output

Question

#include<stdio.h>

int main() {
   int y=3 ;

int z= (--y) + (y=10);

printf("%d  ",z);
}
 

can someone explain how is it 20 ?

Comments

It's undefined behavior but I am not getting how ans 20. Side effects (--y)   and (y=20)  writes value of y two times in an expression.

If an object is written to within a full expression, any and all accesses to it within the same expression must be directly involved in the computation of the value to be written. 

Answer 1

It's clearly mentioned in C Book by Kerninghan And Ritchie  :

The operands of a binary operator may be evaluated in any order ; it is not specified..

Hence for the given question , the program will show undefined behaviour and answer will be compiler dependent..This is because of following 2 possible scenarios :

Case 1 :

Left operand evaluation followed by right one : If we do so , y is decremented first using predecrement operator , so y now becomes 2..So left operand value = 2..Then in the right part , the assignment "y = 10" is done so right operand value is 10..Hence z = left operand + right operand  =  2 + 10  =  12

Case 2 :

Now if we consider right one being evaluated first , so y becomes 10 first and hence right operand = 10..Then on predecrement y becomes 9 first and then the value is used for left operand..

So z here  = 10 + 9  =  19..

Hence in no case u will get 20..Had it been a post decrement operator then , yes the value would have been 13 for case 1 and 20 for case 2..

In short , the given question shows undefined behaviour..

Comments

@Anu

how it could be 20?

If u r taking left to right associativity, first y wiil drecrement the value in memory location, where 2nd y is an assignment of 10

So, both y value could not be same

---

why not....

---

ok I think it will be compiler dependent
https://www.allinterview.com/showanswers/238/what-is-difference-between-initialization-and-assignment.html
https://stackoverflow.com/questions/16217839/is-there-a-difference-between-initializing-a-variable-and-assigning-it-a-value-i
that is why different value comes as ans