C variable naming rules

Rules For Constructing Variable Name:-

• Characters Allowed :
• Underscore(_)
• Capital Letters ( A – Z )
• Small Letters ( a – z )
• Digits ( 0 – 9 )
• Blanks & Commas are not allowed
• No Special Symbols other than underscore(_) are allowed
• First Character should be alphabet or Underscore
• 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;
}

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Remember following Tricks:-

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