Unit 3 · Heavy
Doing something many times without writing it many times. Most early loop bugs are off by exactly one, which is why counting the iterations deliberately matters more than the syntax.
Know
A for loop sets a starting value, a condition to keep going, and a step. It checks the condition before each pass, so it may run zero times.
Apply
List the first and last values the counter takes before predicting anything.
Watch out
Going one step too far by using less-than-or-equal against a size.
Know
A while loop repeats until its condition becomes false, and you are responsible for making that happen inside the body.
Apply
Check that every path through the body moves toward stopping.
Watch out
Forgetting the update, or putting it inside an if so it sometimes does not happen.
Know
The inner loop finishes completely for every single pass of the outer one, so the total work multiplies.
Apply
Say what each loop is counting before you trace.
Watch out
Using the outer counter inside the inner loop by accident.
Know
A running total must start at a value that is correct when nothing has been added yet: zero for a sum, one for a product.
Apply
Say in words what the total holds after one pass, then check the starting value fits.
Watch out
Starting a running largest-value at zero, which breaks when every value is negative.
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 loop that adds up only the even numbers from 1 to 20 and say what it produces if the range is empty.
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