#include < stdio.h >
int main()
{
int a = 20, b = 30, c;
if( !(a > b) )
b = 50;
c = 70;
printf("b = %d and c = %d\n", b, c);
return 0;
}
In this c source code, a is 20 and b is 30. Inside if condition we check if a is greater than b. But a is less than b, so it returns false(0). But we’ve a Logical NOT operator(!) surrounding the false result. So it converts it to true. Hence the next line of code that is, b = 50 gets executed.
So the output is: b = 50 and c = 70
C Programming Interview / Viva Q&A: 4 (Logical NOT Operator)
In this C program we’ve 2 operands and one AND operator(&&). We use Logical NOT Operator to change the true and false conditions inside if statement. Check the source code below and guess the output.
#include < stdio.h >
int main()
{
int a = 20, b = 30;
if( a && b )
{
printf("a = %d\n", a);
}
else
{
printf("b = %d\n", b);
}
return 0;
}
Output: a = 20
In above C program, a = 20 and b = 30. In C, any non-zero number is considered as true. So the condition is if statement returns true and the block of code inside if statement gets executed.
#include < stdio.h >
int main()
{
int a = 20, b = 30;
if( !(!a) && a )
{
printf("a = %d\n", a);
}
else
{
printf("b = %d\n", b);
}
return 0;
}
Lets evaluate the if condition in above c source code. a = 20, which is non-zero number, so its considered as true(1). So negating the true(1) value !a gives false(0). Negating that false(0) value !(!a) gives true(1).
So !(!a) returns true(1). So both sides of &&(AND) is true, so the whole thing returns true. So the block of code inside if condition gets executed.
Output: a = 20
C Programming Interview / Viva Q&A: 3 (Logical Operator)
Important: 1. Logical &&(AND) Operator returns true when both sides operands have true(non-zero number) value. In any other case it returns false(0) value. 2. Any non-zero number is considered as true and zero(0) is considered as false in C programming language.
Guess the output of this simple C program. You can check these 2 video tutorials listed below so that you get to know todays interview / viva question.
#include < stdio.h >
int main()
{
int num;
printf("Enter a number\n");
scanf("%d", &num);
if(num = 5)
{
printf("You entered number 5\n");
}
else
{
printf("You entered %d \n", num);
}
return 0;
}
If user enters num value as 10. What does the above C Program output?
Answer: Did you answer – You entered 10 as output? If yes, then its wrong.
Note: 1. If at first glance of the source code you didn’t get the answer right, then go through the videos I’ve posted in ‘Related Read” section above. 2. After going through the 2 videos I’ve posted in ‘Related Read’ section above, if you still didn’t get the answer, then please open your editor and type the program and execute it one line at a time.
Hint: assignment operator(=) and equality operator(==) are different and work differently in C programming.
As you can see in the condition of if, it’s written as num = 5. = is assignment operator. So here 5 is being assigned to variable num, and thus it returns 5. In C program, any non-zero number is considered as true. So every time the condition in if becomes true, so the block of code present in if gets executed. i.e., value of num is 5.
#include < stdio.h >
int main()
{
int num;
printf("Enter a number\n");
scanf("%d", &num);
if(num == 5)
{
printf("You entered number 5\n");
}
else
{
printf("You entered %d \n", num);
}
return 0;
}
Output 1: Enter a number 10 You entered 10
Output 2: Enter a number 5 You entered number 5
Output 3: Enter a number 1000 You entered 1000
Note: Both programs are correct syntactically. That’s the reason compiler did not throw any error. It’s a logical error, so we need to be very careful while writing programs. If we do this simple logical error in a big program, it might take a lot of time to get to know the bug and fix it. So careful. Happy coding.
