Readability:

the ease with which programs can be read and understood

Because ease of maintenance is determined in large part by the readability of programs,
readability became an important measure of the quality of programs and programming languages.

• Overall simplicity
  • A manageable set of features and constructs
  • Minimal feature multiplicity
    Multiplicity: having more than one way to accomplish a particular operation.
    Example of multiple ways in C to increase a value of a variable:  (It becomes much difficult to understand it.)
    COUNT = COUNT + 1 ;
    COUNT += 1 ;
    COUNT++;
    ++COUNT;
  • Minimal operator overloading
    Operator Overload:  A single operator symbol has more than one meaning.
                      For example,  3 + 7     vs.        Vector a + Vector b
                                            10-5      vs. -7
              It is easy to read but it may confuse people.
  • Why does the simplicity of the assembly language make program less readable?
• Orthogonality
  • A relatively small set of primitive constructs can be combined in a relatively small number of ways
  • Every possible combination is legal
  • Compare assembly languages of IBM mainframe computers and VAX series of minicomputers on pages 10-11.  (VAX fits orthogonality.)
  • Orthogonality: independent of context. 
        For example, pointers should be able to point to any type of variable or data type.

    The more orthogonal, the fewer exceptions the language rules require. 
    That is, the fewer exception makes the language easier to learn, read and understand.

• Data types
  • Adequate predefined data types

    If there is a Boolean data type, it will increase the readability.
    For example,

    sum_is_too_big = 1
    sum_is_too_big = true

    The struct data type is much easier to pass through a function than several arrays.

    Ada, Pascal check data type.  C has no strong type checking.  (see Chapter 4)
    Out of range subscripts cause errors.

• Syntax considerations
  • Identifier forms: flexible composition.
    The length of the name of a variable vs. the meaning of a variable
  • Special words and methods of forming compound statements
    Readability is influenced by the forms of a language's special words.
    {  }, begin/end, end if/end loop
    Block style:   begin end in Pascal
                        { } in C++
                        end if, end loop in Ada
  • Form and meaning: self-descriptive constructs, meaningful keywords
    Designing statements so that their appearance at least partially indicates their purpose is an obvious aid to readability.
    static in C:
    If used on the definition of a variable inside a function, it means the variable is created at compile time.
    If used on the definition of a variable that is outside all functions, it means the variable is visible only in the file in which its definition appears.
    Shell commands of UNIX: grep  - can you see the meaning from command?