ICS 101: Computer Programming in FORTRAN

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ICS 101 Computer Programming in FORTRAN

• Lecture 8 One-Dimensional Arrays

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Objectives

• Definition declaration
• Initialization
• Array processing
• Arrays subprograms

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Definition

• In many programming problems, several operations
  need to be applied on a collection of data. For
  examples, scores of 50 students.
• Having a variable for each single data is not
  efficient.
• How can we
• Read the scores.
• Find the average scores.
• Display the scores higher than the average.
• Doing these operations on the variables is
  cumbersome for the programmer.

REAL S1, S2, ..., S50
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Definition

• FORTRAN provides a data structure (called array)
  that can hold these data together under one
  variable.
• An array is a collection of related data of the
  same type.
• An array consists of elements (50 in this
  example) that can be processed as a normal
  variable.
• Whenever an array is declared, the computer will
  allocate consecutive memory locations of the
  array size.

SCORES
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Declaration

• There are two ways to declare an array in
  FORTRAN.
• Where
• TYPE the data type of the elements. INTEGER,
  REAL, LOGICALetc.
• name is the name of the array (rules of naming
  variables apply here)
• no of elements is the size of the array. It can
  be constant or expression. This value should not
  be REAL.
• Example
• To declare more than one array

TYPE name (no of elements)
INTEGER SCORES(50)
INTEGER LIST1(10), LIST2(20), LIST3(30)
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Declaration

• The other way is as follows
• Example
• If TYPE is omitted, then the array type follow
  the implicit declaration.
• What is the type of ALIST?

DIMENSION name (no of elements) TYPE name
Optional
DIMENSION SCORES(50) INTEGER SCORES
DIMENSION ALIST(100), KLIST(200),
XLIST(300) INTEGER XLIST REAL KLIST
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Declaration

• Array elements are indexed from 1 up to the size
  of the array.
• Example
• FORTRAN allows elements to be indexed from any
  value. for that, the declaration should indicate
  from where the index starts and ends.
• Where m is the index of the first element and n
  indicates the index of the last element.
• The programmer must make sure that m lt n.

SCORES(1) the 1st element SCORES(2) the 2nd
element ... SCORES(50) the 50th element
TYPE name (m n)
DIMENSION name (m n) TYPE name
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Declaration

• The number of elements of arrays declared as
  before is
• Examples

n m 1
REAL YEAR (19831994)
DIMENSION TEMP(-2020)
TEMP(-20) the 1st element TEMP(-19) the 2nd
element ... TEMP(20) the 41st element
YEAR(1983) the 1st element YEAR(1984) the 2nd
element ... YEAR(1994) the 12th element
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Array Initialization

• Initialization of an array is to assign values to
  the array elements.
• A Do loop and READ statement can be used for
  initialization. The choice of the method depends
  on the situation.
• To initialize all the elements to a certain
  value, a Do loop is used.

REAL LIST(3) DO 5 K 1, 3 LIST(K)
0.0 5 CONTINUE
INTEGER POWER2(010) DO 7 K 0, 10
POWER2(K) 2 K 7 CONTINUE
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Array Initialization

• If the values of the array elements should be
  read from the user, then READ statement is used
  in two different ways.
• First method (array name)
• Second method (implied loop)
• Example
• For arrays declared as a range (m, n), the
  implied loop should start from m to n.
• Example

READ, name
READ, (name (K), K 1, array_size)
READ, (SCORES(K), K 1, 50)
READ, (TEMP(K), K -20, 20)
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Array Initialization

• In the previous two methods, only one READ
  statement is executed. This means that values are
  read from the beginning of the input line. If the
  line is not enough for the array elements,
  reading will continue the next line.
• All these inputs will initialize the array in the
  same way
• The difference between the two methods is that
  implied loop can be used to initialize part of
  the array (e.g. first 10 elements).

INTEGER LIST(4) READ, LIST
10 20 30 40
10 20 30 40
10 20 30 40
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Array Initialization

• Using a Do loop will have a different effect.
• This is because we have 4 READ statements and
  each one starts reading from a new line.

DO 7 I 1, 4 READ, LIST(I) 7 CONTINUE
10 20 30 40
10 20 30 40 50 60 70 80
10 20 30 40 50 60
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Array Initialization

• To summarize

Array Initialization
To a known value Use a Do loop
Values from the user
Implied loop
Do loop
READ, name
All elements initialized
All or part of elements initialized.
All or part of elements initialized. Input must
be in each line
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Array Processing

• Common processing on arrays include displaying
  the elements, searching for specific elements and
  doing operations on array elements.
• To display array elements, there are two methods.
• First method (array name)
• Second method (implied loop)
• If the array declared as a range, the implied
  loop should start from m and ends at n.
• Example

PRINT, name
PRINT, (name (i), i 1, array_size)
PRINT, (TEMP(i), i -20, 20)
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Array Processing

• A Do loop can also be used, but each element will
  appear in a single line.
• The output will be

DO 12 I 1, 4 PRINT, LIST(I) 12
CONTINUE
10 50 70 80
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Array Processing

• Whenever dealing with arrays, follow these rules
• Elements indices (subscripts) should not go
  beyond array boundaries.
• Indices must always appear as integer
  expressions.
• The value assigned to an array element must match
  in type with array type.
• Array must be declared before its elements are
  initialized.
• When declaring an array, its size must be integer
  constant except in subprograms.
• The first three errors may not be detected by the
  compiler. It is the programmer responsibility to
  make sure that these rules are satisfied.

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Array Processing

• Consider

INTEGER GRADE(25), LIST(3) LOGICAL
FLAG(20) CHARACTER NAMES(5) 3
Statements Comments
GRADE(26) 0.0 26 is out of range
LIST(2.0) X 3 2.0 is not an integer
NAMES(4) 100 NAMES(4) is a character
FLAG(3) Ali FLAG(3) is logical
READ, (GRADE(K), K 1, 100) GRADE has only 25 elements
NEW(2) 3 NEW is not declared as an array
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Array Processing

• The following program reads a list of numbers and
  then search for the maximum integer in this list.
• Assume the input is

INTEGER LIST(7), MAX READ, LIST MAX
LIST(1) DO 1 K 2, 7 IF(LIST(K) .GT.
MAX) THEN MAX LIST(K) ENDIF 1
CONTINUE PRINT, MAXIMUM IS , MAX END
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2
33
3
88
4
97
5
97
6
97
7
33 20 2 88 97 5 71
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Array Subprograms

• Arrays can also be used in subprograms and rules
  discussed before apply here.
• One exception occur where the array is passed as
  an argument to the subprogram. In this case, you
  have to pass the array name and its size as
  arguments to the subprogram.
• Example

INTEGER LIST(10), SUM, K READ, (LIST(K), K
1, 4) PRINT, SUM(LIST, 4) END INTEGER
FUNCTION SUM(MARK, N) INTEGER N, MARK(N)
SUM 0 DO 13 J 1, N 13 SUM SUM
MARK(J) RETURN END
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Array Subprograms

• Subprograms can also modify an array passed from
  the caller program.

SUBROUTINE ARRABS(A, N) INTEGER K, N
REAL A(N) DO 44 K 1, N 44 A(K)
ABS(A(K)) RETURN END INTEGER J, N
REAL A(100) READ, N, (A(J), J 1, N)
PRINT, THE ORIGINAL ARRAY, (A(J), J 1, N)
CALL ARRABS(A, N) PRINT, THE NEW ARRAY ,
(A(J), J 1, N) END
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Exercise

• Write a program to read an array X and check if
  all the elements of the array are in increasing
  order. Print a proper message.
• Write a subroutine REVRSE to reverse an array
  with 5 elements. Write a main program to test the
  subroutine.
• The sum and difference of two vectors a (a1,
  a2, , an) and b (b1, b2, , bn) is defined as
  follows
• Write a program that reads the two vectors and
  calculates their sum and difference.

5 12 23 45 65 its reverse is 65 45
23 12 5
a b (a1 b1, a2 b2, , an bn) a b
(a1 b1, a2 b2, , an bn )