Visual Prolog: String Handling & Predicates

• Visual Prolog offers standard predicates for string manipulations, divided into basic string-handling and type conversion.
• Strings can also be compared.

Basic String-Handling Predicates

• These predicates form the foundation for string handling in Visual Prolog.
• They divide a string into components/tokens.
• They build strings from specified strings/tokens.
• They verify a string's composition.
• They return a string, token, or list from a given string.
• They verify/return the string's length.
• They create blank strings of a specific length.
• They verify if a string is a valid Visual Prolog name.

frontchar/3

• It concatenates Char and String2 to equal String1.
• Format: frontchar(String1, Char, String2).
• Example goal: frontchar("ahmed", H, R) results in H = 'a' and R = "hmed".
• It takes three arguments: a string, a char (first char of the string), and the rest of the string.
• frontchar splits a string or creates one, and tests characters within.
• It fails if String1 is a zero-length string.
• It is used to change a string to a list of characters.
• Example goal: string_chlist("ABC", Z) binds Z to ['A', 'B', 'C'].

fronttoken/3

• fronttoken has three functions based on the flow pattern used.
• Format: fronttoken(String1, Token, Rest).
• With (i,o,o) flow, it finds the first token of String1, binds it to Token, and the remainder to Rest.
• The (i,i,o), (i,o,i), and (i,i,i) flow variants are tests.
• The (o,i,i) variant constructs a string by concatenating Token and Rest, binding String1 to the result.
• Tokens are grouped as names, numbers or non-space characters based on Visual Prolog Syntax.
• Suited for decomposing a string into lexical tokens.
• It divides a sentence into a list of names.

frontstr/4

• frontstr splits String1 into two parts.
• Format: frontstr(NumberOfChars, String1, StartStr, EndStr).
• StartStr contains the first NumberOfChars of String1, and EndStr contains the rest.
• When called, the first two parameters must be bound, and the last two must be free.

concat/3

• concat concatenates String1 and String2 to equal String3.
• Format: concat(String1, String2, String3).
• At least two parameters must be bound.
• It gives only one solution.
• Example: concat("ali", "hits", W) results in W = "alihits".

str_len/2

• str_len returns or verifies string length, or returns a blank string of a given length.
• Format: str_len(StringArg, Length).
• Example: str_len("ali", L) results in L = 3.
• It binds Length to the length of StringArg or tests if StringArg has the given Length.
• Length is an unsigned integer.
• It can return a string of spaces.
• makebinary is preferable for allocating buffers, especially for binary data.

isname/1

• isname verifies if the argument is a valid name according to Visual Prolog syntax.
• Format: isname(String).
• A name starts with a letter or underscore, followed by letters, digits, and underscores.
• Spaces are ignored.

subchar/3

• subchar returns the character at a given position in a string.
• Format: subchar(String, Position, Char).
• The first character is at position 1.
• It exits with an error if the position is beyond the string's end.

substring/4

• substring returns a part of another string.
• Format: substring(Str_in, Pos, Len, Str_out).
• Str_out is bound to the string starting at Pos with length Len from Str_in.
• It exits with an error if Pos and Len specify a string outside of Str_in.
• It is not an error to ask for 0 bytes at the extreme end of the string.

searchchar/3

• searchchar returns the position of the first occurrence of a character in a string.
• Format: searchchar(String, Char, Position).
• If the character isn't found, it fails and is not re-satisfiable.

searchstring/3

• searchstring returns the position of the first occurrence of a string in another string.
• Format: searchstring(SourceStr, SearchStr, Pos).
• It fails if the search string isn't found or is longer than the source.
• It is not re-satisfiable.

Type Conversion Predicates

• Predicates in this category include: char_int, str_char, str_int, str_real, and upper_lower.
• They facilitate conversions between different data types and strings.

char_int/2

• char_int converts a character to an integer or vice versa.
• Format: char_int(Char, Integer).
• The arguments must correspond when both are bound. If one is bound, the other is converted and bound.

str_char/2

• str_char converts a string containing one character into a character or converts a single character into a string of one character.
• Format: str_char(String, Char).
• In the (i,i) flow, it succeeds if String is bound to the single character string equivalent of Char, otherwise fails.

str_int/2

• str_int converts a string containing an integer into an integer, or converts an integer into its textual representation.
• Format: str_int(String, Integer).
• With the (i,i) flow, it succeeds if Integer is bound to the integer equivalent of the string.

str_real/2

• str_real converts a string containing a real number to a real number, or vice versa.
• Format: str_real(String, Real).
• With the (i,i) flow, it succeeds if Real is bound to the real equivalent of the real number represented by the string.

upper_lower/2

• upper_lower converts a string to all lower/upper case.
• Format: upper_lower(Upper, Lower).
• It succeeds when both arguments, except for the case of letters, are identical.