Sprite index

Explanation
The sprite_index local variable is the variable that defines the Sprite that the object is currently using. When no sprite index is selected for an object, the value of -1 is assigned to sprite_index.

The variable sprite_index contains a real value and not a string value.

Examples
Since it is hard to memorize index numbers for each sprite, Game Maker automatically converts any sprite name used outside a comment or a string into a real number, which is the sprite index.

Therefore: sprite_index=spr_player; Would assign the the sprite spr_player to object this code has been performed with.