CODEWITHSUNDEEP
c++bitwise-operators
user5802701
user5802701

Reputation:

What is the difference between if(x^1!=1) and if(int(x^1)!=1) in C++?

I am trying to find if x's first bit from right is 1, so I check if the value of x^1 is 1. However, 

int x=6; 
if (x^1!=1)

gives wrong answer, but 

if (int(x^1)!=1)

gives the correct answer.

I am not sure why. Could someone clarify this for me?

Upvotes: 8

Views: 226

Answers (3)

shock_gone_wild
shock_gone_wild

Reputation: 6740

@Cornstalks is right with his answer.

I just had that in mind ( it does'nt in fact answer your problem , but can make it more readable) :

Another approach for solving this problem is simply using the modulo operator:

if(x%2 == 0)  // then first bit is 0
else  // first bit is 1

In the end your task is simply a check for even or odd values.

Upvotes: 0

Kulamani
Kulamani

Reputation: 517

Simply, != (Not equal relational operator) has high precedence than ^ (XOR bitwise operator). Check precedence

int x=6;

case 1: if (x^1!=1)

First, 1!=1 is 0; then 6^0= 6. (110 ^ 000 = 110); Check XOR table

Case 2: if (int (x^1)!=1)

First, x^1= 7; then 7!=1 is 1 (true).

Upvotes: 0

Cornstalks
Cornstalks

Reputation: 38228

It's a trap of operator precedence. Operator precedence determines how operations are "grouped" (like how 2*3+4 results in the 2*3 being "grouped" together). Adding parentheses changes how things are "grouped" (for example, 2*(3+4) causes 3+4 to be "grouped" together).

x^1!=1 is equivalent to x^(1!=1), which can be simplified to x^0.

int(x^1)!=1 is equivalent to (x^1)!=1 (because you've manually added parentheses here; the int part isn't very relevant; it's the parentheses that are important).

As you can see, x^(1!=1) and (x^1)!=1 are not the same.

If your goal is to check the first bit, I might suggest using a bitwise-AND (&). You can then just do if (x & 1) (but beware, mixing & and == will result in the same issues as you were having before, so use parentheses if you want to write if ((x & 1) == 1)).

Upvotes: 13

Related Questions