Unit 5 · Heavy
Holding many values under one name. Arrays are a fixed size; lists grow and shrink. This is also the first place two names can point at the same data, which surprises everyone the first time.
Know
An array holds a fixed number of values, reached by position from zero. Its size is decided when it is made and cannot change.
Apply
State the size and the valid positions before writing a loop.
Watch out
Mixing up the size property on arrays with the length operation on text.
Know
A counting loop can both read and change items. The simpler for-each loop reads a copy and cannot replace anything.
Apply
Use the counting loop whenever you need to change values.
Watch out
Assigning to the for-each variable and expecting the array to change. It does not.
Know
Assigning to a position changes the stored value. Passing an array to a method passes the array itself, so the method can change your data.
Apply
Trace the contents after each change, and note when two names refer to one array.
Watch out
Assuming a method got a copy. It did not, and its changes are permanent.
Know
A list can grow and shrink. Removing an item slides everything after it down, which is why removing inside a loop needs care.
Apply
When removing while looping, either go backwards or do not advance after a removal.
Watch out
Removing forwards with a normal step, which silently skips the item after each removal.
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 code that removes every score below 40 from a list and explain why your loop does not skip any.
Work through questions with a tutor and get help with the steps you find difficult.
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