Legal and illegal declaration and initializations in C

Problem

Mention some of the legal and illegal declarations and initializations while doing C programming?

Before discussing the legal and illegal statements let’s see how to declare and initialize the variables in C.

Variable declaration

Following is the syntax of variable declaration −

Syntax

Datatype v1,v2,… vn;

Where v1, v2,...vn are names of the variables.

For example, int sum;

float a,b;

Variable can be declared in two ways −

• local declaration

• global declaration

The ‘local declaration’ is declaring a variable inside the main block and its value is available within that block.

The ‘global declaration’ is declaring a variable outside the main block and its value is available throughout the program.

For example,

int a, b; /* global declaration*/
main ( ){
   int c; /* local declaration*/
   - - -
}

Variable initialization

Following is the syntax of variable initialization −

Syntax

Datatype v1=number;

For example,

int sum=0;
float a=1,b=4.5;

Declare the variables by using data types, we can initialize the value at the time of declaration. So, while initializing and declaring the values we need to follow the rules

Let’s see some of the examples of legal and illegal declarations and initializations in C.

Example

• Char a=65;

  It is a legal statement because we can initialize a variable with a constant.

• Static int p=20, q=p*p

  It is an illegal statement because static variable has to be initialized by a constant but here q is not initialized with a constant.

• Double x=30 *PI

  It is a legal statement because here we initialized a variable with a constant expression.

• Double diameter []={1,PI/2, PI, 2*PI/2}

  It is a legal statement, here we initialized array elements with constant.

Sample program

With legal declaration & initialization −

#include<stdio.h>
void main ( ){
   int a,b;
   a= 10, b = 20;
   printf (" %d", a<b);
   printf (" %d", a<=b);
   printf (" %d", a>b);
   printf (" %d", a>=b);
   printf (" %d", a = =b);
   printf (" %d", a ! =b);
}

Output

1 1 0 0 0 1

Example

With illegal declaration & initialization −

#include <stdio.h>
int main(){
   static int p=20, q=p*p;//illegal initialization
   printf("%d%d",p,q);
   return 0;
}

Output

error will be occurred
error: initializer element is not constant
   static int p=20, q=p*p;