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Identifier Names:

The names of variables, functions, labels, and various other user-defined items are called identifiers.

The length of these identifiers can vary from one to several characters.

The first character must be a letter or an underscore, and subsequent characters must be either letters, digits or underscores.

correct and incorrect identifier names

 

In C, identifiers may be of any length. However, not all characters will necessarily be significant.

C defines two kinds of identifiers: external and internal.

An external identifier will be involved in an external link process. These identifiers, called external names, include function names and global variable names that are shared between source files. If the identifier is not used in an external link process, then it is internal. This type of identifier is called an internal name and includes the names of local variables. For example, in C89, at least the first 6 characters of an external identifier and at least the first 31 characters of an internal identifier will be significant. C99 has increased these values.

In C99, an external identifier has at least 31 significant characters, and an internal identifier has at least 63 significant characters. As a point of interest, in C++, at least the first 1,024 characters of an identifier are significant. These differences may be important if you are converting a program from C89 to C99, or from C to C++.

In an identifier, upper- and lowercase are treated as distinct. Hence, count, Count, and COUNT are three separate identifiers.

An identifier cannot be the same as a C keyword and should not have the same name as functions that are in the C library.

Variables:

A variable is a named location in memory that is used to hold a value that can be modified by the program.

All variables must be declared before they can be used.

The general form of a declaration is

Here, type must be a valid data type plus any modifiers, and variable_list may consist of one or more identifier

names separated by commas. Here are some declarations:

In C the name of a variable has nothing to do with its type

 

Types of variables:

Local variables:  Variables that are declared inside a function.

Variables as formal parameters: A function`s parameters are known as variables that will accept the values of the arguments.

Global variables: Global variables are known throughout the program and may be used by any piece of code. Also, they will hold their value throughout the program`s execution. You create global variables by declaring them outside of any function. Any expression may access them, regardless of what block of code that expression is in.

C89 five foundational data types:

 

These types form the basis for several other types. The size and range of these data types may vary among processor types and compilers. However, in all cases an object of type char is 1 byte. The size of an int is usually the same as the word length of the execution environment of the program.

 

For most 16-bit environments, such as DOS or Windows 3.1, an int is 16 bits. For most 32-bit environments, such as Windows 95/98/NT/2000, an int is 32 bits. However, you cannot make assumptions about the size of an integer if you want your programs to be portable to the widest range of environments. It is important to understand that C stipulates only the minimal range of each data type, not its size in bytes.

 

The use of signed on integers is allowed, but it is redundant because the default integer declaration assumes a signed number. The most important use of signed is to modify char in implementations in which char is unsigned by default.

 

Signed and unsigned integers differ in the way that the high-order bit of the integer is interpreted. If you specify a signed integer, the compiler generates code that assumes the high-order bit of an integer is to be used as a sign flag. If the sign flag is 0, the number is positive; if it is 1, the number is negative.

 

If the high-order bit were set to 1, the number would be interpreted as –1. However, if you declare this to be an unsigned int, the number becomes 65,535 when the high-order bit is set to 1.

 

When a type modifier is used by itself (that is, when it does not precede a basic type), then int is assumed. Thus, the following sets of type specifiers are equivalent:

 

 

Although the int is implied, it is common practice today to specify the int anyway.

 

 

 

Here simple code is given to print data type`s size and range.  Right shift is used for calculating powers of 2 to get range and `sizeof` operator is used for calculation of size. 

The classical method of learning English is to first learn the alphabets used in the language, then learn to combine these alphabets to form words, which in turn are combined to form sentences and sentences are combined to form paragraphs.

Learning C is similar and easier. Instead of straight-away learning how to write programs, we must first know what alphabets, numbers and special symbols are used in C, then how using them constants, variables and keywords are constructed, and finally how are these combined to form an instruction. A group of instructions would be combined later on to form a program.

Steps in learning English language:

Steps in learning C:

A character denotes any alphabet, digit or special symbol used to represent information. below table shows the valid alphabets, numbers and special symbols allowed in C.

 

 

History of C

C was invented and first implemented by Dennis Ritchie on a DEC PDP-11 that used the UNIX operating system.

C is the result of a development process that started with an older language called BCPL.

BCPL was developed by Martin Richards, and it influenced a language called B, which was invented by Ken Thompson. B led to the development of C in the 1970s.

OR

C is a programming language developed at AT & T’s Bell Laboratories of USA in 1972. It was designed and written by a man named Dennis Ritchie. In the late seventies C began to replace the more familiar languages of that time like PL/I, ALGOL, etc. No one pushed C. It wasn’t made the ‘official’ Bell Labs language. Thus, without any advertisement C’s reputation spread and its pool of users grew. Ritchie seems to have been rather surprised that so many programmers preferred C to older languages like FORTRAN or PL/I, or the newer ones like Pascal and APL.

C Is a Middle-Level Language

C is thought of as a middle-level language because it combines the best elements of high-level languages with the control and flexibility of assembly language.

 

Initially, C was used for systems programming. A systems program forms a portion of the operating system of the computer or its support utilities, such as editors, compilers, linkers, and the like. As C grew in popularity, many programmers began to use it to program all tasks because of its portability and efficiency- and because they liked it! At the time of its creation, C was a much longed-for, dramatic improvement in programming languages. In the years that have since elapsed, C has proven that it is up to any task.

Difference between Compiler and Interpreter

In its simplest form, an interpreter reads the source code of your program one line at a time, performing the specific instructions contained in that line. This is the way earlier versions of BASIC worked. In languages such as Java, a program`s source code is first converted into an intermediary form that is then interpreted. In either case, a run-time interpreter is still required to be present to execute the program.

A compiler reads the entire program and converts it into object code, which is a translation of the program`s source code into a form that the computer can execute directly. Object code is also referred to as binary code or machine code. Once the program is compiled, a line of source code is no longer meaningful in the execution of your program.

Keywords in C

Keywords are predefined, reserved words used in programming that have special meanings to the compiler. Keywords are part of the syntax and they cannot be used as an identifier. Below is the list of c keywords:

 

 

Linker

A linker is a computer program that takes one or more object files generated by a compiler and combines them into one, executable program.

 

The Form of a C Program

The N Queens Problem is the problem of placing N chess queens on an N×N chessboard so that no two queens threaten each other. Thus, a solution requires that no two queens share the same row, column, or diagonal. The eight queens puzzle is an example of the N queens problem of placing N non-attacking queens on an N×N chessboard, for which solutions exist for all natural numbers N with the exception of N=2 and N=3. In the code, backtracking method is used to solve the problem. A queen is placed in a column that is known not to cause conflict. If a column is not found the program returns to the last good state and then tries a different column by incrementing or decrementing the column index.

A random password generator is software program or hardware device that takes input from a random or pseudo-random number generator and automatically generates a password. Random passwords can be generated manually, using simple sources of randomness such as dice or coins, or they can be generated using a computer software program. Here we have a simple program by using it you can generate a strong password of any predefined length. As you increase the length of password, it becomes stronger.

Here we demonstrate how can we show the star (*) for an input password. It may be used when we are developing an application that has a login form in which we use a user name and password as logging information. For security purpose it is needed to show (*) instead showing the plain text for the password. The source code is given below to show how we can do it using c.

String manipulation is the action of the fundamental operations on strings, including their creation, concatenation, the extraction of string segments, string matching, their comparison, discovering their length, replacing substrings by other strings. Here we demonstrate some example of string manipulation.