Ada.Text_IO.Look_Ahead

Motivation for Look_Ahead

• Allows viewing an input character but not consuming it


• The character can be viewed again later by the same routine or a different routine
• Example: compiling 123+456 allows + to signify that the end of the number is reached, and the the plus to be recognized later


• Allows the proper input routine to be selected based on the type of character seen by look_ahead
• Example:

                look_ahead(c, EOL);
                if EOL then 
                    skip_line;
                else -- not EOL
                    if is_digit(c) then
                        -- do a numeric get
                    else
                        -- do a character get
                    end if;
                end if;
                

Procedure Look_Ahead

• Declaration: look_Ahead(C: out Character; EOL: out Boolean)


• Parameters
• c: if EOL is false, c returns, but does not consume, the next character in the input stream
• c: if EOL is true, c is NOT set
• EOL: returns true if current input stream is at the end of line, false otherwise


• Warning: For Look_Ahead to work properly your data file should be created on the same OS as your program is compiled under

Procedure Look_Ahead Behavior

• If look_Ahead is called when the input marker is at the end of the line, EOL will return true and C will not be valid


• If look_Ahead is called when the input marker is NOT at the end of the line, EOL will return false and C will contain the next value of input (ie the character that will be returned by the next get of a character), without moving the input marker


• Repeated calls to look_Ahead without intervening calls to get (or other input routines) return the same values

Procedure Look_Ahead Examples

• Example 1 demonstrates how look_ahead processes input characters: lademo.adb and prettified


• Example 2 shows a typical use of look_ahead to select an appropriate input routine: lookaheaddemo.adb and prettified

Other Languages: Java

• java.io.PushbackInputStream(InputStream) - constructs a stream that can be used to push back one byte


• java.io.PushbackInputStream(InputStream, int size) - constructs a stream that can be used to push back size bytes


• void unread(int b) pushes back the byte b which will be input again by the next read. Only one byte can be pushed back at a time.

Other Languages: C++

• C++ has unget and putback

Other Languages: C

• C has ungetc