A variable is a named location in a memory where a program can manipulate the data.

Rules for naming c variable:
1. Variable name can contain letters, digits and the underscore. Variable name must begin with letter or underscore
2. Variable names are case sensitive. Upper and lowercase letters are distinct.
3. Variable name must not be the same as C reserved words or keywords

Examples of valid C variable names
• foo
• Bar
• BAZ
• foo_bar
• _foo42
• _
• QuUx

Examples of invalid C variable names:
• 2foo (must not begin with a digit)
• my foo (spaces not allowed in names)
• $foo ($ not allowed)
• while (keywords cannot be used as names)

Each variable in C has a specific type, which determines the size (the range of values that can be stored within that memory) and layout of the variable’s memory (the set of operations that can be applied to the variable).

Basic Data Types in C

Macros defined in limits.h header define constants with the limits of basic data types.

#include <stdio.h>
#include <limits.h>

int main() {

   printf("The number of bits in a byte %d\n", CHAR_BIT);

   printf("The minimum value of SIGNED CHAR = %d\n", SCHAR_MIN);
   printf("The maximum value of SIGNED CHAR = %d\n", SCHAR_MAX);
   printf("Storage size for SIGNED CHAR : %lu \n", sizeof(signed char));

   printf("The maximum value of UNSIGNED CHAR = %d\n", UCHAR_MAX);
   printf("Storage size for UNSIGNED CHAR : %lu \n", sizeof(unsigned char));

   printf("The minimum value of SHORT INT = %d\n", SHRT_MIN);
   printf("The maximum value of SHORT INT = %d\n", SHRT_MAX);
   printf("Storage size for SHORT INT : %lu \n", sizeof(short int));

   printf("The maximum value of UNSIGNED SHORT INT = %d\n", USHRT_MAX);
   printf("Storage size for UNSIGNED SHORT INT : %lu \n", sizeof(unsigned short int));

   printf("The minimum value of INT = %d\n", INT_MIN);
   printf("The maximum value of INT = %d\n", INT_MAX);
   printf("Storage size for INT : %lu \n", sizeof(int));

   printf("The maximum value of UNSIGNED INT = %d\n", UINT_MAX);
   printf("Storage size for UNSIGNED INT : %lu \n", sizeof(unsigned int));

   printf("The minimum value of LONG INT = %ld\n", LONG_MIN);
   printf("The maximum value of LONG INT = %ld\n", LONG_MAX);
   printf("Storage size for LONG INT : %lu \n", sizeof(long int));

   printf("The maximum value of UNSIGNED LONG INT = %lu\n", ULONG_MAX);
   printf("Storage size for UNSIGNED LONG INT : %lu \n", sizeof(unsigned long int));

   printf("The minimum value of LONG LONG INT = %lld\n", LLONG_MIN);
   printf("The maximum value of LONG LONG INT = %lld\n", LLONG_MAX);
   printf("Storage size for LONG LONG INT : %lu \n", sizeof(long long int));

   printf("The maximum value of UNSIGNED LONG LONG INT = %llu\n", ULLONG_MAX);
   printf("Storage size for UNSIGNED LONG LONG INT : %lu \n", sizeof(unsigned long long int));

   return 0;
}

Float data types in C

Macros defined in float.h header define constants with the limits of float data types.

#include <stdio.h>
#include <float.h>

int main() {
   printf("FLT_RADIX    = %d\n", FLT_RADIX);
   printf("DECIMAL_DIG  = %d\n", DECIMAL_DIG);

   printf("FLT_MIN      = %e\n", FLT_MIN);
   printf("FLT_MAX      = %e\n", FLT_MAX);
   printf("FLT_EPSILON  = %Le\n", FLT_EPSILON);
   printf("FLT_DIG      = %d\n", FLT_DIG);
   printf("FLT_MANT_DIG = %d\n", FLT_MANT_DIG);
   printf("FLT_MIN_EXP  = %d\n",  FLT_MIN_EXP);
   printf("FLT_MIN_10_EXP  = %d\n",  FLT_MIN_10_EXP);
   printf("FLT_MAX_EXP     = %d\n",  FLT_MAX_EXP);
   printf("FLT_MAX_10_EXP  = %d\n",  FLT_MAX_10_EXP);
   printf("FLT_ROUNDS      = %d\n",  FLT_ROUNDS);
   printf("FLT_EVAL_METHOD = %d\n",  FLT_EVAL_METHOD);
   printf("FLT_HAS_SUBNORM = %d\n",  FLT_HAS_SUBNORM);

   printf("DBL_MIN      = %e\n", DBL_MIN);
   printf("DBL_MAX      = %e\n", DBL_MAX);
   printf("DBL_EPSILON  = %Le\n", DBL_EPSILON);
   printf("DBL_DIG      = %d\n", DBL_DIG);
   printf("DBL_MANT_DIG = %d\n", DBL_MANT_DIG);
   printf("DBL_MIN_EXP  = %d\n",  DBL_MIN_EXP);
   printf("DBL_MIN_10_EXP  = %d\n",  DBL_MIN_10_EXP);
   printf("DBL_MAX_EXP     = %d\n",  DBL_MAX_EXP);
   printf("DBL_MAX_10_EXP  = %d\n",  DBL_MAX_10_EXP);
   printf("DBL_HAS_SUBNORM = %d\n",  DBL_HAS_SUBNORM);

   printf("LDBL_MIN      = %Le\n", LDBL_MIN);
   printf("LDBL_MAX      = %Le\n", LDBL_MAX);
   printf("LDBL_EPSILON  = %Le\n", LDBL_EPSILON);
   printf("LDBL_DIG      = %d\n", LDBL_DIG);
   printf("LDBL_MANT_DIG = %d\n", LDBL_MANT_DIG);
   printf("LDBL_MIN_EXP  = %d\n",  LDBL_MIN_EXP);
   printf("LDBL_MIN_10_EXP  = %d\n",  LDBL_MIN_10_EXP);
   printf("LDBL_MAX_EXP     = %d\n",  LDBL_MAX_EXP);
   printf("LDBL_MAX_10_EXP  = %d\n",  LDBL_MAX_10_EXP);
   printf("LDBL_HAS_SUBNORM = %d\n",  LDBL_HAS_SUBNORM);

   return 0;
}

Before using a variable, you must declare it. To declare a variable, you must specify its data type and its name. The variable declaration statement always ends with a semicolon (;).

Example

int count;
char ch;

C also allows you to initialize a variable when you declare it.

int count = 10;
char ch = 'a';