IO

• In my opinion this is the last review item.
• How do you picture an input stream?
  • Assume the file contains

    Hello world!
    How are you?

  • We can represent it this way:

  •  H e l l o _ w o r l d ! \n
    ↑
     H o w _ a r e _ y o u ? \n 

  • Spaces are represented as _
  • The file pointer is ↑
  • The spaces in the example don't exist, they are just to allow space for the file pointer.
• There are three or four operations we should discuss.
  • Stream extraction operator (>>)
    • Reference
    • This operator has a target
    • cin >> target
    • The type of the target determines how the operator works.
    • In general

    • skip all whitespace until a non-whitespace character is encountered.
      while the next character is part of a valid target
          extract the character and add it to the new input value.
      
      If valid data was read
          set the target to the new input value
      else
          set the stream in a state of error 

    • What will happen

    • int i;
      cin >> i
      
      stream
      ------------
       1 2 3 4 _ ...  
       

    • int i;
      cin >> i
      
      stream
      ------------
       1 2 . 3 4 _ ... 
       

    • int i;
      cin >> i
      
      stream
      ------------
       1 2 + 3 4 _ ...  
       

    • int i;
      cin >> i
      
      stream
      ------------
       + 1 2 3 4 _ ...
       

    • int i;
      cin >> i
      
      stream
      ------------
       s 1 2 3 4 _ ... 
       

    • How would this be different for characters?
    • How would this be different for strings?
    • How would this be different for floats?
  • getline
    • Reference
    • Takes
      • a stream
      • a string
      • possibly a delimiter
    • getline(cin, line);
    • getline(inFile, itemName, '|');
    • By default the delimiter is the end of line marker.
    • reads all data, including white space, until the delimiter is encountered.
    • It consumes, but does not include the delimiter if it is found.
    • It stops on end of file.
    • The contents of the string are replaced.
    • Look at getLineLoop.cpp
  • ignore
    • reference
      takes
      • A positive integer size (n)
      • And an optional character delimiter
      • cin.ignore(100,'\n');
      • reads in and discards characters until
        • n characters have been read
        • the delimiter is encountered.
        • the end of file is reached.
      • if the end of file is reached, before n or delimiter it is an error.
      • If the delimiter is reached, it is discarded, the stream pointer points to the character immediately following the delimiter
    • get
      • char c = cin.get()
      • reference
      • There are several versions.
      • The no parameter version extracts the next character from the stream.
      • Whatever that character is.
      • no whitespace is skipped.
  • In all cases
    • When actions on streams are successful, the stream will "test" true.
    • Regardless of the fail condition, when actions on streams are not successful, the stream will "test" false.
    • Some loop for simple input.

      
      // good
      input operation
      while (stream)  {
          process
          input operation
      }
      
      // good
      while (input operation that returns a stream) {
           process
      }
      
      // flaky  you are relying on no \n at the end of the last line
      while (not stream.eof) {
           input operation
           process
      }
      
      // bad why would you do this
      while (stream) {
          input operation
          if (stream)
             process
      }
         
      
      // wrong wrong wrong wrong
      while (stream) {
           input operation
           process
      } 

    • For more complex input, you must understand why these are good and bad.