Preheat the oven to 350°F (175°C).

In a large bowl, mix together the flour, sugar, baking powder, baking soda, cinnamon, and salt.

In another bowl, combine the vegetable oil, eggs, grated carrots, and crushed pineapple.

Pour the wet ingredients into the dry ingredients and mix until well combined.

Stir in the chopped walnuts if using.

Pour the batter into a greased cake pan and bake for 30 minutes or until a toothpick inserted comes out clean.

Allow the cake to cool completely before frosting.

For the frosting, beat together the cream cheese and butter until smooth.

Gradually add the powdered sugar and vanilla extract, mixing until creamy.

Spread the cream cheese frosting evenly over the cooled cake.