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String Handling

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In this section, we will explore how to handle strings in Fortran. Strings are sequences of characters and are essential for various programming tasks, such as reading user input, displaying messages, and manipulating text data. We will cover the following topics:

  1. Declaring and Initializing Strings
  2. String Operations
  3. String Intrinsic Functions
  4. Practical Examples
  5. Exercises

  1. Declaring and Initializing Strings

In Fortran, strings are declared using the CHARACTER data type. You can specify the length of the string using the LEN attribute.

Syntax:

CHARACTER(LEN=length) :: variable_name

Example:

PROGRAM StringExample
  IMPLICIT NONE
  CHARACTER(LEN=20) :: greeting
  greeting = 'Hello, Fortran!'
  PRINT *, greeting
END PROGRAM StringExample

Explanation:

  • CHARACTER(LEN=20) :: greeting declares a string variable greeting with a maximum length of 20 characters.
  • greeting = 'Hello, Fortran!' initializes the string with the value 'Hello, Fortran!'.
  • PRINT *, greeting prints the string to the console.

  1. String Operations

Concatenation:

You can concatenate strings using the // operator.

Example:

PROGRAM ConcatenateStrings
  IMPLICIT NONE
  CHARACTER(LEN=10) :: firstName, lastName
  CHARACTER(LEN=21) :: fullName
  firstName = 'John'
  lastName = 'Doe'
  fullName = firstName // ' ' // lastName
  PRINT *, fullName
END PROGRAM ConcatenateStrings

Explanation:

  • fullName = firstName // ' ' // lastName concatenates firstName, a space, and lastName to form fullName.

Substring:

You can extract a substring using the substring notation string(start:end).

Example:

PROGRAM SubstringExample
  IMPLICIT NONE
  CHARACTER(LEN=20) :: sentence
  CHARACTER(LEN=5) :: word
  sentence = 'Fortran Programming'
  word = sentence(1:5)
  PRINT *, word
END PROGRAM SubstringExample

Explanation:

  • word = sentence(1:5) extracts the substring from position 1 to 5, resulting in 'Fortr'.

  1. String Intrinsic Functions

Fortran provides several intrinsic functions for string manipulation. Here are some commonly used ones:

Function Description Example Usage
LEN(string) Returns the length of the string LEN('Hello') returns 5
TRIM(string) Removes trailing spaces from the string TRIM('Hello ') returns 'Hello'
ADJUSTL(string) Left-justifies the string ADJUSTL(' Hello') returns 'Hello'
ADJUSTR(string) Right-justifies the string ADJUSTR('Hello ') returns ' Hello'
INDEX(string, substring) Returns the position of the substring in the string INDEX('Fortran', 'tran') returns 4

Example:

PROGRAM StringFunctions
  IMPLICIT NONE
  CHARACTER(LEN=20) :: text
  INTEGER :: position
  text = '   Fortran   '
  PRINT *, 'Length:', LEN(text)
  PRINT *, 'Trimmed:', TRIM(text)
  PRINT *, 'Left-justified:', ADJUSTL(text)
  PRINT *, 'Right-justified:', ADJUSTR(text)
  position = INDEX(text, 'For')
  PRINT *, 'Position of "For":', position
END PROGRAM StringFunctions

Explanation:

  • LEN(text) returns the length of text.
  • TRIM(text) removes trailing spaces.
  • ADJUSTL(text) left-justifies the string.
  • ADJUSTR(text) right-justifies the string.
  • INDEX(text, 'For') returns the position of the substring 'For' in text.

  1. Practical Examples

Example 1: Reversing a String

PROGRAM ReverseString
  IMPLICIT NONE
  CHARACTER(LEN=20) :: original, reversed
  INTEGER :: i, len
  original = 'Fortran'
  len = LEN_TRIM(original)
  DO i = 1, len
    reversed(i:i) = original(len-i+1:len-i+1)
  END DO
  PRINT *, 'Original:', TRIM(original)
  PRINT *, 'Reversed:', TRIM(reversed)
END PROGRAM ReverseString

Explanation:

  • LEN_TRIM(original) returns the length of original without trailing spaces.
  • The loop reverses the string by assigning characters from the end of original to the beginning of reversed.

Example 2: Counting Vowels in a String

PROGRAM CountVowels
  IMPLICIT NONE
  CHARACTER(LEN=100) :: text
  INTEGER :: i, len, count
  CHARACTER(LEN=1) :: ch
  text = 'Fortran is a powerful language.'
  len = LEN_TRIM(text)
  count = 0
  DO i = 1, len
    ch = text(i:i)
    IF (ch == 'A' .OR. ch == 'E' .OR. ch == 'I' .OR. ch == 'O' .OR. ch == 'U' .OR. &
        ch == 'a' .OR. ch == 'e' .OR. ch == 'i' .OR. ch == 'o' .OR. ch == 'u') THEN
      count = count + 1
    END IF
  END DO
  PRINT *, 'Number of vowels:', count
END PROGRAM CountVowels

Explanation:

  • The loop iterates through each character in text.
  • The IF statement checks if the character is a vowel and increments the count if true.

  1. Exercises

Exercise 1: Palindrome Checker

Write a program that checks if a given string is a palindrome (reads the same forwards and backwards).

Solution:

PROGRAM PalindromeChecker
  IMPLICIT NONE
  CHARACTER(LEN=100) :: text, reversed
  INTEGER :: i, len
  LOGICAL :: isPalindrome
  text = 'madam'
  len = LEN_TRIM(text)
  reversed = ''
  DO i = 1, len
    reversed(i:i) = text(len-i+1:len-i+1)
  END DO
  isPalindrome = (TRIM(text) == TRIM(reversed))
  IF (isPalindrome) THEN
    PRINT *, 'The string is a palindrome.'
  ELSE
    PRINT *, 'The string is not a palindrome.'
  END IF
END PROGRAM PalindromeChecker

Exercise 2: String Length without Using LEN

Write a program to find the length of a string without using the LEN function.

Solution:

PROGRAM StringLength
  IMPLICIT NONE
  CHARACTER(LEN=100) :: text
  INTEGER :: i, length
  text = 'Fortran'
  length = 0
  DO i = 1, 100
    IF (text(i:i) == ' ') EXIT
    length = length + 1
  END DO
  PRINT *, 'Length of the string:', length
END PROGRAM StringLength

Conclusion

In this section, we covered the basics of string handling in Fortran, including declaring and initializing strings, performing string operations, and using intrinsic functions. We also provided practical examples and exercises to reinforce the concepts. Understanding string manipulation is crucial for effective programming in Fortran, and these skills will be useful in various applications. In the next section, we will delve into array handling, which is another fundamental aspect of Fortran programming.

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