Presentation Number 7

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Presentation Number 7

• C Programming Course

2
Last Time in Class

• Recursion - a method for defining functions in
  which the function being defined is used within
  its own definition.
• Two things you must remember about recursions
• Change the parameters from call to call!!!
• Make sure you defined a boundary condition!!!

3
Merge Sort

• Given two arrays of sorted numbers, how do you
  merge the two into one sorted array?

Left array
Right array
Mergerd array
4
Merge Sort

• Loop across the array and copy the smaller number
  into the merged array.

Is the current left number smaller then the
current right number?
Copy the left number into the merged array
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Merge Sort

• Loop across the array and copy the smaller number
  into the merged array.

Is the current left number smaller then the
current right number?
Copy the left number into the merged array
6
Merge Sort

• What happens when one of the arrays has reached
  the end?

Copy the rest of the other array into the merged
array
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Merge Sort

• Given the merge algorithm that we just saw, how
  do we sort an array?
• Sort the left half.
• Sort the right half.
• Merge the two halves.
• What is the boundary condition?

Sort the left half
Sort the right half
Merge into one sorted array
8
Pointers
9
Declaring variables - Reminder

• Before using a variable, one must declare it.
• The declaration introduces the variable type,
  then its name.
• When a variable is declared, its value is
  undefined.

type variable_name
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Declaring arrays - Reminder

• An array is a block of variables of the same
  type.
• The array variable holds the address in memory of
  the beginning of the sequence.
• The nth element of an array is specified by
  Arrn-1
• We start counting from 0.
• The ith element of an array can be treated as any
  other variable.

type variable_nameSIZE
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Declaring a pointer variable

• A pointer is declared by adding a before the
  variable name.
• A pointer is a variable that contains an address
  in memory.
• The address should be the address of a variable
  or an array that we defined.

type variable_name
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Pointers

• int a 1
• int b 2
• int ptr NULL

ptr
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Referencing

• The unary operator gives the address of a
  variable
• The statement
• ptr a
• assigns the address of a to the pointer
  variable ptr.
• The variable pointer ptr now points to variable
  a.

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Pointers

• int a 1
• int b 2
• int ptr NULL
• ptr a

ptr
ptr
P is assigned the address of variable a
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Dereferencing

• The unary operator is the dereferencing
  operator.
• It is applied on pointers.
• It accesses the object the pointer points to.
• Follow the pointer and perform the operation on
  the actual memory that the pointer points to.

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Pointers

• int a 1
• int b 2
• int ptr NULL
• ptr a
• ptr b

ptr
ptr
The physical address to which ptr points is
assigned the value of b.
ptr
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Pointers - Illustration
memory

num
int num
p
5
int p
p num
p 5
int arr5
arr
p arr
p 7
0
7
p3 0
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Common errors

• It is impossible to define pointers to constants
  or expressions.
• i 3
• j (k 5)
• k (a b)
• It is impossible to change a variables address
• It is not ours to change!!!!!!!!!
• a 150
• a b

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Wrong Swap
A swap that gets integers as variables does not
change the value in the original variables.

• void swap(int x, int y)
• int tmp x
• x y
• y tmp
• int main()
• int x 5, y 7
• swap( x, y )
• printf( x d and y d\n, x, y )
• return 0

x 5 and y 7
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How can we fix it?

• void swap(int x, int y)
• int temp x
• x y
• y temp
• int main()
• int x 5, y 7
• swap( x, y )
• printf( "x d and y d\n", x, y )
• return 0

We define the swap function so it gets pointers
to integers instead of integers.
x 7 and y 5
21
Scanf

• We can now understand the in scanf(d, x)
• The argument list in scanf is simply passed by
  reference, so scanf can change its content.

22
Arrays and Pointers

• An array arr holds the address of its first
  element arr0.
• arr is actually a pointer to arr0.
• char arr10char iptr NULLiptrs
• Both iptr and arr now point to arr0.

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Pointer-array equivalence

• An array is a kind of pointer.
• When an array is defined, a fixed amount of
  memory (the size of the array) is allocated.
• The array variable is set to point to the
  beginning of that memory segment.
• When a pointer is declared, it is uninitialized
  (like a regular variable) and it points to
  somewhere.

The address to which an array variable points
cannot be changed!
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Arrays and Pointers

• for ( p a p lt a N p )
• sum p
• is equivalent to
• for ( i 0 i lt N i )
• sum ( a i )
• is equivalent to
• p a
• for ( i 0 ilt N i )
• sum p i

Declarations int p int aN
Illegal expressions
a p a a 2 a
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Pointer arithmetic

• Pointers can be incremented and decremented
• If p is a pointer to a particular type, p1
  yields the correct address of the next variable
  of the same type.
• p, pi, and p i also make sense.
• If p and q are of the same type and point to
  elements in an array, q-p is the number of
  elements between p and q.

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my_strncpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

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my_strncpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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my_strcpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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my_strcpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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my_strcpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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my_strcpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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my_strcpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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my_strcpy

• void my_strncpy(char dest, const char src, int
  size)
• int i
• for ( i0 i lt size i )
• dest src
• dest
• src

src
dest
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Examples
35
Example

• We need a function that gets a number of type
  double and returns its integer and fraction
  parts.
• What parameters should the function have?

36
Example

• We need a function that gets a number of type
  double and returns its integer and fraction
  parts.
• What parameters should the function have?
• void split(double num, int int_part, double
  frac_part)
• int_part (int)num
• frac_part num - int_part

37
Example

• How would you write a function like this?
• void replace_char(char str, int size, char c1,
  char c2)
• The function replaces each appearance of c1 by c2
  in the string str.
• How do you do this without the operator?

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Solution

• void replace_char(char str, int size, char c1,
  char c2)
• int i
• if (str NULL)
• return
• for ( i0 i lt size i, str )
• if (str c1)
• str c2