CMP 131Introduction to Computer

Programming

Violetta Cavalli-Sforza

Week 10

TODAY

• Finish loops (or almost)

• Homework #5 was posted on Monday Don’t wait to start on it!!!

Equivalent Statements• FOR loop

FOR Count := StartValue TO StopValue DO BEGIN ... END {FOR}

• Equivalent WHILECount := StartValue;WHILE Count <= StopValue DO

BEGIN ... ; Count := Count + 1; END {WHILE}

• Equivalent REPEATCount := StartValue;IF StartValue <= StopValue THEN

REPEAT ... Count := Count + 1;UNTIL Count > StopValue

Simple Applications of FOR Lopps

• Printing patterns– E.g. diagonal band– E.g. tree

• Generating numbers (e.g. tables)– E.g. table of powers of numbers

(sqr, sqrt, cubes)– E.g. Fibonacci numbers

Task: Draw Diagonal Band

***

***

***

***

***

***

***

***

Program: Diagonal Band

PROGRAM DiagonalBand;

VAR J : integer;

BEGIN

FOR J := 1 TO 8 DO

writeln('***':J + 3);

readln

END.

Task: Draw Tree

* * * * * * ***** **** * * * * ***

Program: Tree

PROGRAM Tree;VAR J : integer;BEGIN writeln('*':5); FOR J := 1 to 3 DO writeln('*':5 - J,'*':2 * J); writeln('**** ****'); FOR J := 1 to 2 DO writeln('* *':6); writeln('***':6); readlnEND.

Task: Fibonacci Numbers

• Print the first 20 Fibonacci numbers

• Fibonacci numbers function:

Program: FibonacciPROGRAM Fibonacci;CONST Number = 20;VAR J , {counter variable} N1, N2, N3 {counter variable} : integer;BEGIN N1 := 0; writeln('Fibonacci 0:', N1:5); N2 := 1; writeln('Fibonacci 1:', N2:5); FOR J := 2 TO Number – 1 DO BEGIN N3 := N1 + N2; writeln('Fibonacci',J:3,':',N3:5); N1 := N2; N2 := N3; END; readlnEND.

Output: FibonacciFibonacci 0: 0Fibonacci 1: 1Fibonacci 2: 1Fibonacci 3: 2Fibonacci 4: 3Fibonacci 5: 5Fibonacci 6: 8Fibonacci 7: 13Fibonacci 8: 21Fibonacci 9: 34Fibonacci 10: 55Fibonacci 11: 89Fibonacci 12: 144Fibonacci 13: 233Fibonacci 14: 377Fibonacci 15: 610Fibonacci 16: 987Fibonacci 17: 1597Fibonacci 18: 2584Fibonacci 19: 4181

Simple Applications of WHILE/REPEAT loops

• Accumulators– E.g. sums, means,

• Simulations

Loop Design

• Terminology– Loop control:

• Making sure that the loop exits when it is supposed to

– Loop processing:• Making sure that the body performs the required

operations

• To formulate loop control & loop processing, it is useful to list what we know about the loop

Loop Design

• Loop design can be approached in two ways:– Analyze requirements:

• Determine needed initialization, testing, and updating of the loop control variable

• Then formulate the loop control and loop processing steps

– Develop templates:• Make templates for frequently running forms• Use these templates as the basis for the new loop

Sentinel-Controlled Loops

• Sentinel:– An end marker that follows the last data item

• Frequently, you will not know exactly how many data items a program will process before it begins execution

• One way to handle this is to instruct the user to enter a unique data value (sentinel value) as the last data item. The program then tests each data item and terminates when the sentinel value is read.

• Sentinel value should be carefully chosen and must be a value that could not normally occur as data.

Sentinel-Controlled Loops

• Template:Read the first value of input variable

WHILE input variable is not equal to sentinel do

BEGIN

...

Read next value of input variable

END

Sentinel-Controlled Loops

• Example: Collecting Exam Scores:1.Initialize Sum to 0

2.Read the first score into Score

3.WHILE Score is not the sentinel (e.g. -1) do

BEGIN

4. Add Score to Sum

5. Read the next score into Score

END

Loops Controlled by Boolean Flags

• Flag: A Boolean variable whose value is changed from False to True when a particular event occurs

•  Boolean Flags Template:Initialize flag to false

WHILE flag is still false DO

BEGIN

...

Reset flag to true if event being monitored occurs

END

Loops Controlled by Boolean Flags

• Example: Reading data characters and save the first digit character read

DigitRead := false;WHILE ( not DigitRead ) DO BEGIN write ('Enter another data character >'); readln (NextChar); DigitRead := ('0'<=NextChar) AND (NextChar<='9') END {WHILE}

Task: Dropping Objects

• Problem definition:– Your physics professor wants you to write a

program that displays the effect of gravity on a free-falling object. You should build a table showing the height from a tower to every second.

Analysis: Dropping Objects

• Observations:– Assume

• time of free fall = t– At t = 0.0 {initialization}

• object height = tower height– While falling {updating-loop

body}• object height =

tower height - distance traveled– Free fall ends {testing -exit}

• when height <= 0.0

Program Segment: Dropping Objects

writeln ('Time' :10:2, 'Height' 10:2);{initialization}T := 0.0;Height := Tower;WHILE Height > 0.0 DO BEGIN {body}

writeln (T : 10:2, Height :10:2);T := T + DeltaT;Height := Tower - 0.5 * G *

Sqr(T) END {WHILE}writeln;writeln ('SPLATT!!!');

Example: Dropping Objects

• Observations – Before the loop is entered, a message

displaying the heading is printed.– The number of lines in the table depends on

the time interval between lines (DeltaT) and the tower height (Tower).

– During each loop iteration, the current elapsed time, t, and the current object height are displayed and new values are assigned to these variables.

Simple Loops Summary• Use FOR loop as a counting loop. The loop control

variable must belong to an ordinal type (not real).• REPEAT and WHILE-loops are conditional loops. Their

number of iteration depends on whether the value of a condition is true or false.

• WHILE loop is repeated as long as its loop repetition condition is true.

• REPEAT loop is repeated until its loop-termination condition becomes true.

• Usually a REPEAT loop can be written as a WHILE loop by complementing the condition.

• Not all WHILE loops can be written as REPEAT loops, because REPEAT loops executes at least once, whereas a WHILE loop body may be skipped entirely.

• WHILE loop is preferred over a REPEAT loop unless you are certain that at least one loop iteration must always be performed.

Exercises

• Write a while loop that counts the number of digits entered in a stream of characters. Exit when the character ‘X’ is entered. Make sure you don’t count the X

• Write a while loop that reads negative or positive numbers, keeping track of the largest in absolute terms. Exit when a zero is entered.

• Write a while loop that reads lines of text, and counts the characters entered. Exit when the string ‘Quit’ is entered. (Don’t count the characters in ‘Quit’).

Nested Loops

• It is possible to nest loops• Each time the outer loop is repeated, the inner

loops are reentered, their loop-control parameters are reevaluated, and all required iterations are performed

• Very useful in processing multi-dimensional arrays

Task: Draw Isosceles Triangle

*

***

*****

*******

*********

Program: Isosceles Triangle

PROGRAM Triangle;{Draw an isosceles triangle} CONST

NumLines = 5; {number of rows in triangle}Blank = ' '; {output character}Star = '*'; {output character}

VARRow, {control for outer loop}LeadBlanks, {control for first inner loop}CountStars: integer; {control for 2nd inner loop}

 {Continued}

Nested Loops ExampleIsosceles Triangle

BEGIN {Triangle}

FOR Row := 1 TO NumLines DO {outer loop}

BEGIN {Draw each row}

{first inner loop}

FOR LeadBlanks := NumLines - Row DOWNTO 1 DO

write (Blank); {Print leading blanks}

{second inner loop}

FOR CountStars := 1 TO 2 * Row - 1 DO

write (Star); {Print asterisks}

writeln;

END {for Row}

end. {Triangle}