Checkpoint 21.1.1.
- 3 2 1 Blastoff!
- 1 2 3 Blastoff!
- Blastoff! 3 2 1
- Blastoff! 1 2 3
Section 21.1 Intro to Recursion
A recursive function is simply one that calls itself. The function calls itself is called a recursive call.
Consider the following example of the recursive function
countdown. If it is called with an n value of 0, it displays the word Blastoff!. Otherwise, it displays the number and then makes a recursive call to countdown, passing n - 1 as the argument.
Run the codelens to watch it execute:
What happened when we invoked
countdown(3) from main?-
The execution of
countdownbegins withn == 3, and sincenis not 0, it displays the value 3, and then invokescountdown(2) -
countdown(2)begins, and sincenis not 0, it displays the value 2, and then invokescountdown(1) -
countdown(1)begins, and sincenis not 0, it displays the value 1, and then invokescountdown(0) -
countdown(0)begins, and sincenis 0, it displays the wordBlastoff!.
Now let’s swap lines 9 and 10. Run this new version:
What was different this time? The work of displaying the number was done after the recursive call. So level 3 did not print until after level 2 was done. And level 2 did not print until after level 1 was done...
Insight 21.1.1.
A recursive call breaks the execution of a function into two parts. The code before the recursive call runs before the next recursive call executes. The code after the recursive call runs after the recursive call completes.
We can more clearly see the timing in this final example. Notice this
countdown prints both before and after the recursive call.
What happened this time when we invoked
countdown(3) from main?-
The execution of
countdownbegins withn == 3, and sincenis not 0, it displays the start message, and then invokescountdown(2) -
countdown(2)begins, and sincenis not 0, it displays the start message, and then invokescountdown(1) -
countdown(1)begins, and sincenis not 0, it displays the start message, and then invokescountdown(0) -
countdown(0)begins, and sincenis 0, it displays the wordBlastoff!. It returns to where it was called (countdown(1)). -
Back in
countdown(1)execution resumes after the recursive call and displays the finish message. It then returns to where it was called (countdown(2)). -
Back in
countdown(2)execution resumes after the recursive call and displays the finish message. It then returns to where it was called (countdown(3)). -
Back in
countdown(3)execution resumes after the recursive call and displays the finish message. It returns to where it was called (main).
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