Pre- and Post-Conditions
Definition
Pre-conditions
The requirements that must be met before a function or algorithm is executed.
Definition
Post-conditions
The outcomes that must be true after the function or algorithm has completed.
Note
- Pre-conditions describe the starting state before the execution of an algorithm.
- Post-conditions describe the final state after the execution of an algorithm.
Example
- Cooking a meal
- Pre-condition: All ingredients are available.
- Post-condition: The meal is cooked and ready to be served.
- Calculating the square root
- Pre-condition: The input number is non-negative.
- Post-condition: The output is the square root of the input number.
Why Are Pre- and Post-Conditions Important?
In computer science, such conditions are helpful for:
- Algorithm design: Ensuring that each step of an algorithm is logically sound.
- Software development: Defining clear interfaces between modules or components.
- Testing and debugging: Identifying and fixing errors by verifying that pre- and post-conditions are met.
More examples
Pre-conditions:
- Mathematical Operations: Before calculating the square root of a number, a pre-condition might be that the input must be non-negative.
- Data Structures: When accessing an element in an array, a pre-condition could be that the index is within the array's bounds.
- Algorithmic Assumptions: A sorting algorithm might assume that the input is a list of comparable elements.
Analogy
- Think of pre-conditions as the rules of a game.
- Just as players must understand the rules before playing, algorithms require certain conditions to be met before they can execute correctly.
Post-conditions:
- Output Validation: A sorting algorithm guarantees that the output array is sorted in ascending order.
- State Changes: A function that adds an item to a list guarantees that the list's size has increased by one.
Handling Violations of Pre-Conditions
Often in code, you need to be able to handle edge cases and invalid input.
In that case, pre-conditions would be useful for:
- Error Handling:
- Implement checks to verify that pre-conditions are met before executing the algorithm.
- If not, provide meaningful error messages or fallback solutions.
- Documentation:
- Clearly document pre-conditions in code comments or documentation to ensure users understand the requirements.
Hint
- Ignoring pre-conditions can lead to runtime errors, incorrect results, or even system crashes.
- Always validate inputs and assumptions before executing an algorithm.
Self review
- What are the pre-conditions for a function that calculates the area of a circle?
- How do post-conditions help ensure the reliability of a program?
- Why is it important to define pre- and post-conditions when designing algorithms?
- What strategies can you use to handle violations of pre-conditions in your code?
