y: Int)
 
     fun inside(coordinate: Coordinate) = coordinate.x in 0..<width && coordinate.y in 0..<height
 }
 
+operator fun <T> Grid<T>.set(coordinate: Grid.Coordinate, value: T) = set(coordinate.x, coordinate.y, value)
 operator fun <T> Grid<T>.get(coordinate: Grid.Coordinate) = get(coordinate.x, coordinate.y)
 operator fun <T> Grid<T>.contains(coordinate: Grid.Coordinate) = inside(coordinate)
 
-class ListGrid<T>(override val width: Int, override val height: Int, private val content: List<T>) : Grid<T> {
+class ListGrid<T>(override val width: Int, override val height: Int,  content: List<T>) : Grid<T> {
     init {
         assert(width * height == content.count())
     }
 
-    override fun get(x: Int, y: Int): T = content[x + y * width]
+    private val content = content.toMutableList()
 
-    override fun <U> map(fn: (T) -> U): Grid<U> = ListGrid(width, height, content.map(fn))
+    override fun get(x: Int, y: Int): T = content[x + y * width]
+    override fun set(x: Int, y: Int, value: T) {
+        content[x + y * width] = value
+    }
+
+    override fun <U> map(fn: (T) -> U): Grid<U> = ListGrid(width, height, content.map(fn).toMutableList())
 }