CIS 400 Lecture 9

Functional programming, (skipping chapter 5 for now, will come back to it)

Sequence control

· Used in expressions, (e.g. Precedence rules, parenthesis)

· Between statements or blocks, (e.g. Iteration and conditionals)

· Between sub-programs, (e.g. Calls)

Control sequences

· Implied, (physical statement order)

· Explicit, (parenthesis or goto's)

Example:

Root = -b ± (b²-4ac)^1/2

2a

In Fortran:

Root = ( -b + Sqrt ( b**2 - 4*a*c))/2*a

In some instances, parenthesis are essential, in other they are not.

(a + b)*(c - a) in-fix notation

* + a b - c a pre-fix notation

a b + c a - * post-fix notation

What is the difference between unary and binary minus?

(i.e. how does "-b" and "b²-4ac" in the quadratic formula differ in terms of language definition?)

Tree representation: issues

-a * b + c in-fix notation

· Binary vs. unary operator confusion

· Precedence rules

· Associativity

v left to right

v right to left, (eliminates the need for parenthesis for the "2a" in the denominator of the quadratic)

· Operators with varying number of operands, ( i.e. (+ 2 3 4))

Problem areas:

· Uniform evaluation-once a tree is formed, all operands are treated equally

· Side effects:

· Error conditions: arithmetic over- or under-flow, (i.e. 4/ (200 * 10,000,000))

· Short circuited boolean evaluation:

(a = 0) OR (b / a > c) in short circuit evaluation, the "OR" is evaluated until a "true" is found.

With full boolean evaluation, the "b/a", where a = 0, is evaluated first. This results in a run-time error.

With an "and", the first "false" stops evaluation.

Statement level control structures:

· Composition, (sequence)

Example:

Block statements

· Alternating (conditional statements)

v If-then/else or case, (switch)

"dangling else" issue:

if x = 3 then

y = 5

if x = 5 then

y = 6

else y = 7 // WHICH "IF" GETS THE "ELSE"?

in C++ {} is used to define where the else belongs

v If-then/else or case, (switch) continued

Case design issues:

· What type of selector expression?

· What types of case labels?

· Can you branch to case labels from outside?

· Mutually exclusive labels?

· Exhaustive label case coverage? ("default", etc.)

Iteration issues:

In COBOL;

Perform {body} k times

How often is k evaluated?

If k is re-evaluated, when?

Can k < 0?

· When is termination test made?

· When are the loop expression variables evaluated?

(Pre and post test loops)

Counter incrementing loop:

do i = start to end by increment until (cond)

begin

(body)

end;

Is this pre- or post-test evaluation?

In Fortran 4 it was post-test!!

In Fortran 77 it was pre-test!!

When is i evaluated?

Can "start" or "end" be altered?