Thinking Ahead

Thinking ahead means identifying what is needed before you start solving a problem — the inputs, outputs, pre-conditions, and post-conditions. It also involves considering caching and reusable components to make solutions more efficient. This is a key part of OCR H446 section 2.1.

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Inputs and Outputs

Before writing any code or designing any algorithm, you must clearly identify:

Concept	Definition	Example (sorting problem)
Inputs	The data the system receives	An unsorted list of integers
Outputs	The data the system produces	A sorted list of integers
Processing	The transformation applied to inputs to produce outputs	A sorting algorithm (e.g., merge sort)

Why identify inputs and outputs first?

• It defines the scope of the problem — what exactly are you solving?
• It clarifies what data you need to collect or receive
• It determines what the user expects to see
• It helps you design test cases (you know what should go in and what should come out)

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Pre-conditions

A pre-condition is a condition that must be true before a function, algorithm, or system is executed for it to work correctly.

Example	Pre-condition
Binary search	The data must be sorted
Division function	The divisor must not be zero
Array access at index i	i must be within bounds (0 <= i < length)
User login	The user must have a registered account
File read operation	The file must exist and be accessible

Pre-conditions and defensive programming:

Some programmers handle pre-conditions by checking them inside the function (defensive programming):

FUNCTION divide(a, b)
    IF b = 0 THEN
        RAISE error "Division by zero"
    END IF
    RETURN a / b
END FUNCTION

Others document pre-conditions and assume they are met (design by contract):

// Pre-condition: b != 0
FUNCTION divide(a, b)
    RETURN a / b
END FUNCTION

Both approaches have trade-offs:

Approach	Advantage	Disadvantage
Defensive	Safer — handles invalid input gracefully	Slower — checks run every time, even when unnecessary
Design by contract	Faster — no redundant checks	Riskier — if pre-conditions are violated, the result is undefined

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Post-conditions

A post-condition is a condition that must be true after a function or algorithm has completed.

Example	Post-condition
Sorting algorithm	The output list is in ascending order and contains the same elements as the input
Search function	If the item exists, its position is returned; if not, -1 (or similar) is returned
File write operation	The data has been written to disk and the file is closed
Bank transfer	The sender's balance has decreased by the transfer amount; the receiver's balance has increased by the same amount

Post-conditions are used to verify that a function has done its job correctly. They can be checked using assertions in code:

result = sort(myList)
ASSERT result is sorted
ASSERT length(result) = length(myList)

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Caching

Caching means storing the results of expensive operations so they can be reused without recalculating.

Concept	Detail
Cache	A temporary storage area for frequently accessed data
Cache hit	The requested data is found in the cache
Cache miss	The requested data is not in the cache and must be fetched/computed from the original source
Cache eviction	Removing old data from the cache to make room for new data

Examples of caching: