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, –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.