Increment operator

Question

#include <stdio.h>

int main() {
    
    int a = 5, b;
    
    a = ++a + a++ + ++a;
    printf("%d",a);

    return 0;
}

What will be the output of the code?

Comments

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Answer 1

Answer is 20

Explaination :

 

so a= 6+6+8 = 20

 

Comments

obviously. Infact 3 * undefined behaviour.

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are you sure about that error because i am getting the answer when i am running that  code

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Why should “undefined behaviour” produce “error”? Still if you run in gcc with “-Wall -O3” it’ll probably warn you when it can identify undefined behaviour.

Answer 2

The answer will be

a =21

While all the calls are on the same line then increment will be done on every call.so in first call a=6, in second a=7 and in third call a=8.

Answer 3

20

Comments

I too solved and got 20 but,

Run it in the online compiler, it will print 21, that’s why I am confused and was thinking whether I am missing something or not.

Answer 4

Rule 1 : unary having higher precedence than binary : So lets solve all unary first then take binary

Rule 2: The precedence of post-increment is significantly higher than that of pre-increment in C. Furthermore, when it comes to associativity, pre-increment has right-to-left associativity, whereas post-increment has left-to-right associativity.

Ref: https://www.upgrad.com/tutorials/software-engineering/c-tutorial/pre-increment-and-post-increment-in-c/#:~:text=The%20precedence%20of%20post%2Dincrement,left%2Dto%2Dright%20associativity.

a=5

a= ++a + (a++) + ++a; [post increment higher]

a= ++a + 6 + (++a); [pre increment right to left associativity]

a=  (++a) + 6 + 7;

a= 8 + 6 + 7;

a= 21

So the ans is 21.

Open for any corrections

Comments

As post increment then how can you take (a++) value as 6, it would be 5 no then. After giving the value as 5 it will be incremented right?

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a++ itself will be solved first like at first as u told it takes 5 then post increments then it becomes 6 ( so only I have precided then in brackets) later takes pre increment (on right) and with that  6 becomes 7, again comes to first that 7 will be resolved to 8 after all the unary operations it takes up binary and sums up all in + associativity (left to right rule).

even though its post or pre the precedence between them is won by post so the 5 gets converted to 6 and then further....

Though u might find ans being approached in a different way of getting 21 but this method is having proof of resolving precedence first and then associativity and then binary…

U can apply to other sums and even check this method… to my knowledge this works.

Let me know if there’s any correction :))

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@Viggy_27 May be you can google “undefined behaviour in C”. 

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Okay Arjun sir, but whenever this undefined behavior of C topic comes in… the whole compiler design subject looks doubtful to me… as we write so many grammars for the order of syntax and maintain the precedence and associativity in analyzing operations by using syntax trees and semantic rules how come this c behaves as undefined, whether its because of the ambiguity in satisfying too many operations at once or is it not yet resolved as how we write grammars to some operations like these….

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@Viggy_27 the above undefined behaviour is connected to the backend part of compilers. In other words, it has nothing do with the syntax analysis or the semantic analysis parts. When you try to do code optimisation or code generation - - in Master level courses, you can understand why it makes sense. 

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oh okay sir...

Thanks for responding Sir and hope with your guidance and blessings I would study this topic in Masters through GATE !!