Loops

1
Loops

• When you want a computer to perform the same
  operation many times you use a loop.
• Looping is one of the keys to computer
  programming.
• First we will look at the FOR / DO Loop.
• Later in the semester we will look at the WHILE
  loop provided by Pascal.

2
Loops (cont)

• Reminder A compound statement is a group of
  statements beginning with BEGIN and ending with
  END.

BEGIN writeln (I will not pledge
allegiance to Bart) readln
(answer) END Note no period - only at the end
of the program
3
FOR-DO Loop
FOR Index 1 to 100 DO writeln(Nobody
likes sunburn slappers ) FOR Index1 to 10
DO BEGIN write(Index) writeln(There
was no Roman god named "Farticus) END
4
FOR-DO loops (cont)

• Control variable is the variable which controls
  the loop. In the previous examples the integer
  variable Index was used as the control variable.
  
• The lower and upper limits of the loop must be
  the same type as the control variable. (all
  three must be an ordinal type - more on this next
  class)

5
FOR-DO loops (cont)

• As long as the types match (and are an ordinal
  type), the loop is legal. This does not mean the
  loop is executed.
• This loop is legal but will not be executed
• FOR Index 3 TO 2 DO
• write(I will not sleep through my
  education)

6
Averaging numbers (example from book)

• Suppose you want to write a program that will
  average grades in general (ie. You may use a
  different number of grades each time you run the
  program.)
• requirements specification
• analysis
• design
• implementation / coding
• testing

7
Requirements Specification

• Since the English description for this problem is
  straightforward, the requirements specification
  step is trivial.

8
Analysis

• Inputs
• of grades to be averaged integer
• the grades to be averaged integers
• Outputs
• the average real
• other variables
• sum of the grades read so far

9
Design First version of pseudocode

• Read the number of grades to be averaged
• Sum all the grades to be averaged
• Compute and print the average

10
Design Pseudocode refinement

• Read the number of grades to be averaged
• Sum all the grades to be averaged
• Set Sum to zero
• FOR GradeNum 1 TO TotalGrades DO
• Read the Grade
• Add the grade to Sum
• Compute the Average
• divide the Sun by TotalGrades
• Print the sum and average

11
Implementation

• Show Pascal program

12
testing
13
FOR DOWNTO

• Suppose you want to write a FOR-DO loop where the
  control variable is decreased with each
  repetition of the loop. Pascal provides the
  reserved word DOWNTO for this situation. (The
  first limit value should be greater than the
  second.) FOR Index 10 DOWNTO 1 DO

14
FOR DOWNTO (cont)

• Remember that the loop FOR Index 4 TO 3
  DOwill never be executed
• Similarly, the loop FOR Index 3 DOWNTO 4
  DOwill never be executed

15
FOR DO and the buffer

• The FOR DO loop can be used to read several
  characters, one at a time, from the buffer.

16
Nested loops

• When one loop is completely contained in another
  loop, it is called a nested loop.
• We call the nested loop the inner loop
• The loop it is placed in is called the outer
  loop
• The inner loop is executed in full for each value
  of the outer loop.

17
Homework 5

• Problem 15 A and B from page 204 in the text
  (the problem suggests you read the Social
  Security number as a series of characters)
• Explain how you could do this problem if the
  social security number is read instead as a
  longint.

18
Homework 5 (cont)

• For the longint part. you can use the digits in
  reverse order (b/c it is easier)
• Dont do the program - just explain
• If ss is 987654321 your output could
  be 9999999999 OR 11 888888888 222 777777
  77 3333 6666666 44444 555555 555555 4
  4444 6666666 3333 77777777 222 888888
  888 11 9999999999