Which are valid identifiers?

Answer 1

Identifiers can be composed of letters, numbers, _ and $. Identifiers can only begin with a letter, _ or $. The identifer can be of mixed case.

also. read page 51, paragraph titled"User- Defined Identifiers"

in the book ITT gave you. The answer to your homework is clearly stated there.

Answer 2

1.the name of the identifier should not begin with a letter or a special character

2.the only special character that was allowed in the name of an identifier is underscore

3.there should be no spaces in the identifier name

4.the name of identifier should not begin with a keyword

Answer 3

Each programming language has its own conventions regarding the naming of user-defined identifiers (variables and types). In C there are only two rules:

1. All names must be composed from alphanumeric and underscore characters only. Leading digits are not allowed.*
2. A user-defined name must not clash with any reserved name or keyword.**

* Some language may permit other tokens such as the $ symbol in Java.

** Some languages may use contextual keywords which are new keywords added to the language but which may clash with existing identifiers in older code. For instance, C++ added the "final" and "override" keywords in C++11. To avoid breaking older code that used these names, both keywords were defined as being contextual. Their usage in new code is discouraged.

Most other "rules" are conventional or "rules of thumb" rather than mandatory and will vary from language to language:

1. All uppercase identifiers typically denote macros and/or constants.
2. Use descriptive names for important identifiers and single-character names for trivial identifiers such as loop counters. Use (localised) aliases to reduce verbosity with long but frequently-used names.
3. Some languages, such as Pascal, make prevalent use of "CamelType" (capitalising each word of a multi-word identifier). Others, such as C++, prefer underscores to separate each word. C++ also prefers that user-defined types begin with a leading capital to differentiate them from the built-in and standard library types which are all lower-case.
4. Avoid specifying an identifier's type in the name. E.g., an integer should be named "num" rather than "inum". Revealing implementation details should be kept to a minimum as it makes it easier to change those details at a later date. For instance, we may wish to redefine "num" as a real number (float), so "inum" would need to be renamed "fnum" to maintain consistency. Prefixing pointers with "p", null-terminated strings with "sz" and classes with "c" are commonly accepted conventions but it is not necessary to know the underlying type of an identifier in order to use it. The compiler knows the type and that's all that really matters.
5. In C++ and most other object-oriented languages, the private data members of a class are typically prefixed with "m" or "m_" to differentiate the members from their public accessors which typically use the same name without the prefix.

The most important thing about naming your identifiers is that you do so in a consistent style because consistency improves readability and assists maintainers.