1. Iteration Implemented through loop constructs (e.g., in C++)
E.g., in C++, while, for, do loops
A continue statement skips rest of that iteration of the loop
A break statement exits the loop
Iterators also provide helpful abstractions for iteration
While loop
Most basic construct: test guards each iteration of a block
For loop
Encodes special case of a while loop (can emulate an enumeration controlled loop using logical control)
Do loop
Ensures execution of the block at least once

2. Recursion Functional languages severely limit side effects
Iteration relies on side effects to make progress, terminate
Recursion is a natural alternative in those cases
Or where avoiding side effects simplifies flow control logic
Recursive functions can support lazy evaluation
E.g., packaging up remaining work as a continuation and then only performing that work if it’s needed
Normal order vs. applicative order evaluation
Operands may not be evaluated until needed (applicative)

3. Nondeterminacy Features and Sources
Dijkstra’s original guarded if statement
Multiple alternate execution branches
Each guarded by conditional expression, enabled if true
Non-deterministic selection from among enabled statements
Dijkstra’s guarded do statement
Multiple guarded statements, enabled if guard evaluates true
Repeated non-deterministic selection of enabled statement
Multi-threading
Deterministic sequencing within but not between threads
Can synchronize to re-introduce inter-thread happens-before

4. Today’s Studio Exercises
We’ll code up ideas from Scott Chapter
Looking at iteration, recursion, non-determinacy
Today’s exercises are again in C++
C++ allows us to explore control flow issues for statements
Functional languages rely more on function calls for control
Please take advantage of the on-line tutorial and reference manual pages that are linked on the course web site
As always, please ask us for help as needed
When done, your answers with subject line “Control Flow Studio II” to