What is wrong with the following code snippet: int size = 42; cost = 9.99;?

The code snippet contains a problem related to variable declaration. In programming, every variable must be declared with a type before it can be used. The code shows a declaration for an integer variable named size but fails to declare the variable cost. As a result, attempting to use cost without declaring it first leads to an issue.

Proper syntax would require declaring cost explicitly by specifying its type, such as double or float, to indicate that it will hold a decimal value. By doing this, the code adheres to the rules of variable scope and type safety, allowing the program to compile and run correctly. Therefore, the correct identification of a variable that hasn't been declared is crucial in understanding programming language syntax and ensuring code validity.