High-level Programming Language (Pascal)

Basic Operations on Strings

Assignment

• for example,
  

  　Name := 'Chan Tai Man';
  　Class := 'F4A';
  　Address := 'Rm 5, 20 / F, 108 School Road, Hong Kong.';

  • the value being assigned may be a string shorter than the declared length of the variable, the lengths of Name and Address are 12 and 40 characters respectively
  • if the length of the value being assigned is longer than the declared length of the variable, the value is truncated to the size of the variable, the actual value of Class is 'F4'

   

Input/Output

• Inuput
  • use the readln statement to read a value from the keyboard for a string variable
  • input string value does not need to be enclosed in quotation marks
  • if the input string contains more characters than the declared length, any excess characters are ignored
  • for example, readln (Name);
  • use two readln statements to read and assign two values to two different string variables,

    for example,
    
    　readln (Name);
    　readln (Class);

• Output
  • using the write or writeln statements to output the string constant or the value of a string variable
  • for example, writeln ('My name is ', Name);

   

  Accessing individual characters of a string

• for example,
  • Name[2] is used to access the second character of the string variable Name
  • the following is used to output the characters of a string one by one
    
    　for i := 1 to 5 do
    　　writeln (Name[i]);
    　
• string variable of length 1 and a character variable are not compatible (i.e. a string variable of length 1 cannot be assigned to a character variable)

  
  Comparison

• strings can be compared with the following relational operators : <, >, =, <=, >= and <>
• a character is said to be greater than another if its ASCII value is larger,

  for example,
  
  　'G' is greater than 'A'
  
  　since the ASCII value of the letter 'G' is 71 and that of 'A' is 65

• two strings are compared, character by character, using their ASCII values

  for example,
  
  　'GRAPES' > 'APPLE' is true because 'G' follows 'A'
  　'Computers' < 'Computing' is true because 'e' precedes 'i'

• uppercase letters have smaller ASCII values than lowercase letters

for example,

'DAVID' is less than 'David', because 'A' is less than 'a'

• leading and trailing blank spaces are significant in string comparisons

  for example,
  

  'computer' is less than 'computer' because the blank space comes before the letter 'c' in the ASCII table
  'computer' is greater than the string 'computer' because the former one is longer
  
  

Initialization of string variable

• uninitialized string may contain unpredictable value
• Name := '';
  {initialize a string variable Name with a null string, there is no space between the quotation marks}
• Null string (empty string) is a string with zero character
  Name := ' '; {initialize a string variable Name with one space}