diff --git a/README.md b/README.md
index 7987e64..ea5bf4c 100644
--- a/README.md
+++ b/README.md
@@ -436,25 +436,17 @@ $result = A::findLast(fn($x) => $x["a"] === -4, $data);
   #### calls the given function on the elements of an array and returns the key for the first match. if there's no match it will return `null`
 
 ```php
-$data = [
-  ["a" => 8],
-  ["a" => 10],
-  ["a" => 12]
-];
+$data = [1, 1, 1, 0, 0, 0, 0, 0];
 
-$result = A::findIndex(fn($x) => $x["a"] === 10, $data);
+$result = A::findIndex(fn($x) => $x === 0, $data);
 
-// $result = 1
+// $result = 3
 ```
 
 ```php
-$data = [
-  ["a" => 8],
-  ["a" => 10],
-  ["a" => 12]
-];
+$data = [1, 1, 1, 0, 0, 0, 0, 0];
 
-$result = A::findIndex(fn($x) => $x["a"] === -4, $data);
+$result = A::findIndex(fn($x) => $x === 2, $data);
 
 // $result = null
 ```
@@ -467,25 +459,17 @@ $result = A::findIndex(fn($x) => $x["a"] === -4, $data);
   #### calls the given function on the elements of an array and returns the key for the last match. if there's no match it will return `null`
 
 ```php
-$data = [
-  ["a" => 8],
-  ["a" => 10],
-  ["a" => 12]
-];
+$data = [1, 1, 1, 0, 0, 0, 0, 0];
 
-$result = A::findLastIndex(fn($x) => $x["a"] > 3, $data);
+$result = A::findLastIndex(fn($x) => $x === 1, $data);
 
 // $result = 2
 ```
 
 ```php
-$data = [
-  ["a" => 8],
-  ["a" => 10],
-  ["a" => 12]
-];
+$data = [1, 1, 1, 0, 0, 0, 0, 0];
 
-$result = A::findLastIndex(fn($x) => $x["a"] > 500, $data);
+$result = A::findLastIndex(fn($x) => $x === 2, $data);
 
 // $result = null
 ```
@@ -881,6 +865,17 @@ $point2d->y = 20;
 $point3d = O::assoc("z", 30, $point2d); // {"x": 10, "y": 20, "z": 30}
 ```
 
+```php
+$point2d = [
+  "x" => 10,
+  "y" => 20
+];
+
+$point3d = O::assoc("z", 30, $point2d); // ["x" => 10, "y" => 20, "z" => 30]
+```
+
+**Does not work on arrays with numeric keys!**
+
 </details>
 
 <details>
@@ -897,6 +892,18 @@ $point3d->z = 30;
 $point2d = O::dissoc("z", 30, $point3d); // {"x": 10, "y": 20}
 ```
 
+```php
+$point3d = [
+  "x" => 10,
+  "y" => 20,
+  "z" => 30
+];
+
+$point2d = O::dissoc("z", 30, $point3d); // ["x" => 10, "y" => 30]
+```
+
+**Does not work on arrays with numeric keys!**
+
 </details>
 
 <details>
diff --git a/src/A.php b/src/A.php
index f7f3823..637e7a2 100644
--- a/src/A.php
+++ b/src/A.php
@@ -108,13 +108,13 @@ public static function ensureArray($data): array
     // A::append([4, 5, 6], [1, 2]) -> [1, 2, 4, 5, 6]
     public static function append($value, array $data): array
     {
-        return self::concat($data, $value);
+        return self::concat($value, $data);
     }
 
     // A::prepend([4, 5, 6], [1, 2]) -> [4, 5, 6, 1, 2]
     public static function prepend($value, array $data): array
     {
-        return self::concat($value, $data);
+        return self::concat($data, $value);
     }
 
     // A::pluck('color', [['color' => 'red', ...], ['color' => 'green', ...]]) -> ['red', 'green']
@@ -267,7 +267,7 @@ public static function findLast(callable $fn, array $data) {
         return self::find($fn, self::reverse($data));
     }
 
-    // A::findIndex(x => x.a === 10, [['a' => 8], ['a' => 10]]) -> 1
+    // A::findIndex(x => x === 1, [1, 1, 1, 0, 0, 0, 0, 0]) -> 3
     public static function findIndex(callable $fn, array $data): ?int {
         if (self::isEmpty($data)) {
             return null;
@@ -282,10 +282,12 @@ public static function findIndex(callable $fn, array $data): ?int {
         return null;
     }
 
-    // A::findLastIndex(x => x.a > 3, [['a' => 8], ['a' => 10]]) -> 1
+    // A::findLastIndex(x => x === 1, [1, 1, 1, 0, 0, 0, 0, 0]) -> 2
     public static function findLastIndex(callable $fn, array $data): ?int
     {
-        return self::findIndex($fn, self::reverse($data));
+        $size = A::length($data);
+        $idx = self::findIndex($fn, self::reverse($data));
+        return $idx === null ? null : $size - $idx - 1;
     }
 
     // A::any(x => x.a === 10, [['a' => 8], ['a' => 10]]) -> true