CMP 131 Introduction to Computer Programming

1
CMP 131Introduction to Computer Programming

• Violetta Cavalli-Sforza
• Week 8, Lecture (Monday)

2
MONDAY

• Midterm results
• Statistics (next slide)
• Go over midterm

3
Midterm Statistics
81.3 35
49 32.7
48.3 32.5
44.3 29.3
39.6 28.1
38.2 22
36.5 21.1
36 20

Average 37.11875
Median 35.5
4
WEDNESDAY

• Multiple choice conditionals
• Review extended IF statement
• CASE statement
• Standard Functions

5
Multiple Choice Alternatives

• Do something different if Grade A, B, C, D, F,
  
• Compute frequencies (histogram)
• If Input is lt Val1 increment Interval0
• If Input is in range Val1..Val2 increment
  Interval1
• If Input is in range Val2..Val3 increment
  Interval2
• If Input is in range ValN-1.. ValN increment
  IntervalN-1
• If Input gt ValN increment IntervalN

6
Example

• Input A letter grade A, B, C, D, or F
• Output An English grade description
• A Excellent
• B Good
• C Average
• D Passing
• F Failing

7

• PROGRAM EnglishGrade File EXTIF1.pas
• VAR Grade char
• BEGIN
• write('Enter a grade ')
• readln(Grade)
• write('Your grade is ')
• IF Grade 'A' THEN
• writeln('Excellent')
• ELSE IF Grade 'B' THEN
• writeln('Good')
• ELSE IF Grade 'C' THEN
• writeln('Average')
• ELSE IF Grade 'D' THEN
• writeln('Passing')
• ELSE IF Grade 'F' THEN
• writeln('Failing')
• ELSE
• writeln('NOT RECOGNIZED')
• readln

8
Testing the Program

• Test input should include
• A test for each legal input A, B, C, D,
  or F
• At least one test for an illegal input e.g. E
• Maybe the program should be more robust and
  accept upper and lower case letter grades?

9

• PROGRAM EnglishGrade File EXTIF2.pas
• VAR Grade char
• BEGIN
• write('Enter a grade ')
• readln(Grade)
• write('Your grade is ')
• IF (Grade 'A') OR (Grade 'a') THEN
• writeln('Excellent')
• ELSE IF (Grade 'B') OR (Grade 'b') THEN
• writeln('Good')
• ELSE IF (Grade 'C') OR (Grade 'c') THEN
• writeln('Average')
• ELSE IF (Grade 'D') OR (Grade 'd') THEN
• writeln('Passing')
• ELSE IF (Grade 'F') OR (Grade 'f') THEN
• writeln('Failing')
• ELSE
• writeln('NOT RECOGNIZED')
• readln

10

• PROGRAM EnglishGrade File EXTIF2.pas
• VAR Grade char
• BEGIN
• write('Enter a grade ')
• readln(Grade)
• write('Your grade is ')
• IF (Grade 'A') OR (Grade 'a') THEN
• writeln('Excellent')
• ELSE IF (Grade 'B') OR (Grade 'b') THEN
• writeln('Good')
• ELSE IF (Grade 'C') OR (Grade
  'c') THEN
• writeln('Average')
• ELSE IF (Grade 'D') OR
  (Grade 'd') THEN
• 
  writeln('Passing')
• ELSE IF (Grade
  'F') OR (Grade 'f') THEN
• 
  writeln('Failing')
• ELSE
• 
  writeln('NOT RECOGNIZED')
• readln

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Complex Condition

• Why IF (Grade 'A') OR (Grade 'a') THEN
  and not IF Grade 'A' OR Grade 'a' THEN
  ?
• Try compiling IF Grade 'A' OR Grade 'a'
  THEN
• Result
• Error 41 Operand types do not match operator

12
Condition Evaluation

• The condition IF Grade 'A' OR Grade 'a'
  THEN
• is valuated as
• IF Grade ( 'A' OR Grade ) 'a' THEN
• A and Grade are operands of data type char.
• They are unsuitable for operator OR which wants
  boolean operands.

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Operator Precedence Arithmetic, Relational and
Logical

• Operator Precedence
• ( ) parentheses Highest (evaluated first)
• - NOT (unary operators)
• / DIV MOD AND
• - OR
• lt lt ltgt gt gt Lowest (evaluated last)

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Example Exam Score Intervals

• Exam score Grade Assigned
• 90 above A
• 80-89 B
• 70-79 C
• 60-69 D
• Below 60 F

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Example Exam Scores Intervals (2)

• Correct grade assignment Incorrect grade
  assignment
• if score gt 90 then if Score gt 60 then
• Write ('A') Write ('D')
• else if Score gt 80 then else if Score gt 70
  then
• Write ('B') Write ('C')
• else if Score gt 70 then else if Score gt 80
  then
• Write ('C') Write ('B')
• else if Score gt 60 then else if Score gt 90
  then
• Write ('D') Write ('A')
• else else
• Write ('F') Write ('F')

16
Multiple Choice Conditionals The CASE Statement

• Syntax
• CASE ltselectorgt OF
• ltlabel list 1gt ltstatement 1gt
• ltlabel list 2gt ltstatement 2gt
• ltlabel list Ngt ltstatement Ngt
• END

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Case Statement Example

• Compute gross pay for a particular day
• CASE DayNumber OF
• 1, 7 BEGIN 1,7 - compound statement
• WeekendRate 1.5 DailyRate
• Gross Hours WeekendRate
• END 1,7
• 2 .. 6 Gross Hours DailyRate
• END case
• What should the type of DayNumber be?

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Case Statement

• More readable than nested if statements when the
  nesting is deep
• About selectors and label lists
• The selector expression is evaluated and compared
  to each of the case labels
• The type of each constant in a case label value
  must correspond to the type of the selector
  expression
• Each labeli is a list of one or more possible
  constants, separated by commas
• A particular selector may appear in at most one
  of the labels
• About statements and their execution
• Statementi will be executed if selector value
  lies in labeli
• Each statement may be a single or a compound
  Pascal statement
• Only one statement will be executed
• If the value of the selector is not listed in any
  case label, an error message is printed and
  program execution stops
• After execution of the CASE statement, control is
  passed to the first statement following the CASE
  end

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Guarding Case Statements

• Use an if-statement to prevent the occurrence of
  Out of range error of case expression.
• Compute gross pay for a particular day
• IF (DayNumber gt 1) AND (DayNumber lt 7) THEN
• CASE DayNumber OF
• 1, 7 BEGIN 1,7
• WeekendRate 1.5 DailyRate
• Gross Hours WeekendRate Rate
• END 1,7
• 2 . . 6 Gross Hours DailyRate
• END case
• ELSE
• WriteLn (DayNumber1, ' is an invalid day
  number')

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Guarding Case Statements

• Some compilers provide the else clause to avoid
  the out of range errors.
• Compute gross pay for a particular day
• case DayNumber of
• 1, 7 begin 1,7
• WeekendRate 1.5 DailyRate
• Gross Hours DailyRate
• end 1,7
• 2 . . 6 Gross Hours DailyRate
• else
• WriteLn (DayNumber1, ' is an invalid day
  number')
• end case

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The CASE Statement with ELSE or OTHERWISE clause

• Syntax
• CASE ltselectorgt OF
• ltlabel list 1gt ltstatement 1gt
• ltlabel list 2gt ltstatement 2gt
• ltlabel list Ngt ltstatement Ngt
• ELSE / OTHERWISE
• ltstatement 1gt
• ltstatement 2gt
• ltstatement Ngt
• END

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Example Tax Rate

• read(GrossIncome) Real typeCASE
  round(GrossIncome) OF 0 null
  statement, do nothing 1..90 Tax
  GrossIncome 0.15 91..320 Tax
  GrossIncome 0.28ELSE optional Tax
  GrossIncome 0.28 Tax Tax
  (GrossIncome 0.0725) END case statement

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Multiple-AlternativeCASE or extended IF?

• Use CASE when the condition is a single
  multi-valued expression
• If several variables are involved in the decision
  use Boolean expressions OR IF-THEN-ELSE statement
  instead
• Example to see if various criteria are met

Using nested if statements Using Boolean expressions
AllMet FalseIF single THEN IF (Gender 'M') THEN IF Age gt 18 THEN IF Age lt 26 THEN AllMet True AllMet Single AND (Gender 'M') AND (Age gt 18) AND (Age lt 26)
IF AllMet THEN WriteLn ('Current person satisfies the criteria.') ELSE WriteLn ('All Criteria are not satisfied') IF AllMet THEN WriteLn ('Current person satisfies the criteria.') ELSE WriteLn ('All Criteria are not satisfied')
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These are almost equivalent

• Cascaded IF-THENIF ltcondition1gt THEN IF
  ltcondition2gt THEN IF ltcondition3gt THEN
  IF ltcondition4gt THEN If
  ltcondition Ngt is false ltcondition N1gt will never
  be evaluated
• Single IF-THEN with complex Boolean
  expressionIF ltcondition1gt AND ltcondition2gt AND
  ltcondition3gt AND ltcondition4gt THEN
• If ltcondition Ngt is false ltcondition N1gt will
  never be evaluated if the compiler is using
  short-circuit evaluation

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Short-Circuit Evaluation

• Used to speed up evaluation of Boolean
  expressions
• Boolean conditions combined with AND must all be
  true for the entire expression to be true
• Boolean conditions combined with OR must all be
  false for the entire expression to be false
• The compiler can stop evaluating a Boolean
  expression as soon as its value can be determined
• We can control the evaluation by using compiler
  directives

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Short-Circuit Evaluation

• ExampleSingle AND (Gender 'M' ) AND (Age gt
  18) AND (Age lt 26)
• If Single False
• The whole expression will be false regardless of
  the values of the rest of the expression
• There is no need to evaluate the other conditions

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Short-Circuit Evaluation

• Example
• Let X 0
• If ( X ltgt 0.0 ) and (Y / X gt 5.0 ) then . . .
• Short circuit evaluation here is important to
  prevent dividing by zero
• Otherwise the condition should be split into two
  parts
• If ( X ltgt 0.0 ) then
• If (Y / X gt 5.0 ) . . .

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Short-Circuit Evaluation

• Remember
• True or anything is always true
• False and anything is always false
• Turbo Pascal uses short-circuit by default
• Standard Pascal doesnt
• Enable full evaluation by using B
• This is a compiler directive

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Short-Circuit Evaluation

• Compiler directives
• Comments beginning with the symbol that
  provides instructions to the compiler.
• B
• Causes complete evaluation of Boolean expressions
• B-
• Causes short-circuit evaluation to take place

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Case Studies

• A couple of long programs in Chapter 4
• Example 4.10, p.171 177
• p. 190-196
• Read through these carefully, understand them!!!
  The next quiz may draw from them!
• Case studies use user-defined procedures and
  functions (Ch 3)
• We dont study these in the course
• You dont need to know how to write them
• You need to understand them

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User-Defined Functions Procedures

• Used to
• encapsulate operations performed frequently,
  possibly with different input
• to structure programs.
• Both can take arguments
• Functions usually do
• Procedures may not (occasionally)
• Functions return results
• They are used in the same place as you would put
  a value, variable, or expression
• Procedures do not return results
• They accomplish their work through side effects
• E.g. writeln has the side effect of printing to
  the screen (or a file)
• E.g. readln has the side effect of setting its
  argument to the input

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User-Defined Functions Procedures

• The argument list (or parameter list contains 2
  types of parameters)
• Value parameters these are only used for
  inputting information into the function/procedure
• Functions usually only have these
• VAR parameters these can be used to transmit
  information back to the caller by storing into
  them as if they were variables
• Procedures often use VAR parameters
• VAR parameters used when need to transmit back
  more than one piece of information or data with a
  complex structure
• Storing into a VAR parameter is a kind of side
  effect

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Standard or Built-In Functions

• Standard operations provided by the programming
  language
• Computation performed more efficiently than if
  the user were to define them
• Fundamentally 2 types in the language Pascal
• Numeric Standard Functions
• Character Standard Functions
• The particular Pascal implementation (e.g.
  Borlands TurboPascal) may provide more

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Numeric Standard Functions
Function Name Function Value Argument Type Result Type
sqr square real or integer same as argument
Examples sqr(5) ? 25, sqr(5.0) ? 25.0 sqr(5) ? 25, sqr(5.0) ? 25.0 sqr(5) ? 25, sqr(5.0) ? 25.0
sqrt square root real or integer (non-negative) real
Examples sqrt(25) ? 5.0 sqrt(-25) ? error sqrt(25) ? 5.0 sqrt(-25) ? error sqrt(25) ? 5.0 sqrt(-25) ? error
abs absolute value real or integer same as argument
Examples abs(5) ? 25, abs(-5.0) ? 5.0 abs(5) ? 25, abs(-5.0) ? 5.0 abs(5) ? 25, abs(-5.0) ? 5.0
round rounding real integer
Examples round(2.0) ? 2, round(2.1) ? 2, round(2.5) ? 3, round(2.8) ? 3round(-2.0) ? -2, round(-2.1) ? -2, round(2.5) ? -3, round(2.8) ? -3 round(2.0) ? 2, round(2.1) ? 2, round(2.5) ? 3, round(2.8) ? 3round(-2.0) ? -2, round(-2.1) ? -2, round(2.5) ? -3, round(2.8) ? -3 round(2.0) ? 2, round(2.1) ? 2, round(2.5) ? 3, round(2.8) ? 3round(-2.0) ? -2, round(-2.1) ? -2, round(2.5) ? -3, round(2.8) ? -3
trunc truncation real integer
Examples trunc(2.0) ? 2, trunc(2.1) ? 2, trunc(2.5) ? 2, trunc(2.8) ? 2trunc(-2.0) ? -2, trunc(-2.1) ? -2, trunc(2.5) ? -2, trunc(2.8) ? -2 trunc(2.0) ? 2, trunc(2.1) ? 2, trunc(2.5) ? 2, trunc(2.8) ? 2trunc(-2.0) ? -2, trunc(-2.1) ? -2, trunc(2.5) ? -2, trunc(2.8) ? -2 trunc(2.0) ? 2, trunc(2.1) ? 2, trunc(2.5) ? 2, trunc(2.8) ? 2trunc(-2.0) ? -2, trunc(-2.1) ? -2, trunc(2.5) ? -2, trunc(2.8) ? -2
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Using Standard Functions

• Very much the same way you use an (arithmetic)
  operator in an expression
• Functions return values so
• Function calls are used like values
• E.g.
• Result 5 2 x / 3 3
• Result sqrt(10)
• Result 2 sqrt (10)
• Result Result (1 sqrt(10)) / N

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Using Standard Function (2)

• Arguments of standard functions can themselves be
  constants, variables, or expressions. E.g.
• round(3.5)
• round(Amount)
• round(Amount1.065)
• Make sure they are of the right type
• This wont work trunc(Var lt 5)

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Character Standard Functions

• Ordering a character requires associating an
  integer with each character
• The type character is an ordinal type
• The ASCII standard is used for encoding (among
  others)
• p. 78, Table 2.9
• See also Appendix 4 Character Sets

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ASCII Character Encoding

• Codes 0 to 127 for most things on a Roman
  keyboard
• Codes 0-31 are non-printable control characters
• Represented in Pascal as 0 .. 31
• Code 10 is ENTER
• Code 32 is (space, blank)
• Code 33-47 punctuation
• 0 48 9 57
• Codes 58-64 more punctuation
• A 65 Z 90
• Codes 91-96 more punctuation
• a 97 z 122
• Codes 123-126 more punctuation
• Code 127 is non-printable

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Character Standard Functions
Function Name Function Value Argument Type Result Type
ord Returns ASCII value char integer
Examples ord(C) ? 67 ord(C) ? 67 ord(C) ? 67
chr Returns associated character integer chr
Examples chr(67) ? C, chr(ord(C)1) ? D chr(67) ? C, chr(ord(C)1) ? D chr(67) ? C, chr(ord(C)1) ? D
pred Returns the predecessor of the number any ordinal type same as argument
Examples pred(C) ? B, pred(67) ? 66, pred(ord(C)) ? 66 pred(C) ? B, pred(67) ? 66, pred(ord(C)) ? 66 pred(C) ? B, pred(67) ? 66, pred(ord(C)) ? 66
succ Returns the successor of the number any ordinal type integer
Examples succ(C) ? D, succ(67) ? 68, succ(ord(C)) ? 68 succ(C) ? D, succ(67) ? 68, succ(ord(C)) ? 68 succ(C) ? D, succ(67) ? 68, succ(ord(C)) ? 68
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Example Counting Characters

• see program CHRCOUNT.PAS
• CASE Ch OF 0..9 Digits
  Digits 1 A..Z,a..z Alpha
  Alpha 1 0..31 Control
  Control 1
• ELSE optional Other Other 1
• END case statement

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Non-printable Characters

• You can represent them inside the program by
  using the before the character number
• You cant enter them that way however

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Self Check 1

• Write the Boolean expression to check if a
  character variable Ch is a digit.
• Write a Boolean expression to check if a
  character variable Ch is an upper-case letter.
• Write a Boolean expression to check if a
  character variable Ch is a lower-case letter.
• Using a Boolean test, convert upper-case
  character Ch to a lower-case character.

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Self-Check 2

• Write Boolean assignment statements
• Assign true to variable Between if the value of N
  is in the range -K to K, inclusive otherwise,
  assign a value of false
• Assign a value of true to variable Uppercase if
  Ch is an uppercase letter otherwise, assign
  false
• Assign a value of true to variable Divisor if M
  is a divisor of N otherwise, assign false

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Self-Check 3

• What do these statements display?
• IF 12 lt 12 THEN writeln(Less)ELSE writeln(No
  t less)
• Var1 25.12Var2 15.00IF Var1 lt Var2
  THEN writeln(Less or equal)ELSE writeln(Grea
  ter than)

45
Self-Check 4

• What value is assigned to X when Y is 15.0?
• X 25.0IF Y ltgt (X - 10.0) THEN X X -
  10.0ELSE X X / 2.0
• IF (Y lt 15.0) AND (Y gt 0.0) THEN X 5
  YELSE X 2 Y

46
Self-Check 5

• Write Pascal statements to perform the following
• If Item is nonzero, then multiply Product by Item
  and save the result in Product otherwise, skip
  the multiplication. In either case, print the
  value of Product.
• Store the absolute difference of X and Y in Y,
  where the absolute difference is (X - Y) or (Y -
  X), whichever is positive.
• Dont use the abs() function in your solution

47
Self-Check 6

• Evaluate the following expressions, with and
  without short-circuit evaluation
• Assumptions
• X 6
• Y 7
• (X gt 5) and (Y div X lt 10)
• (X lt 10) or (X / (Y - 7) gt 3)

48
Self-Check 7

• Implement the following decision table using a
  nested if statement then using a case statement.
• 0.0-0.99 Failed - registration suspended
• 1.0-1.99 On probation for next semester
• 2.0-2.99 (No message)
• 3.0-3.49 Deans list for semester
• 3.5-4.0 Highest honors for semester
• Assumption
• GPA is within the range 0.0 through 4.0.

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Self-Check 8

• Write a case statement that prints a message
  indicating whether NextCh (of type char) is
  either one of the following
• An operator symbol (, -, , , lt, gt, /)
• A punctuation mark (comma, semicolon,
  parenthesis, brace, bracket, or colon)
• A digit.
• Your program should print the category selected
• Guard the case statement
• Write the same program using a nested if statement

50
Common Programming Errors

• Use semicolons carefully inside if statements.
• A semicolon before else will indicate the end of
  the if statement.
• Don't forget to bracket (with begin-end) compound
  statements, otherwise only the statement
  preceding the semicolon will be considered.