Presentation Number 5

1
Presentation Number 5

• C Programming Course

2
The Switch Statement

• many if's in one

3
The Switch Statement

• You can write
• if (letter 'a')
• printf("The letter is a\n")
• else
• if (letter 'b')
• printf("The letter is b\n")
• else
• if (letter 'c' letter 'C')
• printf("The letter is either c or C\n")
• else
• printf("This is not a familiar
  letter\n")

4
The Switch Statement
A switch statement is a multi-way conditional
statement.

• switch ( letter )
• case 'a'
• printf("The letter is a\n")
• break
• case 'b'
• printf("The letter is b\n")
• break
• case 'c'
• case 'C'
• printf("The letter is either c or C\n")
• break
• default
• printf("This is not a familiar letter\n")

There is an expression to be evaluated. It must
have an integer value.
The cases are labels to jump to according to the
value of the expression. There is a defalut label.
5
The Switch Statement

• switch ( letter )
• case 'a'
• printf("The letter is a\n")
• break
• case 'b'
• printf("The letter is b\n")
• break
• case 'c'
• case 'C'
• printf("The letter is either c or C\n")
• break
• default
• printf("This is not a familiar letter\n")

When a switch jumps to a chosen case, it executes
the statements from that point on.
Unless stopped it will fall through to the next
case.
break causes the program to jump to the first
statement after the switch.
6
Simple Calculator

• int main()
• double x 0.0, y 0.0
• char oper 0
• printf("Enter a simple math operation\n")
• scanf("lf c lf", x, oper, y)
• switch( oper )
• case ''
• printf("Value lf\n", x y )
• break
• case '-'
• printf("Value lf\n", x - y )
• break
• case ''
• printf("Value lf\n", x y )
• break
• case '/'
• printf("Value lf\n", x / y )

Sometimes you might decide not to use a default
label (although it does not hurt to have one even
if it is empty).
SimpleCalc.c
7
Thinking About Algorithms
8
Advanced Subsequences

• You are given a set of numbers.
• How do you calculate the length of the longest
  increasing sub-sequence when the sequence does
  not have to be of successive numbers?
• Example input
• Sequence length 8

9
Advanced Subsequences

• Here are a few questions that you should answer
  first
• Do you have to store the input numbers?
• What is the brute force solution? How bad is
  it?
• Is it feasible to solve the problem in one pass
  over the number?
• How did YOU find the length in this input set?

10
Advanced Subsequences

• Input
• Generally, it does not matter how many numbers
  are in the input set. Lets assume that we have n
  numbers.
• Lets assume that part of the problem is solved.
  We will figure out later what this means.
• Lets assume that we are now working on the ith
  element and we want to add it to a subsequence.
• We can add it at the beginning or at the end of a
  subsequence

11
Solution 1 Adding The First Element

• Elements
• If we are adding the ith element as first element
  in a subsequence, what kind of information about
  the next elements can we use to make the
  calculation easier?

12
Solution 1 Adding The First Element

• Elements
• Lengths
• For each element j (where j gt i) the Lengths
  array stores the length of the longest
  subsequence whose first number is the jth number.
• If (element j gt element i) then
• length i 1 length j
• If (Lengthsi lt length i) then
• Lengthsi length i
• The maximal length is the biggest number in the
  Lengths array.

13
Solution 2 Adding The Last Element

• Elements
• Lengths
• For each element j (where j lt i) the Lengths
  array stores the length of the longest
  subsequence whose last number is the jth number.
• If (element j lt element i) then
• length i 1 length j
• If (Lengthsi lt length i) then
• Lengthsi length i
• The maximal length is the biggest number in the
  Lengths array.

14
Multi-Dimensional Arrays

• Arrays allow us to store a block of many
  variables of the same type and access them with a
  single name.
• Arrays can have more than one dimension.
• int numbers1002
• double values10319
• float mat33

The first element is mat00 and the last is
mat22
mat
15
Functions
16
Function Definition

• type name(type name1, type name2, )
• function body
• return value
• A function is a group of declarations and
  statements.
• It has a name.
• It has a type.
• It has a value.
• It can have parameter variables.
• main is a function.

17
Function Definition

• double abs_value( double x )
• if ( x gt 0.0 )
• return x
• else
• return -x

18
The Return Statement

• double abs_value( double x )
• if ( x gt 0.0 )
• return x
• else
• return -x

return ends the execution of the function and
returns a value to the calling function.
The type of the value returned must be the same
as the return-type defined for the function.
19
Example

• include ltstdio.hgt
• double abs_value( double x )
• if ( x gt 0.0 )
• return x
• else
• return -x
• int main()
• double num 0.0, absNum 0.0
• printf("enter a number\n")
• scanf("lf",num)
• absNum abs_value(num)

Here we define two functions abs_value and
main. A function must be defined or at least
declared before it can be used.
AbsValue.c
20
Example

• include ltstdio.hgt
• double abs_value( double x )
• if ( x gt 0.0 )
• return x
• else
• return -x
• int main()
• double num 0.0, absNum 0.0
• printf("enter a number\n")
• scanf("lf",num)
• absNum abs_value(num)

These are the function calls. The variable absNum
is assigned the value that was returned by the
function abs_value with parameter num.
AbsValue.c
21
Why use functions?

• They break a problem down into smaller sub-tasks.
• makes it easier to solve complex problems.
• They generalize a repeated set of instructions.
• We dont have to keep writing the same thing over
  and over again.
• printf and scanf are good examples
• They make a program much easier to read, maintain
  and debug.

22
What Does This Program Do?

• int main()
• int numArrayMAX_NUMBERS
• int numValues 0
• numValues readNumbers( numArray, MAX_NUMBERS
  )
• if ( numValues ! 0 )
• sortNumbers( numArray, numValues )
• printUniqueNumbers( numArray, numValues)
• else
• printf("No numbers were read!\n")
• return -1
• return 0

23
The Great Void

• Sometimes there is no reason for a function to
  return a value
• In these cases, the function return type should
  be void.
• If the return keyword is used within such a
  function it exits the function immediately. A
  value should not be specified.
• Calling return in a function returning void is
  not obligatory.
• If the function receives no parameters, the
  parameter list should be replaced by void (or
  be left empty).

24
Example
The function does not recieve parameters and does
not return a value.

• void ShowHelp()
• printf("This function explains what the program
  does\n")
• printf("It prints this explanation!!")
• int main()
• char choice 0
• printf("Please enter your selection ")
• scanf("c", choice)
• if (choice 'h')
• ShowHelp()
• .
• .
• .

25
Function Declaration
26
Function Declaration

• All of the software projects in C are composed of
  multiple functions.
• Most software projects in C are composed of more
  than one file
• We want to define a function once, and use it
  many times.
• To do so, the function must be declared in every
  file in which its called, before its called for
  the first time

27
Function Declaration

• include ltstdio.hgt
• / Function Declaration! /
• int factorial(int a)
• int main()
• int num, factValue
• printf("enter a number\n")
• scanf("d", num)
• factValue factorial(num)
• printf(The factorial of d is d\n", num,
  factValue)
• return 0

A function is declared as follows type name(type
name1, type name2, )
Factorial.c
28
Function Declaration

• int factorial(int a)
• int i, b 1
• for (i 1 i lt a i)
• b i
• return b

A function declaration allows you to use the
function ... however, you must also define the
function
Factorial.c
29
Function Declaration

• stdio.h actually contains a large set of function
  declarations
• The include directive tells the compiler to
  insert these declarations into the file, so that
  these functions could be called

30
Developing large programs
pgm.h
includes defines list of prototypes
main.c
fct.c
prn.c
includes pgm.h
includes pgm.h
includes pgm.h
31
Variables and Functions
32
Call By-Value

• Function arguments are passed to the function by
  copying their values rather than giving the
  function direct access to the actual variables.
• A change to the value of an argument in a
  function body will not change the value of
  variables in the calling function.

33
Call By-Value

• include ltstdio.hgt
• int compute_sum(int num)
• int main()
• int num3, sum0
• printf("d\n", num)
• sum compute_sum(num)
• printf("d\n", num)
• printf("d\n", sum)
• return 0

Function declaration
3 is printed
what happens to n and sum here?
ByValue.c
34
Call By-Value

• int compute_sum( int num )
• int sum 0
• for ( num gt 0 --num )
• sum num
• return sum

sum the integers from 1 to n
This num is local. Its value is the same as the
value of the variable that was sent to the
function.
sum is local to the function.
local num and sum are changed
ByValue.c
35
Riddle me this
int main() int n 0, fact 0
printf("enter a number\n") scanf("d",n)
fact factorial(n) printf("d!d\n", n,
fact) / What will this print? /
printf("n d\n", n) return 0

• include ltstdio.hgt
• int factorial(int n)
• int fact 1
• while (ngt1)
• fact n
• n--
• return fact

36
Scope Rules

• Scope rules apply also to functions.
• A variable declared within a function is
  unrelated to variables declared elsewhere, even
  if they have the same name
• A function cannot access variables that are
  declared in other functions

37
Magic Square Example
38
Example Magic-Square
39
Magic Square

• include ltstdio.hgt
• int IsMagic (int mat33)
• / Input A 3x3 matrix as nine integers ordered
  by columns /
• / Output is it a magic square /
• int main (void)
• int mat33
• int row, col, ret
• for (row0 rowlt3 row)
• for (col0 collt3 col)
• ret scanf ("d", matrowcol)
• if ( ret 0 ret EOF )
• printf ("Error, input needs to be nine
  integers.\n")
• return -1

Loop across all the rows
Loop across the elements of the row
Read a number
Check the scanf worked correctly
MagicSquare.c
40
Magic Square
Let the function check for magic squareness

• if (IsMagic (mat) 1)
• printf ("the input is a magic square.\n")
• else
• printf ("the input square is NOT a magic
  square.\n")
• return 0

MagicSquare.c
41
Magic Square

• / Function check if a square is a magic square
  /
• / input a 3x3 matrix of integers /
• / output 1 - if it is a magic square 0
  otherwise /
• int IsMagic (int mat33)
• int i, j
• int sum_r, sum_c, ref_sum, r_diag, l_diag
• / init ref_sum with first row sum /
• ref_sum 0
• for (j0 jlt3 j)
• ref_sum mat0j

The function recieves the matrix as a parameter.
This is known as passing parameters by-reference.
MagicSquare.c
42
Magic Square

• / check rows and cols together /
• for (i0 ilt3 i)
• sum_r sum_c 0
• for (j0 jlt3 j)
• sum_r matij
• sum_c matji
• if (sum_r ! ref_sum sum_c ! ref_sum)
• return 0

Instead of repeating the code once for rows and
once for columns we use the same loops for both.
MagicSquare.c
43
Magic Square

• / check main diagonals /
• r_diag l_diag 0
• for (i0 ilt3 i)
• r_diag matii
• l_diag mati2-i
• if (r_diag ! ref_sum l_diag ! ref_sum)
• return 0
• return 1

MagicSquare.c