Introduction to C Programming CE003121

1
Introduction to C ProgrammingCE00312-1

• Lecture 3
• Control Structures in C

2
Program Structure

• Modularity
• Atomisation of complex problem into manageable
  units possibly reusable in other complex
  problems
• Functions
• Control structures
• Basic mechanics of process deconstruction
• Most problems can be expressed as combinations of
• Sequence
• Selection
• Repetition

3
Modularity in a C Program

• The Function main
• In C, all code is packaged in functions
• main is the function that the system calls when
  you run your program
• Every program must have one and only one main
• All functions may return a value

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Function Components

• A function definition consists of two
• parts
•  A Specification ( or header or interface)
  Specifies the function name, the return value and
  argument list
• A Body ( or block or implementation) Code to be
  executed when the function is called. A sequence
  of statements enclosed in braces

5
Declaring and using a function

• 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

6
Program Control - Sequence

• Default control structure
• Program execution occurs sequentially unless
  otherwise directed
• Each step should be the logical consequence of
  the preceding one
• Statements should be arranged such that they
  occur in the right order

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Program control - Selection

• Selection
• Implemented using
• If..
• Single branch
• If..else..
• Multiple branching
• Ordinal relationship in branch determination
• Switch..case...
• Multiple branching
• Category branch determination

8
If Statements

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

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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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Some Examples

• If the conditional value is non-zero then
  statement1 is executed, otherwise execution
  passes to next statement after if construct.
• e.g. 1
• if (agelt18age gt75)
• printf(Error)
• e.g. 2
•   if (gender !m gender !f)
• printf(Error)
• e.g. 3
• if (xlt0) x-x

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IF.. cf IF.. ELSE

• If the conditional value is non-zero then
  statement1 is executed, otherwise statement2 is
  executed.
•  
• e.g. 4
• if (x3)
• printf(Yes)
• else
• printf(No)

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Compound statements

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

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Compound Statements

• e.g. 7
• 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)
printf(\nData OK\n) else printf(\nData
Error\n) return 0
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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)
•  
• Which condition determines the outcome?
• 2. if (xgt0)
• if (yA)
• printf(Positive A)
• else
• printf(Negative)
• A is only printed if x gt and y is A

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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 c1statements
• case c2statements
• .
• defaultstatements

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switch Statement

• expression is any integer valued expression
• c1, c2 are integer valued constants
• (remember integer include chars!)
• statements are any C statements
• default is optional
• c1, c2 must be unique (i.e same constant cannot
  appear twice)
• each case can have only one value (not range)

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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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Integer example switch

• switch (gradePoint)
• case 1
• printf (You have failed badly)
• break
• case 3
• printf(A narrow fail\n)
• case 2
• printf(Better luck at the resit)
• break
• case 4 case 5 case 6 case 7 case 8
• printf(Well done, you passed)
• break
• default
• printf(Not a valid grade point)

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Switch

• after switch is evaluated control falls to
  matching case
• if no match, go to default if present
• terminate with break or by reaching end of switch
  block
• can group cases together by omitting break -
  careful!
• No need for ... within case