C variable and control Flow questions and Answers

Posted by Stephen thangaraj at 14:43
C variable and control Flow questions and Answers
1. What is the difference between declaring a variable and defining a variable?
Declaration of a variable in C hints the compiler about the type and size of the variable in compile time. Similarly, declaration of a function hints about type and size of function parameters. No space is reserved in memory for any variable in case of declaration.
Example: int a;
Here variable 'a' is declared of data type 'int' Defining a variable means declaring it and also allocating space to hold it. We can say "Definition = Declaration + Space reservation".
Example: int a = 10;
Here variable "a" is described as an int to the compiler and memory is allocated to hold value 10
.
2. What is a static variable?
A static variable is a special variable that is stored in the data segment unlike the default automatic variable that is stored in stack. A static variable can be initialized by using keyword static before variable name.
Example:
static int a = 5;
A static variable behaves in a different manner depending upon whether it is a global variable or a local variable. A static global variable is same as an ordinary global variable except that it cannot be accessed by other files in the same program / project even with the use of keyword extern. A static local variable is different from local variable. It is initialized only once no matter how many times that function in which it resides is called. It may be used as a count variable.
Example:
#include <stdio.h>
//program in file f1.c
void count(void) {
static int count1 = 0;
int count2 = 0;
count1++;
count2++;
printf("\nValue of count1 is %d, Value of count2 is %d", count1, count2);
}/
*Main function*/
int main(){
count();
count();
count();
return 0;
}
Output:
Value of count1 is 1, Value of count2 is 1
Value of count1 is 2, Value of count2 is 1
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Value of count1 is 3, Value of count2 is 1

3. What is a register variable?
Register variables are stored in the CPU registers. Its default value is a garbage value. Scope of a register variable is local to the block in which it is defined. Lifetime is till control remains within the block in which the register variable is defined. Variable stored in a CPU register can always be accessed faster than the one that is stored in memory. Therefore, if a variable is used at many places in a program, it is better to declare its storage class as register
Example:
register int x=5;
Variables for loop counters can be declared as register. Note that register keyword may be ignored by some
compilers.

4. Where is an auto variables stored?
Main memory and CPU registers are the two memory locations where auto variables are stored. Auto variables are defined under automatic storage class. They are stored in main memory. Memory is allocated to an automatic variable when the block which contains it is called and it is de-allocated at the completion of its block execution.
Auto variables:
Storage : main memory.
Default value : garbage value.
Scope : local to the block in which the variable is defined.
Lifetime : till the control remains within the block in which the variable is defined.

5. What is scope & storage allocation of extern and global variables?
Extern variables: belong to the External storage class and are stored in the main memory. extern is used when we have to refer a function or variable that is implemented in other file in the same project. The scope of the extern variables is Global.
Example:
/***************
Index: f1.c
****************/
#include <stdio.h>
extern int x;
int main() {
printf("value of x %d", x);
return 0;
}
Index: f2.c
****************/
int x = 3;
Here, the program written in file f1.c has the main function and reference to variable x. The file f2.c has the declaration of variable x. The compiler should know the datatype of x and this is done by extern definition. Material from Interview Mantra. Subscribe to free updates via email. Global variables: are variables which are declared above the main( ) function. These variables are accessible throughout the program. They can be accessed by all the functions in the program. Their default value is zero.
Example:
#include <stdio.h>
int x = 0;
/* Variable x is a global variable.
It can be accessed throughout the program */
void increment(void) {
x = x + 1;
printf("\n value of x: %d", x);
} int main(){
printf("\n value of x: %d", x);
increment();
return 0;
}


6. What is scope & storage allocation of register, static and local variables?
Register variables: belong to the register storage class and are stored in the CPU registers. The scope of the register variables is local to the block in which the variables are defined. The variables which are used for more number of times in a program are declared as register variables for faster access.
Example: loop counter variables.
register int y=6;
Static variables: Memory is allocated at the beginning of the program execution and it is reallocated only after the program terminates. The scope of the static variables is local to the block in which the variables are defined.
Example:
#include <stdio.h>
void decrement(){
static int a=5;
a--;
printf("Value of a:%d\n", a);
} int main(){
decrement();
return 0;
}
Here 'a' is initialized only once. Every time this function is called, 'a' does not get initialized. so output would be 4 3 2 etc.,
Local variables: are variables which are declared within any function or a block. They can be accessed only by function or block in which they are declared. Their default value is a garbage value.


7. What are storage memory, default value, scope and life of Automatic and Register storage class?
1. Automatic storage class:
Storage : main memory.
Default value : garbage value.
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Scope : local to the block in which the variable is defined.
Lifetime : till control remains within the block.
2. Register storage class:
Storage : CPU registers.
Default value : garbage value.
Scope : local to the block in which the variable is defined.
Lifetime : till control remains within the block.


8. What are storage memory, default value, scope and life of Static and External storage class?
1. Static storage class:
Storage : main memory.
Default value : zero
Scope : local to the block in which the variable is defined.
Lifetime : till the value of the variable persists between different function calls.
2. External storage class:
Storage : main memory
Default value : zero
Scope : global
Lifetime : as long as the program execution doesn't come to an end.


9. What is the difference between 'break' and 'continue' statements?
Differences between 'break' and 'continue' statements
break continue
1. break is a keyword used to terminate the loop or exit from the block. The control jumps to next statement after the loop or block.
1. continue is a keyword used for skipping the current iteration and go to next iteration of the loop
2.Syntax:
{
Statement 1;
Statement 2;
Statement n;
break;
}
2.Syntax:
{
Statement 1;
continue;
Statement 2;
}
3. break can be used with for, while, do- while, and switch statements. When break is used in nested loops i.e. within the inner most loop then only the innermost loop is terminated.
3. This statement when occurs in a loop does not terminate it but skips the statements after this continue statement. The control goes to the next iteration. continue can be used with for, while and do-while.
4. Example:
i = 1, j = 0;
while(i<=5)
{
i=i+1;
if(i== 2)
4. Example:
i = 1, j = 0;
while(i<=5)
{
i=i+1;
if(i== 2)
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break;
j=j+1;
}
continue;
j=j+1;
}

10. What is the difference between 'for' and 'while' loops?
for loop: When it is desired to do initialization, condition check and increment/decrement in a single statement of an iterative loop, it is recommended to use 'for' loop.
Syntax:
for(initialization;condition;increment/decrement)
{/
/block of statements
increment or decrement
}
Program: Program to illustrate for loop
#include<stdio.h>
int main() {
int i;
for (i = 1; i <= 5; i++) {
//print the number
printf("\n %d", i);
}
return 0;
}
Output:
12345
Explanation:
The loop repeats for 5 times and prints value of 'i' each time. 'i' increases by 1 for every cycle of loop. while loop: When it is not necessary to do initialization, condition check and increment/decrement in a single statement of an iterative loop, while loop could be used. In while loop statement, only condition statement is present.
Syntax:
#include<stdio.h>
int main() {
int i = 0, flag = 0;
int a[10] = { 0, 1, 4, 6, 89, 54, 78, 25, 635, 500 };
//This loop is repeated until the condition is false.
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while (flag == 0) {
if (a[i] == 54) {
//as element is found, flag = 1,the loop terminates
flag = 1;
}
else {
i++;
}
}
printf("Element found at %d th location", i);
return 0;
}
Output:
Element found at 5th location
Explanation:
Here flag is initialized to zero. 'while' loop repeats until the value of flag is zero, increments i by 1. 'if' condition checks whether number 54 is found. If found, value of flag is set to 1 and 'while' loop terminates.




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