Why is the following function `impure`?

Question

BlackTom AI · Accepted Answer

To evaluate why the function is impure, we need to inspect what the function does and whether its behavior can vary across calls.
Option 1: 'Because it allocates memory dynamically when creating a vector of integers.' Dynamic memory allocation by itself does not necessarily imply impurity; in many languages and contexts, it can be considered a normal, pure operation as long as it doesn't depend on external state or mutate shared state. On its own, allocating memory repeatedly can still produce the same outputs if inputs are the same.
Option 2: 'Because it prints the content of the vectors using a loop into the console output.' Printing to a console is a side effect observable outside the function. However, impurity is defined by observable effects that can change behavior beyond returning a value. Printing is indeed a side effect, but impurity is more about dependence on changing external state or non-deterministic behavior; console I/O is a side effect, yet it may not be the sole or definitive cause of impurity in all contexts.
Option 3: 'Because it uses loops instead of recursion.' The choice of looping versus recursion does not inherently determine impurity. A function can be pure whether it uses loops or recursion, as long as it produces the same output for the same input and has no external interactions.
Option 4: 'Because it depends on an external data source whose contents may change between calls.' This statement points to dependence on outside state that can vary between invocations, which directly introduces non-determinism and potential state changes visible to callers. If the function reads from a file, database, or any global or external resource whose contents can change, repeated calls with the same input can yield different results, defining impurity.
Overall, the key idea is that impurity arises when the function’s result is not determined solely by its input parameters, due to interactions with changing external data. The option that explicitly mentions such external changing data sources aligns with that concept, while the other options either focus on aspects that do not guarantee impurity (like looping vs recursion) or describe side effects whose presence alone does not fully define impurity in all contexts.

Why is the following function `impure`?单项选择题

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