www.openITis.com | L INUX For YoU | AUgUsT 2008 83

S.G. GaneSh

The Joy ofProgramming

1.  Which operator in C can result in a ‘divide by zero’ error other than the / (division) operator?

2.  The conditional operator (? :) is equivalent to if-then-else, which is a ternary operator. Why is there no if-then binary (?) operator? 

3.  Your nephew has scored 453 out of 500 marks in the SSLC exam, and you write a trivial program to check the percentage—what does it print?  

int main() {

int marks = 453, total = 500;

float percent = (marks/total)*100;

printf(“percentage is = %3.2f!”, percent);

}

4.  You want a method that multiplies an integer by 9; will the following work? 

int mul_by_nine (int x) {

return (x << 3 + x);

}

5.  What is the output of the following program: 

int main(){

printf(“%d \n”, 1 < 2 < 3);

printf(“%d \n”, 3 > 2 > 1);

}

6.  What is wrong with the following program?  

#include <assert.h>

int main(){

int i = 2;

i = -i;

assert (i == -2);

i = +i;

assert (i == 2);

}

Why did this program fail with this assertion: “Assertion failed: i == 2, file tem.c, line 8”? 

1.The modulus operator (%) can result in a ‘divide 

by zero’ error since it is typically implemented using the division (/) operator.

2.  Unlike if-then-else, the conditional operator can be part of expressions; in other words, the conditional operator has a type. The type of the conditional operator is the second and the third operands (they should be of the same type; otherwise, they get promoted to the same type). If there were a binary (?) operator, then it cannot take part in expressions, because it wouldn’t have any type if the condition becomes false! So, it is not possible to have something like a binary ‘if’ operator (?) in C!

3.  It prints: your percentage of marks is 0.00!  The integer division 432/500 results in 0, and 0 * 

100 is 0. The integer value 0 is converted and stored in floating point value and hence the output is 0.00. Note that the % symbol will also not be printed because it has a special meaning in a format string (you have to make it as %% to print the percentage symbol).

4.  No, it won’t. The logic in this code is to left-shift the int by 3- which is equivalent to ‘multiply by 8 - and add x once’ so that it becomes ‘multiply by 9’. However, this code has a bug: the operator + has higher precedence than the << operator, so the expression becomes (x << (3 + x)), which is wrong. If you use explicit parenthesis, it will avoid this problem.

5.  It prints:   1   0 The mathematical operations/semantics are not directly 

translated as such in C programs. In mathematics, 1 < 2 < 3 and 3 > 2 > 1 are both true. However, in C, ( 1 < 2 ) is 1 and ( 1 < 3 ) is true; hence 1 is printed. But ( 3 > 2 ) is 1 (‘is true’?) and (1 > 1 ) is false (0) and hence 0 is printed.

6.  The unary plus (+) operator is the only dummy operator in C. Its occurrence is just ignored by the compiler; it has no effect in the expressions. Note that the unary minus (–) operator changes the sign of the value, but unary plus is not a complementary operator to unary minus in that it doesn’t have any effect on the value on which it operates. 

C has a very rich set of operators. In this month’s column, we’ll look at some interesting questions about various operators in C. The issues discussed about C are also applicable to languages based on C, such as Java and C++.

Some Interesting Questions on Operators in C

By: S G Ganesh is a research engineer in Siemens (Corporate Technology). His latest book is “60 Tips on Object Oriented Programming”, published by Tata McGraw-Hill in December last year. You can reach him at sgganesh@gmail.com.