Pascal Programming

Pascal Loops and Debugging

Pascal Programming

Pascal Loops In our first brush with the while do loops,

simple comparisons were used. Now, we add the Boolean expression. As long as the expression evaluates as

true, the loop continues. False stops the looping.

Pascal Programming

The repeat statement, another method of looping.

Syntax . . .– repeat

• Statement or statements;

– until• Boolean-expression

– end

Pascal Programming

The body of a repeat may contain multiple statements . . .

The body of the while statement contains one statement.

The difference holds limited significance because the while statement may be compound.

Pascal Programming

The for loop . . . Syntax . . .

– For Control_variable := Initial_expression to Final_expression do

– Body The value increments by one each time through

the loop. The body of the for loop can not change the loop-

control variable. The loop-control variable is intended for exclusive

use of the loop. (a local variable)

Pascal Programming

The second version of the for loop decrements . . . In the statement downto replaces to Downto is a reserved word. Each time through the loop it decreases by one. Note: Pascal only increments by one or decrements

by one. You must design around this limitation. e g. You want 0, 2, 4, 6, 8, 10 . . .

– For N := 0 to 5 do– Write (2*N)

Pascal Programming

When designing a Pascal loop, design…

The Body The initializing statements The conditions for exiting the loop. We have talked about the first two no

let’s look at terminating the loop.

Pascal Programming

Terminating a loop . . .– Terminating input

• List is headed by size (list size is known)• Ask before iterating (ask the user)• List ended by sentinal value.• Input runs out.

Pascal Programming

Terminating the loop . . .– Count the controlled loops.– Ask before iterating.– Exit on sentinel value.– Input runs out.– Exit on a flag condition.

A flag is a variable that changes value to indicate that an event has occurred.

The end of line (eoln) and end of file (eof) are examples. (Eoln does not work well with numeric data. It prefers char type.)

Pascal Programming

What loop should I use? Numeric data, fixed iterations, equal changes

each time . . .use for Generally, the for loop will be best for numeric

data. If the loop may not be used at least once, use

while. It also works when no data may be entered from a list.

If you insist that the loop be used at least once, use repeat.

Pascal Programming

Assertions Comments about pre- and post-

conditions are called assertions. An invariant assertion is true before the

loop runs and true after. The variable changes with each iteration but holds true with each iteration. Thus it is invariant. continue . . .

Pascal Programming

Assertions cont . . . A variant assertion changes after each

iteration. Two conditions . . .

– The value must decrease by a fixed amount (or more) each iteration.

– When the variant equals or falls below the threshold, the loop must stop.

– The threshold is the point to be reached or passed.

Pascal Programming

Debugging Loops For repeat or while loops . . .check the

Boolean expression. Is something reversed?

Be sure that the error is in the loop. Trace the data through the loop. Check intervals in long loops.