Unit 5 · Heavy
Fixed-size indexed storage, and the first place two variable names can refer to the same data. Aliasing is what makes array questions harder than they look.
What a strong answer looks like
A strong Unit 5 answer says which indexes are visited, whether the array is read or mutated, and whether any other name refers to the same array.
Know
An array has a fixed length set at creation and is accessed by index from zero. Its elements start at a default value rather than being empty.
Apply
State the length and the valid index range before writing any loop over it.
Watch out
Confusing length, which is a field on arrays, with length(), which is a method on Strings.
Study move
Declare arrays of three different types and state each default element value.
Know
An indexed loop can read and write elements; an enhanced for loop reads a copy of each element and cannot replace it.
Apply
Choose the indexed form whenever the array must be modified.
Watch out
Assigning to the enhanced for variable and expecting the array to change. It does not.
Study move
Attempt to double every element with an enhanced for loop, then explain why it fails.
Know
Assigning to an index changes the stored value. Because arrays are objects, passing one to a method passes a reference, so the method can change the caller’s array.
Apply
Trace array contents after each assignment, and note when two names alias one array.
Watch out
Assuming a method received a copy. It received a reference, and its changes persist.
Study move
Write a method that modifies its array parameter and confirm the change is visible to the caller.
Know
Linear search inspects elements in order and works on unsorted data. It must handle the not-found case explicitly.
Apply
Return a sentinel such as negative one when nothing matches, and stop as soon as a match is found.
Watch out
Returning from inside the loop on a mismatch, which reports failure after checking only the first element.
Study move
Write a linear search returning the index of the first match, then test it on an array with no match.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Write a method that doubles every even value in an array and leaves odd values alone, then state what the caller sees afterwards.
