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Rules For Constructing Variable Name

1. Characters Allowed :
• Underscore(_)
• Capital Letters ( A - Z )
• Small Letters ( a - z )
• Digits ( 0 - 9 )

2. Blanks & Commas are not allowed
3. No Special Symbols other than underscore(_) are allowed
4. First Character should be alphabet or Underscore
5. Variable name Should not be Reserved Word

Explanation with Example

Tip 1 : Use allowed Characters

Valid Names

num
Num
Num1
_NUM
NUM_temp2

Tip 2 : blanks are not allowed

Invalid Names

number 1
num 1
addition of program

Tip 3 : No special symbols other that underscore

Valid Identifier

num_1
number_of_values
status_flag

Tip 4 : First Character must be underscore or Alphabet

Valid Identifier

_num1
Num
Num_
_
__

Invalid Identifier

1num
1_num
365_days

Tip 5 : Reserve words are not allowed

• C is case sensitive.
• Variable name should not be Reserve word.
• However if we capitalize any Letter from Reserve word then it will become legal variable name.

Valid Identifier

iNt
Char
Continue
CONTINUE

Invalid Identifier

int
char
continue

Tip 6 : Name of Identifier cannot be global identifier

Basic Note :

• Global predefined macro starts with underscore. (_) .
• We cannot use Global predefined macro as our function name.

Example : Some of the predefined macros in C

• __TIME__
• __DATE__
• __FILE__
• __LINE__

Valid Identifier

__NAME__
__SUM__

Invalid Identifier

__TIME__
__DATE__
__FILE__

Tip 7 : Name of identifier cannot be register Pseudo variables

Invalid Example :

#include<stdio.h>
int main(){
    long int _AH = 15;
    printf("%ld",_AH);
    return 0;
}

Tip 8 : Name of identifier cannot be exactly same as of name of another identifier within the scope of the function

Valid Example :

#include<stdio.h>
int main(){
    int ivar = 15;
     { 
     int ivar = 20;
     printf("%d",ivar);
     } 
    return 0;
}

Invalid Example : We cannot declare same variable twice within same scope

#include<stdio.h>
int main(){
    int ivar = 15;
    int ivar = 20;
    printf("%d",ivar);
    return 0;
}

Tip 9 : Constants

1. We know that M_PI constant is declared inside math.h header file.
2. Suppose in our program we have declared variable M_PI and we have not included math.h header file then it is legal variable.

Legal Example :

#include<stdio.h>
int main(){
    int M_PI=25;
    printf("%d",M_PI);
    return 0;
}

Output :

 25

Illegal Example :

#include<stdio.h>
#include<math.h>
int main(){
    int M_PI=25;
    printf("%d",M_PI);
    return 0;
}

Output :

 Compile error

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Remember following Tricks

1. Do not Create unnecessarily long variable name
2. Do not use underscore as first character to avoid confusion between System Variable & user defined variables because many system variables starts with undescore
3. Variable names are case-Sensitive . i.e sum,Sum,SUM these all three are different variable names.
4. Reserve words with one/more Capital letters allowed eg. Int,Float,chAr are allowed but try to skip them.

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