CODEWITHSUNDEEP
ccomparisonbit-manipulation
icedTea
icedTea

Reputation: 495

C programming: How could you compare two types without ==

I was wondering how the C compiler would handle the comparison operator '=='

I wish to create a function (in C) that compares two things without using the ==. I think it can be done some how by using bitwise operators ( &, |, ^, >>) however I can't seem to think of how to do it.

Anyone have ideas?

Thanks!

Upvotes: 0

Views: 518

Answers (6)

asaelr
asaelr

Reputation: 5456

Other answers are good, but if you want only bitwise operators, do something like:

int is_equal(int a,int b) {
 int bit=1;
 while(bit) {
  if ((a&bit)^(b&bit)) return 0;
  bit<<=1;
 }
 return 1;
}

Upvotes: 0

Kendall Frey
Kendall Frey

Reputation: 44316

Since you're wondering how the compiler does it, it translates == into machine code that contains a cmp instruction. This instruction compares two operands, and stores the result of the comparison. The result can be checked to see if the operands were equal, not equal, less than, greater than, etc.

In the case of using == in an if statement, it would probably generate something like this:

cmp op1, op2
jne else_address

which basically means 'Compare the two values. If they are not equal, skip to the else block.'

Upvotes: 0

gtdevel
gtdevel

Reputation: 1513

What if you invert every bit of one of the values and then "&" the two values together. If the result is zero, then they should be equal.

int x=5;
int y=5;
int z=(~x)&y;

if(!(y>0||y<0)){
   //Values are equal. 
}

Why don't you want to use the "==" operator?

Upvotes: 0

hmjd
hmjd

Reputation: 121961

Without using ==:

int is_a_equal_to_b(int a, int b)
{
    if (a > b || a < b) return 0; /* Not equal. */
    return 1;                     /* Equal.     */
}

Upvotes: 1

Jamie Wong
Jamie Wong

Reputation: 18350

For integral types, if (!(a - b)) will work.

You might run into some funny problems with other types.

Maybe if (!(a ^ b))?

Upvotes: 1

Paul
Paul

Reputation: 141829

Here's a simple way to check if two ints are equal without using ==:

int x = 3;
int y = 3;

if(x < y || y < x)
    printf("Not equal");
else
    printf("Equal");

Upvotes: 6

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