CODEWITHSUNDEEP
coutputcompound-assignment
Loris Simonetti
Loris Simonetti

Reputation: 217

Compoud operator (+=) and && in C, weird value

Looking at this code:

#include <stdio.h>
#include <stdlib.h>

int main(){
  int a = -1;
  int b = 0xfc; // = 252
  b+=a && a++;
  printf("%d %d\n", a, b);
}

the output I think should be is:

0 251

Actually, the real output is:

0 253

But why? associativity of && is from left to right, so

  1. b = b - 1(a) (and b should be 251)
  2. the left part is true so: a++ (a = 0)

There's something wrong with my hypothesis, can someone explain to me what?

If can help that the output of gdb (with watchpoints to a and b):

Old value = 0
New value = -1
main () at test.c:7
7         int b = 0xfc;
(gdb) c
Continuing.

Hardware watchpoint 2: b

Old value = 0
New value = 252
main () at test.c:8
8         b+=a && a++;
(gdb) c
Continuing.

Hardware watchpoint 3: a

Old value = -1
New value = 0
0x0000555555555172 in main () at test.c:8
8         b+=a && a++;
(gdb) c
Continuing.

Hardware watchpoint 2: b

Old value = 252
New value = 253
main () at test.c:9
9         printf("%d %d\n", a, b);

Upvotes: 3

Views: 62

Answers (1)

0___________
0___________

Reputation: 67751

The explanation is very simple.

a && a++ is a logical operation and its result can be 0 or 1. As a is non zero and any non zero value is considered as the true the result of this operation is 1.

This value is added to 252. The result displayed is 253.

Upvotes: 5

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