SOFTWARE DEVELOPMENT IN C CM9041

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SOFTWARE DEVELOPMENT IN CCM904-1

• Lecture 2
• Control Structures in C

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Last Week

• C is a procedural language
• Simple
• Compiled
• Cf Java
• Interaction achieved by printf and scanf

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This week

• Modularity
• Control structures
• Sequence
• Selection
• Repetition

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Declaring and using a function (modularity)

• include ltstdio.hgt
• int sum(int,int) /Function prototype /
• int main(void)
• int num14, num27
• int total
• totalsum(num1,num2)
• printf(The sum of the numbers is d\n,
  total)
• return 0
•  
• int sum(int n1, int n2)
• / Function to find the sum of two numbers /
• int sumresult
• sumresultn1n2
• return sumresult

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Conditions and Operators

• Conditions
•  No boolean type
• integer 0 false, 1 true
• e.g.
• 5gt3 has the value 1
• 2gt3 has the value 0
•  Relational Operators
•  lt, lt, , !, gt, gt
•  if (x1) printf(OK)
• if (x1) printf(OK)
•  LogicalOperators
•  ! NOT
• AND
• OR

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If Statements

• if (condition)
• statement
• if (condition)
• statement1
• else
• statement 2
•  
• where a statement can be a compound statement.

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Some Examples

• If the conditional value is non-zero then
  statement1 is executed, otherwise statement2 is
  executed.
• e.g 1.
• if (agelt18age gt75)
• printf(Error)

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IF.. cf IF.. ELSE

• e.g.2
•   if (gender !m gender !f)
• printf(Error)
•   e.g 3.
• if (x3)
• printf(Yes)
• else
• printf(No)
• e.g 4.
• if (xlt0) x-x

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Compound statements

• e.g 5.
• if (altb)
• tempa / compound /
• ab / statement /
• btemp

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Compound Statements

• e.g7.
• if (agtb)
• maxa
• flag1
• else
• maxb
• flag2

e.g 6. if (agtb) maxa else
maxb
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Validation Example

• include ltstdio.hgt
• int main(void)
• int valid,age
• char gender
• scanf(dc,age, gender)
• valid1
• if (agelt18agegt65)
• valid0
• printf(\nAGE ERROR\n)
• if (gender !m gender !f)
• valid0
• printf(\nGENDER ERROR\n)
• if (valid)

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If.. Else Chains

• e.g.
•   if (month1)
• printf(January)
• else if (month2)
• printf(February)
• else if (month3)
• printf(March)
• else
• printf(Error)

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Dangling else problem

• 1. if (xgt0)
• if (yA)
• printf(Positive A)
• else
• printf(Negative)
•  
• 2. if (xgt0)
• if (yA)
• printf(Positive A)
• else
• printf(Negative)

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Switch Statement

• Provides a multi-way decision by testing whether
  an expression matches one of a number of constant
  values.
• General Form
•   switch (expression)
• case const_expr1statements
• case const_expr2statements
• .
• defaultstatements

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Example Switch Statement

• Classify an input character c as digit, white
  space or other.
•  
• switch (c)
• case
• case \n
• case \t nwhitenwhite1
• break
• case 0 case 1 case 2 case 3
• case 4 case 5 case 6 case 7
• case 8 case 9
• ndigitndigit1
• break
• default nothernother1

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Iteration

• Non-deterministic
• No pre-set number of iterations
• Use
• While
• Do While
• Deterministic
• Known number of iterations
• Use
• For

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Non-deterministic Loop While statement

• Format
• while (condition)
• statement
• where the statement can be a compound
• statement
• e.g.
• sum0
• scanf(d,x)
• while (x!-9999)
• sumsumx
• scanf(d,x)

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Read Ahead Technique

• Input data 1 4 5 6 9999
•  
• Unwinding the iteration
• Read num 1
• Process num
• Read num 4
• Process num
• Read num 5
• Process num
• Read num 6
• Process num
• Read num -9999

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• The iterated step is
• Process num
• Read num
•   
• Which leads to
•  
• Read num
• While num ! -9999
• Process num
• Read num

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Example While Loop

• Define Problem
• Problem
• Read a series of integers terminated by 9999.
  Count the number of integers that are greater
  than 150 and find the largest and smallest
  numbers greater than 150.Print a message if none
  of the integers are greater than 150.
•  
• Example Input
• 100 200 10 80 400 150 6 -9999
•  
• Expected Output
•   Number of valuesgt1502
• Max400
• Min200

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• 2. Design
• Initialise
• count0
• max0
• min99999
• Read first number
• Input values and process
• While number ltgt -9999 loop
• If number gt 150 then
• Increment count
• Check if new largest
• If numbergtmax then
• maxnumber
• Check if new smallest
• If number ltmin then
• minnumber
• Read next number
• End loop

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• include ltstdio.hgt
• int main(void)
• /program description/
• int num,max,min,count
• count0
• min99999
• max0
• scanf(d, num)
• while (num !-9999)
• if (numgt150)
• count
• if (numgtmax) maxnum
• if (numltmin) minnum
• scanf(d,num)

3. Code
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Non-deterministic loop Do While loop

• General Form
• do
• statement
• while (expression)
•  
• Unlike the while statement, the expression in the
  do while comes after statement (the loop
  body) which is thus executed at least once.
• Note that it is very easy to make mistakes with
  the do while loop, although it may compile and
  appear to produce valid code.

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Deterministic Loop For statement

• For statement format
• for (initialising list condition altering list)
• statement
• initialising list Executed once
• altering list Executed at the end of each
  iteration
• condition Evaluated and checked at the
• start of each
  iteration i.e. a
• leading decision

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ExampleFor loop

• for (count1countlt10countcount2)
• printf(3d,count)
• Gives output 1 3 5 7 9
• Equivalent While loop
• count1
• while (countlt10)
• printf(3d,count)
• countcount2

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Increment and Decrement Operators

• countcount1 count count
• countcount-1 --count count--
• Increment can occur before or after statement
  actioned
• count3
• xcount count 4, x 4
• count3
• xcount count4, x3
• Similarly with decrement operators.

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Using increment and decrement operators in for
loops

• for (count0countlt10count)
• printf(3d,count)
• Prints 1 2 3 4 5 6 7 8 9 10
• for (count0countlt10count)
• printf(3d,count)
• Prints 0 1 2 3 4 5 6 7 8 9 10
• Classic Error
•  for (i0ilt10i)
• printf(d, i)
•  

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Example 1 Deterministic Loop

• Problem Print a multiplication table of size N.
• e.g.
• 1 2 3 4 5
• 2 4 6 8 10
• 3 6 9 12 15 where N5
• 4 8 12 16 20
• 5 10 15 20 25
•  
• The structure of the data determines the
  structure of the program

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Design

• Data
• Repetition of rows
• Repetition of values (rowcolumn)
•  
• Design
• Read N
• Perform N times
• Print a row
• Perform N times
• Print a value

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Code

• include ltstdio.hgt
• / Reads an integer N and prints the
• N times multiplication table /
• int main(void)
• int row, col, n
• printf(\nPlease enter the value of N )
• scanf(d, n)
• printf(\n\n)
• for (row1rowltnrow)
• for (col1colltncol)
• printf(6d, rowcol)
• printf(\n)
• return 0

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Deterministic Loops Example 2

• Problem
• Write a program to encode lower case letters in a
  message by performing a cyclic shift one place to
  the right.
•  
• Example Input
• The cat sat on the mat
• Example Output
• Uif dbu tbu po uif nbu

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Code

• include ltstdio.hgt
• / Character processing example /
• int main(void)
• char c
• while (scanf(c, ch) ! EOF)
• if (ch z)
• printf(c, a)
• else if (ch gt a ch lty)
• printf(c, ch1)
• else
• printf(c, ch)
• return 0

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Next week

• Simple software development lifecycle
• Program design
• Testing