Constants Variables and Keywords in C

WHY THESE THREE IDEAS COME FIRST

In this lecture, we shall learn constants variables and keywords in C. These are three very small but very important ideas of C programming . A student should not hurry to write a full C program before understanding these three ideas. Given that C is a programming language, it uses some fixed values, some names for memory locations, and some reserved words. A constant is a fixed value. A variable is a named place where a value can be stored. A keyword is a reserved word that already has a special meaning in C. So, constants, variables and keywords in C are the first building blocks of the language.

SEE THE THREE BUILDING BLOCKS

Building Block Map connects the three beginner ideas of C with their main meanings. Click Constant, Variable, Keyword, or Show All to focus on one part of the map. Notice that a constant is a fixed value, a variable is a named memory place, and a keyword is a reserved word. This matters because a student can read simple C statements more calmly after separating values, names, and reserved words in the mind.

BUILDING BLOCK MAP

Dynamic Sandbox

Initializing Workspace

DEFINITION

CONSTANT IN C

3

CHECK THE TYPE OF A CONSTANT

Constant Type Checker helps you test whether a written value looks like an integer constant, real constant, or character constant at the beginner level. Type a value such as 10, 8.5, 4.1e8, or 'A', then click Check Constant. Read the feedback carefully, especially for character constants, because one ordinary character should be written inside single quotes. This matters because a student must first recognize fixed values before using them inside assignments and expressions.

CONSTANT TYPE CHECKER

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EXAMPLE

FIXED VALUE EXAMPLE

100

DEFINITION

VARIABLE IN C

x

EXAMPLE

MEMORY BOX EXAMPLE

x

STORE FIRST VALUE

x = 3;

VALUE STORED IN MEMORY

x

CHANGE STORED VALUE

x = 5;

VARIABLE VALUE CHANGES

3

WATCH A VARIABLE STORE VALUES

Memory Box Simulator shows a variable as a named memory box. Enter a variable name and a value, then click Store Value to put the value into the same box. Try storing 3 first and then 5, and notice that the box name remains the same while the stored value changes. This matters because it separates the variable name from the constants that may be stored inside it.

MEMORY BOX SIMULATOR

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TIP

CONSTANT AND VARIABLE DIFFERENCE

x = 3

TYPES OF CONSTANTS IN C

10

NOTE

QUOTE MARK IN CHARACTER CONSTANT

'A'

DEFINITION

KEYWORD IN C

A keyword in C is a reserved word whose meaning is already fixed in the C language. For example, int, float, char, if, else and while are keywords. We cannot use these words as ordinary variable names. Why? Because C already understands them for special purposes. If we try to give a new meaning to a keyword, the compiler gets confused or rejects the program.

NOTE

RESERVED WORDS IN C

Keywords in C are also called reserved words. The word reserved means already kept for a special purpose. For example, int is reserved for integer type declaration. It should not be used as a variable name. Similarly, float, char, return, if and while are reserved words. So, when a student hears keywords in C or reserved words in C, both ideas point to the same beginner-level concept.

NOTE

CLASSIC C KEYWORD LIST

In the classic beginner-level treatment of C, the common list contains 32 keywords. Some common examples are: auto, break, case, char, const, continue, default, do, double, else, enum, extern, float, for, goto, if, int, long, register, return, short, signed, sizeof, static, struct, switch, typedef, union, unsigned, void, volatile, while. For a first course, learn the idea first. A keyword is not an ordinary name. It is a word already reserved by C. Later, when you use a modern compiler, you may also meet additional reserved words or compiler-specific words. That is not a problem. The basic rule remains the same.

CHECK A WORD BEFORE USING IT

Keyword or Name Checker helps you decide whether a word is one of the classic C keywords or a possible ordinary name. Type a word such as int, float, total, or marks, then click Check Word. If the word is reserved, do not use it as a normal variable name. This matters because keywords already have fixed meanings, so reusing them as ordinary names creates confusion or errors.

KEYWORD OR NAME CHECKER

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VARIABLE NAMES IN C

A variable name is the name given to a memory location. For example, salary, age, total, marks and rate are possible variable names if they follow the naming rules. A good variable name should be meaningful. If possible, let the name itself tell us what the value means. For example, prin may stand for principal. roi may stand for rate of interest. noy may stand for number of years. This is better than using only a, b and c everywhere.

TIP

RULES FOR VARIABLE NAMES

For beginner-level C programming, remember these rules. A variable name may contain letters, digits and underscore. The first character should be a letter or underscore. A variable name should not contain blank spaces. A variable name should not contain commas. A variable name should not contain special symbols other than underscore. A keyword should not be used as a variable name.

TEST A VARIABLE NAME

Variable Name Validator checks a proposed C variable name against the beginner rules. Type a name such as basic_salary, 1rate, student marks, or float, then click Validate Name. Read the rule-by-rule feedback to see exactly why the name is accepted or rejected. This matters because students often know the rule in words but still make small naming mistakes while writing code.

VARIABLE NAME VALIDATOR

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EXAMPLE

VALID VARIABLE NAMES

The following names are valid variable names. Underscore is allowed. Digits may appear after the first character. There is no blank space in these names. Hence, these names are acceptable.

VALID NAMES

si_int m_hra pop_e_89 total marks rate

EXAMPLE

INVALID VARIABLE NAMES

The following names are not valid C variable names.

INVALID NAMES

basic-hra #mean population in 2006 2015_DDay int

SORT THE INVALID NAMES

Name Validity Sorter lets you judge the names from the example before reading the written explanation. Choose valid or invalid for each name, then click Check Answers . The feedback tells you whether the decision agrees with the C naming rules. This matters because seeing the exact reason for rejection helps students avoid the same mistake in their own programs.

NAME VALIDITY SORTER

Dynamic Sandbox

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WHY THESE NAMES ARE INVALID

The name basic-hra is invalid because hyphen is not allowed in a variable name. The name #mean is invalid because # is not allowed. The name population in 2006 is invalid because blank spaces are not allowed. The name 2015_DDay is invalid because it starts with a digit. The name int is invalid as a normal variable name because int is a keyword.

SOLVED PROBLEM

CLASSIFY CONSTANTS VARIABLES AND KEYWORDS

10

CLASSIFY EACH ITEM

Item Classification Practice helps you decide whether each item is a constant, a variable name, or a keyword. Choose the type for each item and click Check Classification. Use the feedback to connect fixed values with constants, ordinary names with variables, and reserved words with keywords. This matters because many C statements contain all three ideas together.

ITEM CLASSIFICATION PRACTICE

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SOLUTION

CLASSIFICATION SOLUTION

10

SOLVED PROBLEM

FIND THE INVALID VARIABLE NAME

Find the invalid variable names from the following list.

NAMES TO CHECK

basic_salary basic-salary rate1 1rate float student_marks student marks

SOLUTION

VARIABLE NAME SOLUTION

basic_salary is valid because underscore is allowed. basic-salary is invalid because hyphen is not allowed. rate1 is valid because a digit may appear after the first character. 1rate is invalid because a variable name cannot start with a digit. float is invalid as a normal variable name because float is a keyword. student_marks is valid because underscore is allowed. student marks is invalid because blank space is not allowed. Therefore, the invalid names are basic-salary, 1rate, float and student marks.

REMARK

COMMON BEGINNER MISTAKE

Many students think that any English word can be a variable name. This is not correct. A variable name must follow the rules of C. Also, a keyword is not free for our own use. For example, int is meaningful in English as a short form of integer in C, but it is already reserved. So, it cannot be used as a normal variable name.

EXERCISE

PRACTICE QUESTIONS

x

PRACTICE NAMES

total_marks total marks 2value value2 char gross_sal gross-sal

ANSWER

PRACTICE ANSWERS

10

FAQS ON CONSTANTS VARIABLES AND KEYWORDS IN C

6 Questions

A constant is a fixed value. A variable is a named memory location whose value may change.

x

5

Keywords already have fixed meanings in C. So, they cannot be reused as ordinary names.

For beginner-level C programming, yes. Keywords are also called reserved words.

Yes. Meaningful names make a program easier to read and understand.

STUDY THE BUILDING BLOCKS BEFORE WRITING PROGRAMS

First understand constants, variables and keywords. Then the first C program becomes much easier.

Next: Types of C Constants Practice Variable Names

BMLabs Mathematics

WHY THESE THREE IDEAS COME FIRST

SEE THE THREE BUILDING BLOCKS

BUILDING BLOCK MAP

CONSTANT IN C

CHECK THE TYPE OF A CONSTANT

CONSTANT TYPE CHECKER

FIXED VALUE EXAMPLE

VARIABLE IN C

MEMORY BOX EXAMPLE

STORE FIRST VALUE

VALUE STORED IN MEMORY

CHANGE STORED VALUE

VARIABLE VALUE CHANGES

WATCH A VARIABLE STORE VALUES

MEMORY BOX SIMULATOR

CONSTANT AND VARIABLE DIFFERENCE

TYPES OF CONSTANTS IN C

QUOTE MARK IN CHARACTER CONSTANT

KEYWORD IN C

RESERVED WORDS IN C

CLASSIC C KEYWORD LIST

CHECK A WORD BEFORE USING IT

KEYWORD OR NAME CHECKER

VARIABLE NAMES IN C

RULES FOR VARIABLE NAMES

TEST A VARIABLE NAME

VARIABLE NAME VALIDATOR

VALID VARIABLE NAMES

VALID NAMES

INVALID VARIABLE NAMES

INVALID NAMES

SORT THE INVALID NAMES

NAME VALIDITY SORTER

WHY THESE NAMES ARE INVALID

CLASSIFY CONSTANTS VARIABLES AND KEYWORDS

CLASSIFY EACH ITEM

ITEM CLASSIFICATION PRACTICE

CLASSIFICATION SOLUTION

FIND THE INVALID VARIABLE NAME

NAMES TO CHECK

VARIABLE NAME SOLUTION

COMMON BEGINNER MISTAKE

PRACTICE QUESTIONS

PRACTICE NAMES

PRACTICE ANSWERS

FAQS ON CONSTANTS VARIABLES AND KEYWORDS IN C

STUDY THE BUILDING BLOCKS BEFORE WRITING PROGRAMS

Introduction to C Programming

Coordinates and Locus