Chapter 5 Array, Pointers and Strings

1
Chapter 5 Array, Pointers and Strings

• By C. Shing
• ITEC Dept
• Radford University

2
Objectives

• Understand how to use arrays (one and
• multiple dimensions)
• Understand how to use pointers
• Understand the relationship between pointers
• and arrays
• Understand how to call function using
• pass-by-reference and pass-by-value
• Understand how to create dynamic memory

3
Objectives (Cont.)

• Understand how to pass function as argument
• Understand how to use strings
• Understand the relationship between pointers
• and strings
• Understand array of pointers
• Understand how to use command line arguments
• Understand how to use Library function scanf

4
Array

• A contiguous memory with the same data type
• Declared as
• 1-dimension
• type array_name array_size
• 2-dimension
• type array_name row_sizecoulmn_size

5
Array (Cont.)

• Has index (or indices) starting from 0
• to access each memory
• 1-dimension specify the content at index
• array_nameindex
• Example
• type array_name array_size
• declare memory array_name0, array_name1,
  ,
• array_namearray_size-1
• 2-dimension specify the content
• at row_index and column_index
• array_namerow_indexcolumn_index

6
Array (Cont.)

• Programmer must make sure that
• indices are not out of bound.
• Otherwise, buffer overflow security problem
• will occur.

7
Array (Cont.)

• Initialization
• row-wise (starts fill in 1st row first)
• Fill in 0 if not enough initial integer values
  specified
• Form
• 1-dimension
• type array_namearray_size initial values
• 2-dimension
• type array_namerow_sizecolumn_size
• row 0 initial values, ,
• row row_size-1 initial values

8
Array (Cont.)

• Initialization (Cont.)
• Example
• 1-dimension
• int a 4 4,3,2,1
• or
• int a4,3,2,1
• Then a04, a13, a22, a31
• 2-dimension
• int a34
• 10,11,12,13, 20,21,22,23,
• 30,31,32,33
• or
• int a 10,11,12,13, 20,21,22,23,
• 30,31,32,33
• or
• int a4 10,11,12,13, 20,21,22,23,
• 30,31,32,33
• Then a0010, a0111, a0212,
  a0313, , ,
• a2030, a2131, a2232,
  a2333

9
Array (Cont.)

• Initialization (Cont.)
• Example (Cont.)
• 1-dimension
• int a 4 4,3
• Then a04, a13, a20, a30
• 2-dimension
• int a34
• 10, 20,21,22,
• 30,31
• Then a0010, a01a02a030,
  a2030,
• a2131, a22a230

10
Array (Cont.)

• Initialization (Cont.)
• Example Initialize the whole array
• 1-dimension
• int a 4 0
• Then a00, a10, a20, a30
• 2-dimension
• int a34
• 0
• Then a00a01a02a03 a20
• a21a22a230

11
Pointers

• Store address using a word
• More efficient than using index
• Declare as
• type pointer_variable
• This request a word size pointer_variable that
• stores an address of the type
• Note do not use pointer to a register type
  variable
• Use in a statement (Addressing operator )
• pointer_variable type_variable
• The pointer_variable stores the address of the
• type_variable
• Note do not use expression

12
Dereferencing Operator

• Find the content that pointed by an address
• Use in a statement
• variable pointer_variable
• Example
• int i , p
• i 10
• p i
• j p
• Then j also contains 10. (Note and cancel
  each other)
• Example

13
Top-Down Design Example - Use Pointer

• Example write a program to convert a series of
• Fahrenheit degrees into Celsius degree (use
• sentinel -1 to stop)
• The Output will look like
• Fahrenheit Celsius
• _________ ______
• 212.00 100.00
• 32.00 0.00

14
Top-Down Design Example - Use Pointer

• Functions
• 1st level main(void)
• 2nd level
• PrintHeading this function prints header
  and __ in screen
• input nothing
• output nothing
• GetInput this function reads data from keyboard
  and
• stores fahrenheit degree
• input fahrenheit address (double)- due to
  pass by value
• output nothing
• ProcessData this function converts the
  fahrenheit to celsius degree
• input fahrenheit (double) and celsius
  address (double ) from main
• output nothing
• OutputResult this function prints out
  fahrenheit and celsius degrees in screen
• input fahrenheit (double), celsius (double)
  from main
• output nothing

15
Top-Down Design Example - Use Pointer

• program
• data
• Top-Down Design Example - Use Array
• Program (sentinel -1)
• data

16
Array and Pointer Relation

• Array name stores address of the array
• 1-dim
• type array_name array_size
• The base address is array_name0
• The array_name stores array_name0
• The (array_namei) stores array_namei
• Example
• int aN, p
• p a // p a0
• p ai // pai
• Note if the address of the array a is 1000, then
  p contains
• the address 1000 isizeof(int), which is not
  1000i
• Example

17
Array and Pointer Relation (Cont.)

• Do not attempt to change the address of the array
• Example
• int a100, p
• 100 is not allowed
• a is not allowed
• a is not allowed
• a (int ) 100 is not allowed
• p (int ) 100 is allowed

18
Array and Pointer Relation (cont.)

• Array name stores address of the array (Cont.)
• 2-dim
• type array_name row_sizecolumn_size
• The base address is array_name00
• The array_name stores array_name0
• beginning address of 1st row
• (different from array_name00)
• The (array_namei) stores array_namei

19
Array and Pointer Relation (cont.)

• Example
• int aMN, p
• p a // p a0, where a0 is an array of N
  elements
• // p is the array a0, which contains N
  elements
• P ai // pai
• Class Example

20
Array and Pointer Relation (cont.)

• Array name stores address of the array (Cont.)
• 2-dim
• Question
• int aMN, p
• Can you use pointer p (where pa00)
• to find aij?

21
Array and Pointer Relation (cont.)

• Array name stores address of the array (Cont.)
• 2-dim (Cont.)
• int aMN, p
• pa00
• Answer
• aij(piNj)

22
Array and Pointer Relation (cont.)

• Array name stores address of the array (Cont.)
• 2-dim
• Question
• int aMN, p
• pa
• Can you use pointer p to find aij?

23
Array and Pointer Relation (cont.)

• Idea
• Find address of 1-dim array ai by (ai)
• Find the 1 dim array ai by (ai).
• Then if you use index now,
• the cell aij content is either ((ai))j or
• Find the address of the cell j of ai by
  (ai)j
• The cell aij content is ((ai)j)
• Note when you cast (int ) on a,
• then it specifies the base address a00
• See Example.

24
Array and Pointer Relation (cont.)

• Array name stores address of the array (Cont.)
• 2-dim (Cont.)
• int aMN, p
• pa
• Answer
• aij ((pi))j or
• aij ((pi)j)
• Is the following correct?
• aij (pij)
• Why?

25
Array and Pointer Relation (cont.)

• See Example1
• or
• Example 2
• Summary Example

26
Operators Priority
27
Operators Priority (Cont.)
28
Passing Array Element to Functions

• Pass-by-value pass a copy of only some or an
• array element
• The array element will not be changed after
• function call
• Syntax functionname(an)
• Example

29
Passing Entire Array to Functions

• Pass-by-reference pass the address of an entire
• array
• Only a word size is passed, very efficient
• ANSI C uses pass by reference for entire array
  only
• The array content can be changed however the
• array address is not changed
• ANSI C provides const type to prevent the whole
  array
• is accidentally changed
• Syntax functionname(a)
• Example

30
Passing Array Element to Functions- by Value

• Example 1-dim
• int a100
• a00
• byValue(a0)
• printf(a0d,a0) // a00 is printed
• void byValue(int b)
• b100

31
Passing Array to Functions- by Reference

• Example 1-dim
• int a100, i
• for (i0ilt100i)
• aii
• byReference(a)
• for (i0ilt100i)
• printf(add,i,ai) // a0100,
  a199, are printed
• void byReference(int a) // a is a pointer
• int i
• for (i0ilt100i)
• ai100-i // use pointerindex to specify
  the cell element

32
Use Array

• Read in data to function inputData and
• returns count of number of data array1.c
• syntax count functionname()
• Read in data to function inputData and
• store count of number of data in a variable
  count
• array3.c
• syntax functionname(, count , )
• Example using 1-dim int array (read in
  character),
• data

33
Use Array (Cont.)

• Linear Search array2.c, Data array2_data.txt
• Run (Use a sentinel -1)
• a.out lt array2_data.txt

34
Class Example

• Example. 2D array vendor
• Write a program to use the following input and
  produce
• the output.
• Input(keyboard)
• For each vendor, prompt for and enter the
  following data
• (by storing them in a file and use redirect input
  from the
• file)
• Vendor Invoice Invoice Discount
• Number Number Amount Rate
• 1239 2309.12 0.10
• 9823 670.00 0.09

35
Class Example (Cont.)

• Example. 2D array vendor (Cont.)
• Output(screen)
• For each vendor, print the following report
• Author ACCOUNTS PAYABLE
• Invoice Number XXXX
• Vendor Number XXX
• Invoice Amount 99999.99
• Discount Amount 999.99
• Amount Due 99999.99
• Example

36
Class Example (Cont.)

• Example. 2D array vendor (Cont.)
• Example 1 program for input and output, data
• a.out lt 2d.txt
• Example2 1 row data only
• Example 2D array input output
• Hint to HW 2, test data

37
Class Example (Cont.)

• Example (Cont.)
• The following segment has a run-time error on
  void input_data (double vendor)
• Why?
• define MAXSIZE 50
• void input_data (double vendor)
• int main (void)
• double vendorMAXSIZE4
• input_data (vendor)
• void input_data (double vendor)
• int i, j
• for (i0iltMAXSIZEi)
• for (j0jlt4j)
• scanf(lf, vendorij)

38
Class Example (Cont.)

• Example (Cont.)
• Answer
• The bug is at
• void input_data (double vendor)
• And
• void input_data (double vendor)
• scanf(lf, vendorij)

39
Class Example (Cont.)

• Example (Cont.)
• Answer (Cont.)
• Reason when you pass vendor into
• function input_data
• It is the address of the array vendor0
• There is no such thing as
• vendorij used in scanf statement.

40
Class Example (Cont.)

• Example (Cont.)
• Answer (Cont.)
• Correction
• define MAXSIZE 50
• void input_data (double vendor4)
• int main (void)
• double vendorMAXSIZE4
• input_data (vendor)
• void input_data (double vendor4)
• int i, j
• for (i0iltMAXSIZEi)
• for (j0jlt4j)
• scanf(lf, vendorij)

41
Bubble Sort

• Sort an array of int a0, , acount
• of passes count -1
• Program, Data
• Another way of using array address
• also use const on formal parameters to prevent
• accidental parameter values changed

42
Passing Array to Functions- by Reference (Cont.)

• Example 1-dim
• int a100, i
• void bubble_sort (int a, int n)
• int i,j
• for (i0iltni)
• for (jn-1jgti--j)
• swap (aj-1, aj) // swap aj-1 and aj

43
Passing Array to Functions- by Reference (Cont.)

• Example (Another version of bubble sort)
• 1-dim
• int a100, i
• void bubble_sort (int a, int n)
• int i,j
• for (i0iltn-1i)
• for (j0jltn-i-1j)
• if (ajgtaj1)
• swap (aj, aj1) // swap aj and
  aj1

44
Passing Array to Functions- by Reference (Cont.)

• Example 1-dim (Cont.)
• void swap (int const a, int const b)
• // a and b are pointers
• int tmp
• tmpa
• ab
• btmp

45
Passing 2D Array to Functions- by Reference

• Example 2-dim
• int a10100, i, j
• for (i0ilt10i)
• for (j0jlt100,j)
• aijij
• byRef (a)
• for (i0ilt10i)
• for (j0jlt100,j)
• printf(addd,i,j,aij) //
  a00100, a0199, are printed
• void byRef (int a100)
• int i, j
• for (i0ilt10i)
• for (j0jlt100,j)
• aij100-i-j

46
Use Pointers only instead of indexing

• Example
• Remember you must define spaces for array
• before using pointers. Otherwise, runtime error
• informs you due to no spaces can be assigned
• beyond the pointer that you declared.

47
Strings

• Array of characters end with \0
• String name is a pointer to the base address
• (i.e. the address of the 1st character)
• Initialize by
• char sstring constant
• // initialization specifies the length of s 16
• or
• char s string constant
• or
• char s s,t,, n, t,\0

48
String and Pointer Relation

• Similar to the array and pointer relation
• Examplechar pconstant
• printf(s\n, p2) // nstant printed
• printf(c\n, p5) // a printed
• printf(c\n, (p4)) // t printed

49
String Functions

• Use string function to work on strings
• String functions are defined in string.h
• String functions (not secure- buffer overflow
  attack)
• strings s and t
• strcat (char s, const char t) ss concatenate
  t
• strcpy (char s, const char t)
• scopy of t until \0 of t is reached
• strcmp (char s, const char t)
• lt0 if sltt
• 0 if st
• gt0 if sgtt
• strlen (const char s) return string length
  before \0,
• an unsigned integer or long int (type size_t)
• strerror (int errornum) returns pointer to
  system dependent error message

50
String Functions (Cont.)

• String functions are defined in string.h
• String functions (More secure) strings s and t
• strncat (char s, const char t, size_t n)
• ss concatenate at most n characters of t
• strncpy (char s, const char t, size_t n)
• scopy of at most n characters of t
• strncmp (char s, const char t, size_t n)
• compare up to n characters of s with t
• lt0 if sltt
• 0 if st
• gt0 if sgtt

51
String Functions (Cont.)

• Example
• char strcpy (char s, register const char t)
• register char ps
• while (pt)
• return s

52
Keyboard Input Function - gets

• Form gets (string_variable)
• Read from keyboard into string_variable
• until end of line or end of file
• Example
• char s80 // remember s is address, do not use
  char s
• gets (s)
• // sample dataa 100 -1.23 This is a sample data
• // sa 100 -1.23 This is a sample data

53
A Safer Keyboard Input Function - getline

• A non-standard function
• Form ssize_t getline
• (char lineptr, size_t n, FILE stream)
• Read from keyboard into a string lineptr at most
  n
• characters until end of line or end of file
• Note A standard function called fgets does a
  similar
• job. See sscanf slide (Slide 64) Example link.
• Example

54
Keyboard Output Function - puts

• Form puts (string_variable)
• Print the content of the address of
  string_variable
• Example
• void output_string (const char s)
• // remember s is address
• puts (s)
• // print out sa 100 -1.23 This is a sample
  data

55
Keyboard Input Format Function - scanf

• Form scanf(format, variable_address)
• Format similar to those used in printf, see
  Chapter 3
• If use s format, it reads all until it reaches
  the first white space
• Example
• char c
• int i
• double db
• char s80 // remember s is address
• scanf(cdlfs, c, i, db, s)
• // sample dataa 100 -1.23 This is a sample data
• // ca, i100, db-1.23, sThis

56
Example - scanf

• Program read characters into an arrays
• (lname and fname) and convert them to strings
• Data

57
Keyboard Input Format Function scanf (Cont.)

• Form scanf(pure_format, variable_address)
  (Cont.)
• Scan set
• range
• Reads every character which belongs to range,
• stop at the character (not read in) which does
  not belong to the range
• range
• Reads every character which does not belongs to
  range,
• stop at the character (not read in) which
  belongs to the range
• 10d
• Read in 10 digit number
• 10c
• Read in 10 characters
• c
• Skip any number of characters
• Example Program, Data
• Hint to Quiz 5, Hint Data

58
Keyboard Output Format Function - printf

• Form printf(format, variable_address)
• Format s, see those used in printf, see Chapter
  2

59
Search Strings

• Find the index of the array if found. Return -1
  if not found
• Example read the following data into
• array of strings (max 8 characters)- no
  duplication
• Data
• ABC 123
• PLO4ME
• 123456
• AL-QADA2
• PLO4ME
• 123V456

60
Search Strings (Cont.)

• int i
• char plate9
• char license10009
• int frequency10000
• int count0, index
• // count of different licenses in license
  array
• for (i0gets(plate)i)
• index search(license, plate, count)

61
Search Strings (Cont.)

• if (index -1)
• strcpy(licensecount, plate)
• else

62
Search Strings (Cont.)

• int search (char license9, char plate, int
  count)
• int i
• for (i0 iltcount i)
• if (strcmp (licensei,plate) 0)
• return i // found plate in license array
• return -1 // not found

63
Input and Output String Format Function sscanf
and sprintf

• Form sscanf (instring, format,
  variable_address)
• Similar to scanf, it inputs data from string
• instead of keyboard
• Format similar to those used in printf, see
  Chapter 4
• If use s format, it reads all until it reaches
• the first white space
• Form sprintf (outstring, format, variable)
• Similar to printf, it outputs results to string
• instead of screen

64
Input and Output String Format Function sscanf
and sprintf (Cont.)

• Example
• char c
• int i
• double db
• char s80, outstring80,
• instringa 100 -1.23 This is a sample data
• sscanf(instring,cdlfs, c, i, db, s)
• //Then ca, i100, db-1.23, sThis
• sprintf(outstring,cdlfs, c, i, db, s)
• // print to outstring a 100 -1.23 This
• Example, Data

65
Class Example

• sscanf and gets
• Example
• Hint to HW
• Substring of string from location 2 to 3 is tr

66
String Search Utilities

• Need to include ltstring.hgt
• char strchr(const char s, int c) return
  pointer to
• the found character c in s. Returns NULL
  otherwise
• Example
• strchr(This is a sunny day., u) returns
  s11
• size_t strcspn(const char s, const char range)
  
• returns length of initial s that are not in the
  range
• Example
• strcspn(This is a sunny day., abcd)
  returns 8

67
String Search Utilities (Cont.)

• size_t strspn (const char s, const char range)
  
• returns length of initial s that are in range
• Example
• strspn(This is a sunny day., hist)
• returns 4 since space is not in hist
• char strpbrk (const char s, const char range)
  
• returns pointer of the 1st character of s found
  
• in range
• Example
• strpbrk(This is a sunny day., abcd)
• returns pointer of a

68
String Search Utilities (Cont.)

• char strrchr (const char s, int c)
• returns pointer of the last found character c
  in s
• Example
• strrchr(This is a sunny day., s)
• returns position of s in sunny
• char strstr (const char s, const char range)
• returns pointer of the 1st occurrence of
• range found in s
• Example
• strstr(This is a sunny day., is)
• returns pointer of is in This

69
String Search Utilities (Cont.)

• char strtok (const char s, const char
  delimiter)
• returns pointer of the token in s using the
  delimiter
• and save the position of the next token
• Example
• char sThis is a sunny day.
• char token
• tokenstrtok(s, )
• do
• printf(tokens\n, token)
• tokenstrtok(NULL, ) // must use this to
  get next token
• while (token ! NULL)
• The tokens printed are
• This
• is
• a

70
Other General String Utilities

• void memcpy
• (void destination, const void source, size_t
  n)
• copies n items of source to destination returns
• pointer to the result
• Note this function cannot overwrite the
  destination object
• Example
• char s This is a sunny day.
• char t80
• memcpy(t,s, 21)
• returns pointer of t This is a sunny day.

71
Other General String Utilities (Cont.)

• void memmove
• (void destination, const void source, size_t
  n)
• copies n items of source to destination returns
• pointer to the result
• Note this function can overwrite the destination
  object
• Example
• char s This is a sunny day.
• memmove(s,s10, 3)
• returns suns is a sunny day.

72
Other General String Utilities (Cont.)

• void memcmp
• (const void s, const void t, size_t n)
• compares the 1st n elements of s and t and
  returns 0 if st,
• -1 if sltt, 1 if sgtt
• void memchr (const void s, int c, size_t n)
• returns pointer of the 1st found c in the 1st n
  elements of s
• Example
• char s This is a sunny day.
• memchr (s, u,16)
• returns pointer of unny day.

73
Other General String Utilities (Cont.)

• void memset (void s, int c, size_t n)
• copies c in the 1st n elements of s and
• returns pointer of s
• Example
• char s This is a sunny day.
• memset (s, c,6)
• returns pointer of sssssss a sunny day.

74
Pass Function as Argument

• The name of the function is a pointer,
• passing a function is just passing a pointer

75
Pass Function as Argument (Cont.)

• Example Write a function that calculate
• the average of any double function f(x),
• where x m, , n (Cont.)
• To find the average of sin (5), .., sin (10), we
  use
• average (sin, 5, 10)
• To find the average of sqrt(10),, sqrt(100),
• we use
• average (sqrt, 10, 100)

76
Pass Function as Argument (Cont.)

• Example Write a function that calculate
• the average of any double function f(x),
• where x m, , n
• // The function header can also be
• // double average (double f(double), int m, int
  n)
• double average (double (f)(double), int m, int
  n)
• int i
• double sum0.0
• for (im iltn i)
• sum f(i)
• return sum/(n-m)

77
Pass Function as Argument (Cont.)

• Example Find
• Using the trapezoidal rule

78
Array Out of Bound

• When array index is out of bound, you will
• The undesired memory
• Example
• Output

79
Dynamic Memory Allocation

• Allocate n memory with a type
• Use function defined in stdlib.h
• malloc (nsizeof(type)) more efficient,
• do not initialize all memory
• calloc(n, sizeof(type)) less efficient,
• will initialize all memory
• Always check request memory is successful.
• If successful, return pointer to the n memory
• base_pointer with the generic type void
• Need to free spaces after finish function call by
• free(base_pointer)

80
Dynamic Memory Allocation (Cont.)

• Example
• int a
• if (acalloc(5, sizeof(int)))
• a010 a120 a230 a340 a450
• else
• printf(Error, request memory are not
  available.\n)

81
Array of Pointers (Array of Strings)

• Store array of strings save space
• Example
• Assume that array w has 5 cells, each stores an
• address of strings as follows
• w0100address of string this
• w1200address of string is
• w2300address of string a
• w3400address of string snow
• w4500address of string day

82
Array of Pointers (Cont.)

• Example (Cont.)
• The addresses of w are as follows
• address of w01000
• address of w11004
• address of w21008
• address of w31012
• address of w41016

83
Array of Pointers (Cont.)

• Example (Cont.)
• There are 3 methods to do bubble sort for array
  w.
• Method 1 is the most efficient one.

84
Array of Pointers Method 1 (Cont.)

• Example (Cont.)
• To use bubble sort to sort the array w
• void sort_strings (char w, int n)
• int i, j
• for (i0 iltn i)
• for (ji1 jltnj)
• if (strcmp(wi,wj)gt0)
• swap (wi, wj)

85
Array of Pointers Method 1 (Cont.)

• Example (Cont.)
• To use bubble sort to sort the array w (Cont.)
• for i0 and j1,
• if (strcmp(wi,wj)gt0)
• This means strcmp(100, 200) which is required by
• Function strcmp to compare the contents of
• addresses 100 and 200
• swap (wi, wj)
• This sends in addresses 1000 and 1004 to function
  swap

86
Array of Pointers Method 1 (Cont.)

• Example The swap function is written as (Cont.)
• void swap (char s, char t)
• char tmp
• tmps
• st
• ttmp

87
Array of Pointers Method 1 (Cont.)

• Example (Cont.) You may also write it as
• void swap (char s, char t)
• char tmp
• tmps
• st
• ttmp

88
Array of Pointers Method 1 (Cont.)

• Example The detailed function call (Cont.)
• void swap (char s, char t)
• /The line above has char , which means
• address of address (or address of pointers).
• For the 2 slides before, s1000, t1004 /
• char tmp // tmp contains address
• tmps // s1000100
• st // t1004200
• ttmp
• // The swap here swaps addresses 100 and 200

89
Array of Pointers Method 1 (Cont.)

• Example (Cont.)
• Instead of swapping address of wi and wj,
• we can swap the string directly (a less efficient
  way)
• void sort_strings (char w, int n)
• int i, j
• for (i0 iltn i)
• for (ji1 jltnj)
• if (strcmp(wi,wj)gt0)
• swap (wi, wj)

90
Array of Pointers Method 1 (Cont.)

• Example The swap function is written as (Cont.)
• void swap (char s, char t)
• char tmp
• strcpy(tmp, s)
• strcpy(s, t)
• strcpy(t, tmp)

91
Array of Pointers Method 1 (Cont.)

• Example (Cont.)
• After bubble sort, the array w contains
• w0300address of string a
• w1500address of string day
• w2200address of string is
• w3400address of string snow
• w4100address of string this

92
Array of Pointers Method 1 (Cont.)

• Example (Bubble sort of words)
• Swap (direct addressing), word static array
• Swap (in-direct addressing), word static array
• efficient
• Swap (in-direct addressing),
• word dynamic array efficient
• Program, Data

93
Array of Pointers Method 2

• Another way to write sort_strings is as below
• To use bubble sort to sort the array w
• void sort_strings (char w, int n)
• int i, j
• for (i0 iltn i)
• for (ji1 jltnj)
• if (wigt wj)
• swap (wi, wj)

94
Array of Pointers Method 2 (Cont.)

• Another way to write sort_strings is as below
• For i1 and j2,
• if (wigt wj)
• This means the content of address 100
• (The 1st character at address 100)gt
• that of address 200
• swap (wi, wj)
• This sends addresses 100 and 200 into
• function swap

95
Array of Pointers Method 2 (Cont.)

• Example The swap function is (Cont.)
• void swap (char s, char t)
• char tmp
• tmps
• st
• ttmp

96
Array of Pointers Method 2 (Cont.)

• Example The detailed function call (Cont.)
• void swap (char s, char t)
• / s100, t200 /
• char tmp
• tmps // s100t
• st // t200i
• ttmp
• // This swaps only the 1st char of
• stings this and is

97
Array of Pointers Method 2 (Cont.)

• Example (Cont.)
• After bubble sort, the array w contains
• w0100address of string ahis
• w1200address of string ds
• w2300address of string i
• w3400address of string snow
• w4500address of string tay

98
Array of Pointers Method 3

• Example (Cont.)
• To use bubble sort to sort the array w
• void sort_strings (char w, int n)
• int i, j
• for (i0 iltn i)
• for (ji1 jltnj)
• if (strcmp (wi,wj)gt0)
• swap (wi, wj)

99
Array of Pointers Method 3 (Cont.)

• Example The swap function is written as (Cont.)
• void swap (char s, char t)
• char tmp
• tmps
• st
• ttmp

100
Array of Pointers Method 3 (Cont.)

• Example It can also be written as (Cont.)
• void swap (char s, char t)
• char tmp
• tmps
• st
• ttmp

101
Array of Pointers Method 3 (Cont.)

• Example The detailed function call (Cont.)
• void swap (char s, char t)
• /The line above has char , which means
• address of address (or address of pointers).
• For the 2 slides before, s1000, t1004 /
• char tmp // tmp contains content of string
• tmps // s1000100t
• st // t1004200i
• ttmp
• // The swap here also swaps the 1st char of
• contents of addresses 100 and 200

102
Sort Array of strings- Method 4

• Problem In the previous license example,
• we want to sort array of strings license
• according to the frquencies of each license
• int main (void)
• sort_strings (license, frequency, count)

103
Sort Array of strings- Method 4 (Cont.)

• void sort_strings (char w9, int f, int n)
• int i, j
• for (i0 iltn i)
• for (ji1 jltnj)
• if (filtfj)
• swapString (wi, wj) // swap license
  array
• swapInt(fi, fj) // swap frequency
  array

104
Sort Array of strings- Method 4

• void swapString (char s9, char t9)
• char tmp9""
• strcpy(tmp,s)
• strcpy(s,t)
• strcpy(t,tmp)

105
Sort Array of strings- Method 4

• void swapInt (int a, int b)
• int tmp
• tmpa
• ab
• btmp
• Complete Example Sort Names and Grades by
  Grades,
• Sort Names and Grades by Lastname,
• Data

106
Command Line Arguments

• 3 arguments for main() function
• argc total number of arguments including the
• command
• argv array of pointers
• argv0 contains command
• argv1 contains 1st argument after command
• etc
• env array of environment variables

107
Command Line Arguments (Cont.)

• Example
• // use the following function header
• // int main(int argc, char argv, char env)
• or
• int main (int argc, char argv, char env)
• int i
• for (i0iltargci)
• printf(argvds\n,i, argvi)
• for (i0envi ! NULLi)
• printf(envds\n,i, envi)
• return 0

108
Command Line Arguments (Cont.)

• Class Example Print All Arguments
• Example1
• Example2
• Example3
• Example4 (Hint to Extra HW 1)
• use strtok function

109
Practice 1

• Given int i3, j5,pi,qj,r double x
• Find the values of the following table
• Expression Value
• ________ _____
• p i
• p
• rx
• 7p/q7
• (rj)p r

110
Practice 1 (Answer)

• Given int i3, j5,pi,qj,r double x
• Find the values of the following table
• Expression Value
• ________ _____
• p i 1
• p 3
• rx 1
• 7p/q7 11
• (rj)p r 15

111
Practice 2

• Given char s1"beautiful big sky country"
• s2"how now brown cow"
• Find the values of the following table
• Expression Value
• ------------- ------
• strlen(s1)
• strlen(s28)
• strcmp(s1,s2)
• printf("s",s110)
• strcpy(s110, s28) strcat(s1,"s!")
  printf("s",s1)

112
Practice 2 (Answer)

• Given char s1"beautiful big sky country"
• s2"how now brown cow"
• Find the values of the following table
• Expression Value
• ------------- ------
• strlen(s1) 25
• strlen(s28) 9
• strcmp(s1,s2) lt 0
• printf("s",s110) big sky country
• strcpy(s110, s28) strcat(s1,"s!")printf("s",s
  1)
• beautiful brown cows!

113
Practice 3

• Given int a35
• Is a a0?
• Is a a00?
• Find which ones in the following are not the
• same as aij?
• (aij)
• (a005ij)
• ai j
• ((ai))j
• (((ai))j)

114
Practice 3 (Cont.)

• Answer
• Yes
• No
• ai j
• Example

115
Practice 4

• Use command line to execute a program
• called mycp using
• mycp indata outresult
• If your main function header is
• int main(int argc, char argv )
• What's the value of the following?
• argc
• argv12
• argv22
• (argv)0
• (argv1)0
• argv

116
Practice 4 (Answer)

• Use command line to execute a program
• called mycp using
• mycp indata outresult
• If your main function header is
• int main(int argc, char argv )
• What's the value of the following?
• argc 3
• argv12 d
• argv22 t
• (argv)0 m
• (argv1)0 i
• argv m

117
Practice 4 (Explanation)

• Example (Cont.)
• The command line arguments are in the array
• argv which contains
• argvargv0100address of string mycp
• (argv1)argv1200address of string indata
• (argv2)argv2300address of string outresult

118
Practice 5

• Given char c'A', s"Blue moon!"
• Show what to be printed?
• statement print out
• ________ _______
• printf("c",c)
• printf("2c",c)
• printf("-3c",c)
• printf("s",s)
• printf("3s",s)
• printf(".6s",s)
• printf("-11.8s",s)

119
Practice 5 (Answer)

• Given char c'A', s"Blue moon!"
• Show what to be printed? ( represents a blank)
• statement print out
• ________ _______
• printf("c",c) A
• printf("2c",c) A
• printf("-3c",c) A
• printf("s",s) Blue moon!
• printf("3s",s) Blue moon!
• printf(".6s",s) Blue m
• printf("-11.8s",s) Blue moo

120
References

• Deitel Deitel C How to Program, 4th ed.,
• Chapter 6, 7 8, Prentice Hall