C Interview Question Bank
Variables & Control Flow
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
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.
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.
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
| 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) break; j=j=1; } | 4. Example:
i = 1, j = 0; while(i<=5) { i=i+1; if(i== 2) 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:
1
2
3
4
5
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.
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.
Operators, Constants & Structures
1. Which bitwise operator is suitable for checking whether a particular bit is ON or OFF? Bitwise AND operator.
Example: Suppose in byte that has a value 10101101 . We wish to check whether bit number 3 is ON (1) or OFF (0) . Since we want to check the bit number 3, the second operand for AND operation we choose is binary
00001000, which is equal to 8 in decimal.
Explanation:
ANDing operation :
10101101 original bit pattern
00001000 AND mask
---------
00001000 resulting bit pattern
---------
The resulting value we get in this case is 8, i.e. the value of the second operand. The result turned out to be a 8 since the third bit of operand was ON. Had it been OFF, the bit number 3 in the resulting bit pattern would have evaluated to 0 and complete bit pattern would have been 00000000. Thus depending upon the bit number to be checked in the first operand we decide the second operand, and on ANDing these two operands the result decides whether the bit was ON or OFF.
2. Which bitwise operator is suitable for turning OFF a particular bit in a number?
Bitwise AND operator (&), one's complement operator(~)
Example: To unset the 4th bit of byte_data or to turn off a particular bit in a number.
Explanation:
Consider,
char byte_data= 0b00010111;
byte_data= (byte_data)&(~(1<<4));
1 can be represented in binary as 0b00000001 = (1<<4)
<< is a left bit shift operator,
it shifts the bit 1 by 4 places towards left. (1<<4) becomes 0b00010000
And ~ is the one's complement operator in C language. So ~(1<<4) = complement of 0b00010000
= 0b11101111
Replacing value of byte_data and ~(1<<4) in
(byte_data)&(~(1<<4));
we get (0b00010111) & (0b11101111) Perform AND operation to below bytes.
00010111
11101111
-----------
00000111
----------- Thus the 4th bit is unset.
3. What is equivalent of multiplying an unsigned int by 2: left shift of number by 1 or right shift of number by 1?
Left shifting of an unsigned integer is equivalent to multiplying an unsigned int by 2. Eg1: 14<<1;
Consider a number 14-----00001110 (8+4+2)is its binary equivalent left shift it by 1--------------00011100(16+8+4) which is 28.
Eg2: 1<<1;
consider the number as 1---00000001(0+0+1).
left shift that by 1------------00000010(0+2+0) which is 2. left shift by 1 bit of a number=2*number
left shift by 1 bit of 2*number=2*2*number left shift by n bits of number=(2^n)*number
Program: Program to illustrate left shift and right shift operations.
#include<stdio.h>
int main(void)
{
int x=10,y=10;
printf("left shift of 10 is %d \n",x<<1); printf("right shift of 10 is %d \n",y>>1); return 0;
}
Output:
left shift of 10 is 20 right shift of 10 is 5
Explanation:
Left shift (by 1 position) multiplies a number by two. Right shift divides a number by 2.
4. What is an Enumeration Constant?
Enumeration is a data type. We can create our own data type and define values that the variable can take. This can help in making program more readable. enum definition is similar to that of a structure.
Example: consider light_status as a data type. It can have two possible values - on or off. enum light_status
{
on, off
};
enum light_status bulb1, bulb2;
/* bulb1, bulb2 are the variables */
Declaration of enum has two parts:
a) First part declares the data type and specifies the possible values, called 'enumerators'. b) Second part declares the variables of this data type.
We can give values to these variables:
bulb1 = on;
bulb2 = off;
5. What is a structure?
A structure is a collection of pre-defined data types to create a user-defined data type. Let us say we need to create records of students. Each student has three fields:
int roll_number; char name[30]; int total_marks;
This concept would be particularly useful in grouping data types. You could declare a structure student as:
struct student { int roll_number; char name[30]; int total_marks;
} student1, student2;
The above snippet of code would declare a structure by name student and it initializes two objects student1, student2. Now these objects and their fields could be accessed by saying student1.roll_number for accesing roll number field of student1 object, similarly student2.name for accesing name field of student2 object.
6. What are the advantages of unions?
Union is a collection of data items of different data types. It can hold data of only one member at a time though it has members of different data types. If a union has two members of different data types, they are allocated the same memory. The memory allocated is equal to maximum size of the members. The data is interpreted in bytes depending on which member is being accessed.
Example: union pen { char name; float point;
};
Here name and point are union members. Out of these two variables, 'point' is larger variable which is of float data type and it would need 4 bytes of memory. Therefore 4 bytes space is allocated for both the variables. Both the variables have the same memory location. They are accessed according to their type. Union is efficient when members of it are not required to be accessed at the same time.
7. How can typedef be to define a type of structure?
typedef declaration helps to make source code of a C program more readable. Its purpose is to redefine the name of an existing variable type. It provides a short and meaningful way to call a data type. typedef is useful when the name of the data type is long. Use of typedef can reduce length and complexity of data types.
Note: Usually uppercase letters are used to make it clear that we are dealing with our own data type.
Example:
struct employee { char name[20]; int age;
};
struct employee e;
The above declaration of the structure would be easy to use when renamed using typedef as:
struct employee { char name[20]; int age;
};
typedef struct employee EMP; EMP e1, e2;
8. Write a program that returns 3 numbers from a function using a structure.
A function in C can return only one value. If we want the function to return multiple values, we need to create a structure variable, which has three integer members and return this structure.
Program: Program with a function to return 3 values
#include<stdio.h>
//sample structure which has three integer variables. struct sample {
int a, b, c;
};
//this is function which returns three values. struct sample return3val() {
struct sample s1;
s1.a = 10; s1.b = 20; s1.c = 30;
//return structure s1, which means return s1.a ,s1.b and s1.c return s1;
}
int main() {
struct sample accept3val;
//three values returned are accepted by structure accept3val. accept3val = return3val();
//prints the values
printf(" \n %d", accept3val.a); printf("\n %d", accept3val.b); printf(" \n %d", accept3val.c); return 0;
}
Output:
10
20
30.
Explanation:
In this program, we use C structure to return multiple values from a function. Here we have a structure holding three int variables and a function which returns it. 'return3val' is a function which assigns 10, 20, 30 to its integer variables and returns this structure. In this program, 'accept3val' is a structure used to accept the values returned by the function. It accepts those values and shows the output.
9. In code snippet below:
struct Date { int yr; int day;
int month;
} date1,date2;
date1.yr = 2004; date1.day = 4; date1.month = 12;
10. Write a function that assigns values to date2. Arguments to the function must be pointers to the structure, Date and integer variables date, month, year.
Date is a structure with three int variables as members. set_date(..) is a function used to assign values to the structure variable.
Program: Program to illustrate a function that assigns value to the structure.
#include<stdio.h>
#include<stdlib.h>
//declare structure Date struct Date {
int yr;
int day;
int month;
} date1, date2;
//declare function to assign date to structure variable void set_date(struct Date *dte, int dt, int mnt, int year) { dte->day = dt;
dte->yr = year;
dte->month = mnt;
}
int main(void) { date1.yr = 2004; date1.day = 4;
//assigning values one by one date1.month = 12;
//assigning values in a single statement set_date(&date2, 05, 12, 2008);
//prints both dates in date/month/year format
printf("\n %d %d %d ", date1.day, date1.month, date1.yr); printf("\n %d %d %d ", date2.day, date2.month, date2.yr); return 0;
}
Output:
4 12 2004
5 12 2008
Explanation:
Two variables of type Date are created and named 'date1', 'date2'. 'date2' is assigned by using the function set_date(..). Address of 'date2' is passed to set_date function.
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