Arrays

1
Arrays

• Simple data types use a single memory
• cell to store a variable int x
• A data structure is a composite of
• related data items stored under a same
• name. Array is one of such data structures.
• For example it is easier to write a program that
  processes exam scores for a class by using an
  array because all scores can be stored in one
  area of the memory and can be accessed together.

2
Declaring and Referencing Arrays

• Both name of the array and its number of cells
  should be defined
• For example double x8 instructs the compiler
  to associate eight adjacent memory cells of
  double type with the name of x.
• Array subscript is a value or expression enclosed
  in the brackets after array name, that shows an
  array element. For example x3 means element 3
  of array x.

3
Declaring and Referencing Arrays

• The range of array subscript is from zero to one
  less than the number of memory cells in the
  array.
• array subscript always starts from 0
• x0 x1 x2 x3 x4 x5 x6
  x7

16.0
12.0
28.0
26.0
2.5
12.0
14.0
-54.0
same type
4
Parallel Arrays

• Two or more arrays with the same number of
  elements for storing related information
• int idNUM_STUDENTS
• double gpaNUM_STUDENTS

5503
2.71
id0
gpa0
4556
3.09
id1
gpa1
5
More on Array Declaration

• One or more arrays can be declared in one single
  type declaration
• int factor12, n, pins6
• Array can be initialized in the time of
  declaration
• int prime_1t_5 2,3,5
• It is same as initializing a variable when we
  declare it int sum0

6
Array Subscript

• Array subscript (or index)
• Can be a constant, integer variable, or integer
  expression
• x0 2 xi 5 x2 3 x
  7
• C does NO bound checking
• Result of exceeding bounds is system dependent!
• Sometimes a runtime error is printed - but
  sometimes subscript error may cause the side
  effect
• Only the existing elements in the array should be
  referenced

7

• i 5
• printf(.1f, xi)
• printf(.1f, xi 1)
• printf(.1f, xi i)
• printf(.1f, xi 1)
• printf(.1f, xi 2)
• printf(.1f, x(int)x4)
• printf(.1f, xi)
• printf(.1f, x--i)
• xi 1 xi
• xi 1 xi

8
Using for Loops for Sequential Access

• counter variable starts from 0 to lt size of array
• define SIZE 11
• int squareSIZE, i
• for (i 0 i lt SIZE i)
• squarei i i

9
Storing Input Values into the Array elements

• The variable i is using as control loop variable
  and array subscript
• define MAX_ITEM 11
• int xMAX_ITEM, i
• for (i 0 i lt MAX_ITEM i)
• scanf("lf", xi)
• output
  parameter

10
Printing the Values of Array Elements

• define MAX_ITEM 11
• double xMAX_ITEM
• int i
• for (i 0 i lt MAX_ITEM i)
• printf (The dth element of array is
  lf\n, i, xi )
• input
  parameter

11

• /
• Computes the mean and standard deviation of an
  array of data and
• displays the difference between each value and
  the mean.
• /
• include ltstdio.hgt
• include ltmath.hgt
• define MAX_ITEM 8 / maximum number of items
  in list of data /
• int
• main(void)
• double xMAX_ITEM, / data list /
• mean, / mean (average) of
  the data /
• st_dev, / standard deviation
  of the data /
• sum, / sum of the data /
• sum_sqr / sum of the squares
  of the data /
• int i
• / Gets the data /
• printf("Enter d numbers separated by
  blanks\ngt ", MAX_ITEM)
• for (i 0 i lt MAX_ITEM i)

12

• / Computes the sum and the sum of the squares of
  all data /
• sum 0
• sum_sqr 0
• for (i 0 i lt MAX_ITEM i)
• sum xi
• sum_sqr xi xi
• / Computes and prints the mean and
  standard deviation /
• mean sum / MAX_ITEM
• st_dev sqrt(sum_sqr / MAX_ITEM - mean
  mean)
• printf("The mean is .2f.\n", mean)
• printf("The standard deviation is .2f.\n",
  st_dev)
• / Displays the difference between each
  item and the mean /
• printf("\nTable of differences between data
  values and mean\n")
• printf("Index Item
  Difference\n")
• for (i 0 i lt MAX_ITEM i)
• printf("3d4c9.2f5c9.2f\n", i, ' ',
  xi, ' ',
• xi - mean)
• return (0)

13

• Enter 8 numbers separated by blanks
• gt 16 12 6 8 2.5 12 14 -54.5
• The mean is 2.00.
• The standard deviation is 21.75.
• Table of differences between data values and mean
• Index Item Difference
• 0 16.00 14.00

14
Using Array Elements as Function Arguments

• The formal parameter is a simple variable
• The actual parameter is the array name followed
  by the particular index in
• Example
• function prototype
•  void do_it(double a1, double a2p, double a3p)
• In the function body
• a2p.
• a3p.
• function call
• do_it(x0, x1, x2)

15
Array Arguments

• The argument declaration of the array does not
  indicate the
• size of the array because compiler does not
  need to know that.
• Array name with no subscript is a pointer to the
  initial array element
• / Sets all elements of its array parameter to
  in_value. /
• void fill_array (int list, / output - list of
  n integers /
• int n, / input - number of list
  elements /
• int in_value) / input - initial
  value /
• int i / array subscript and
  loop control /
• for (i 0 i lt n i)
• listi in_value
• function call fill_array(y, 10,
  num)
• fill_array(x, 5, 1)

16
Array Arguments

• int list also also can be used instead of int
  list in a formal parameter list

Calling function data area
function fill_array data area
list
0
1
2
3
4
17
Arrays as Input Arguments

• / Returns the largest of the first n values in
  the array /
• int get_max(const int list, int n)
• const indicates
  that array is input and can not be modified
• int i,
• cur_large / largest value so
  far /
• / First array element is largest so
  far. /
• cur_large list0
• /Compare remaining element to cur_large,
  save the larger/
• for (i 1 i lt n i)
• if (listi gt cur_large)
• cur_large listi
• return (cur_large)

18
Returning an Array Result

• In C it is not legal for a function return type
  to be an array,
• therefore, array should be used as output
  parameter.
• For example
• void add_arrays(const double ar1, const double
  ar2,
• double arsum, int n)
• int i
• / Adds corresponding elements of ar1 and
  ar2 /
• for (i 0 i lt n i)
• arsumi ar1i ar2i
• Call this function add_arrays(x, y, x_plus_y,
  5)

19
Partially Filled Array

• function
• void fill( int max, double sentinel, double x,
  int size)
• ..
• status scanf( f , data)
• while (status 1 data ! sentinel i
  lt max)
• xi data
• i
• status scanf( f , data)
• .
• size i
• ..
• main program
• double y 1000
• int used
• ..
• fill(1000,-1, y, used)

20
Searching in the Array

• Linear search
• int search (const in arr, int target, int n)
• int i, found0, where
• i0
• while ( !found iltn)
• if (arri target)
• found i
• else
• i
• if (found)
• where i
• else where -1
• return (where)

21
Strings

• printf (Average .2f, avg)
• define ERROR error
• Strings are arrays of char,
• For example
• char str40 value
• Terminated by \0 at str5
• Null character marks the end of string
• Array size must be more than string length plus
  \0
• All C string handling functions ignore whatever
  is stored in the cells following the null
  character

22
String

• Individual elements are char
• char course6 math"

m
a
t
h
\0
char letter1 A
A
\n
23
Strings

• Array of Strings
• char month1210 January, February,
  March, April, May, June, July,
  August, September, October, November,
  December
• Input/output
• printf(Topic s\n, string_var)
• Right justification printf(10s\n, president)
  president
• Left justification printf(-10s\n,president)
  president
• scanf(sdsd, dept, course_num,
• days, time)

24

• include ltstdio.hgt
• define STRING_LEN 10
• int
• main(void)
• char deptSTRING_LEN
• int course_num
• char daysSTRING_LEN
• int time
• printf("Enter department code, course
  number, days and ")
• printf("time like this\ngt COSC 2060 MWF
  1410\ngt ")
• scanf("sdsd", dept, course_num, days,
  time)
• printf("s d meets s at d\n", dept,
  course_num, days, time)
• return (0)
• Enter department code, course number, days and
  time like this
• gt COSC 2060 MWF 1410
• gt MATH 1270 TR 1800 (scanf skips white space
  characters such blank
• MATH 1270 meets TR at 1800
  newline and tab)

25

• Here namesi is an array but agesi is int
• define NUM_NAMES 30
• define NAME_LEN 25
• ..
• char names NUM_NAMESNAME_LEN
• int ages NUM_NAMES output parameters
  
• for (i0 iltNUM_NAMES i)
• scanf(sd, namesi, agesi
• printf(-35s d\n, namesi, agesi)
• 
  input parameters

26
String assignment

• char str20
• str test it is wrong, we can only use in
• string initialization
• For doing that
• include ltstring.hgt
• strcpy(str,test)
• after that
• strlen(str) returns 4

27

• strcpy(destination, source)
• strcpy(one_str, Test string 1)
• Overwrites characters in destination with
  characters from source
• Stops at \0 in source
• strlen(string)
• len strlen(one_string)
• Returns the number of characters in string
• Does not include \0

28
String Comparison

• a lt b is true because the numeric code of a
  is less than
• numeric
  code of character b
• But for strings it is not that easy, For example
• thrill lt throw is true because first
  non-matching characters
• at position three are
  compared
• joy lt joyous is true because the first one
  is shorter, and all
• 3 initial characters of two strings
  match with each others
• strcmp(string1, string2)
• Compares one string to another
• Returns
• lt 0 if string1 lt string2
• 0 if string1 string2
• gt 0 if string1 gt string2

29
Sentinel-controlled loop for string input

• printf("Enter list of words on as many lines as
  you like.\n")
• printf("Separate words by at least one
  blank.\n")
• printf("When done, enter s to quit.\n", SENT)
• for (scanf("s", word)
• strcmp(word, SENT) ! 0
• scanf("s", word))
• / process word /

30
Case Study

• Problem A genetic engineer is developing a
  program to identify palindromes of a certain
  length in strings representing the nucleotide
  sequences of a portion of a DNA molecule.
• A-T-C-G-C-A-T-G-C-G-T-A-G
• T-A-G-C-G-T-A-C-G-C-A-T-C

31
Case Study

• Analysis
• Constant STRANDSIZ 100
• Inputs char strand1STRANDSIZ,
• char strand2STRANDSIZ
• int palin_len
• Outputs index and value of each palindromic
  sequence of length palin_len

32
Case Study

• Design
• Algorithm
• 1. Get input data complementary strands and
  palindrome length
• 2. For each starting subscript of a substring of
  the desired length
• 2.1 if substring from strand1 matches the
  reverse of corresponding substring of strand2
• 2.1.1 Print the position and two substrings

33
Dynamic Allocation of Arrays

• In addition to stack heap is a second allocation
  storage area for specific requests while program
  is running. For example a pointer variable in
  stack that points to
• double nums_list the start of dynamic array
  in heap
• determines array
  size in runtime
• num_lis (double ) calloc(lsize, sizeof
  (double))
• creates array elements that are initialized to
  zero
• finds the
  number of required bytes

34

• include ltstdio.hgt
• include ltstdlib.hgt / gives access to calloc /
• int
• main(void)
• char string1
• int array_of_nums
• int str_siz, num_nums, i
• printf("Enter string length and stringgt ")
• scanf("d", str_siz)
• string1 (char )calloc(str_siz, sizeof
  (char))
• scanf("s", string1)
• printf("\nHow many numbers?gt ")
• scanf("d", num_nums)
• array_of_nums (int )calloc(num_nums,
  sizeof (int))
• array_of_nums0 5
• for (i 1 i lt num_nums i)
• array_of_numsi array_of_numsi -
  1 i
• . . .

35
Common Programming Errors

• Subscript range error with arrays
• Applying to the array name when array is used
  as an argument of a function is wrong even if
  array is output parameter but for using array
  elements as output parameters is required
• Note that int z as formal parameter can be used
  both for array or an output int parameters
• Overflow with strings in strcpy and scanf
• Not considering the null character in string size
• Intention to use dynamic array but forgetting
  calloc function