Loops Brent M. Dingle Texas A&M University Chapter 7 – part C (and some from Mastering Turbo Pascal 5.5, 3 rd Edition by Tom Swan)

For–To-Do Format FOR [ctrl var] := [init val] TO [final val] DO [statement] Notice, there is no explicit [expr] in a for-loop. Basically for-to-loops just count BY 1 from [init val] to [final val]

For-To-Do Example PROGRAM ForCount; VAR Counter : integer; BEGIN FOR counter := 1 TO 10 DO BEGIN writeln(‘count = ‘, counter); END; END.

For-To-Do Example The above program simply counts from 1 to 10. Notice that the control variable is “automatically” initialized and incremented by one for you. Can you create a while loop that will do exactly the same thing? Try it. And TEST run the two programs to check!

While-loop, for the lazy This while loop will count to 10 also: PROGRAM WhileCount; VAR counter : integer; BEGIN counter := 1; while (counter <= 10) do BEGIN writeln(‘count = ‘, counter); counter := counter + 1; END; END.

Observance Check What do you think would happen if we altered the for line to be: for counter := 10 to 1 do What would have happened in the while loop if you initialized counter := 10 and said: while (counter <= 1) do Try it and see if you answered correctly.

For the lazy, Here are the two altered progs PROGRAM ForCount; VAR counter : integer; BEGIN FOR counter := 10 TO 1 DO BEGIN writeln(‘count = ‘, counter); END; END. PROGRAM WhileCount; VAR counter : integer; BEGIN counter := 10; while (counter <= 1) do BEGIN writeln(‘count = ‘, counter); counter := counter + 1; END; END.

Observance Check - Answer For those of you too lazy to try entering the programs yourself, you would have discovered that ‘nothing’ happens. More specifically, if you start at 10 can you count to 1 by ADDING 1 (repeatedly) to 10? Exactly. Since 10 is already >= 1 neither the for- to-do loop nor the while loop would ever execute. So can you use a for-loop to count backwards? Yes, see the next example =)

Another Example For-Downto-Do PROGRAM ForCount; VAR Counter : integer; BEGIN FOR counter := 10 DOWNTO 1 DO BEGIN writeln(‘count = ‘, counter); END; END.

For-Downto-Example Notice the above program counts from 10 down to 1. Notice the control variable is automatically initialized to 10. Notice also the control variable is automatically decremented by 1 (i.e. one is subtracted from it).

Challenge Can you write a program using a for loop to display the capital characters A to Z on the screen? Could you write it using a while-do loop? Recall that succ() allowed you see that Pascal “knows” the character after A is B, and after B is C, and so on. Notice in a for-loop you will NOT need to use the function succ().

Challenge Answer PROGRAM ForAlpha; VAR letter : char; BEGIN FOR letter := ‘A’ TO ‘Z’ DO BEGIN write(letter); END; writeln; END. PROGRAM WhileAlpha; VAR letter : char; BEGIN letter := ‘A’; while (letter <= ‘Z’) do BEGIN write(letter); letter := succ(letter); END; writeln; END.

Challenge two Can you write a repeat-until loop that does the same as the above two loops? Or rather, write a repeat-until loop which displays the characters A to Z using the function succ().

Challenge Two Answer You think you get all the answers for free? Go try that one on your own. =)

NEVER in a for-loop Never increment the control variable of a for-loop inside the loop. Never write a for-loop like this: FOR count := 1 to 10 DO BEGIN writeln(count); count := count + 1; { BAD LINE !!! } END;

When to use WHILE If you know there is a condition where the loop should never execute, not even once, a while loop would be a good choice. While loops also can be used to count by increments other than +1 or –1. Point  some while loops NEVER execute.

When to use REPEAT If you know that a loop while ALWAYS need to execute at least once then a repeat loop might be a good choice. Point  repeat loops ALWAYS execute AT LEAST ONCE.

When to use FOR If it is known in advance exactly how many times a loop should be executed (or it is known that there will be a specific number of times entered or calculated) then a for loop is a good choice. i.e. if you know you will do something 10 times, or N times, a for loop could be a good choice. Note  some for loops NEVER execute.

Suggested Problems (not graded) page 244, 245  1, 2, 3, 4, 6, 7, 8, 9, 11

End part C