Binary AND operator in If statement condition

Question

I Didn't understand this piece of code. Any help?

    for(j = 0x40; j > 0; j = j >> 1) 
    {
     if(0xC9 & j)
       LED = 1;
     else
       LED = 0;
     delay_us(10);
       CLK = 1;
     delay_us(10);
       CLK = 0;
    }

Well it's binary and operator. What exactly do you not understand? — ckruczek, Jul 05 '16 at 10:34
how does it work? What condition is passed in if (condition).? — MICRO, Jul 05 '16 at 10:40
The `if`-statement expression has a bitwise and operator. Is it how this works that you wish to find? Have you read http://stackoverflow.com/questions/3427585/? Or is it how C handles booleans? — Mats, Jul 05 '16 at 10:44

Tim · Answer 1 · 2020-10-15T12:16:52.393

The for loop is a bit tricky. This operation :

j = j >> 1

Is basically an integer division by two. All the bits are shifted from 1 bit to the right. Example :

unsigned char foo = 0x20; //   Binary : 0010 0000, Decimal : 32
foo = foo >> 1; //Hex : 0x10,  Binary : 0001 0000, Decimal : 16

This if statement :

if(0xC9 & j)

is testing if some bits are set to 1 in the loop variable. 0xC9 is a number in which bits 7, 6, 3 and 0 are set to 1 (0xC9 = 0b11001001 = 201). If any of these bits equals '1' in j, then the condition is true.

This is not really a user-friendly code, the one who wrote it should use some #define to make it more readable. The algorithm is also unclear, using bitwise operations is not a pretext to writing unreadable code.

Kusalananda · Answer 2 · 2016-07-05T10:49:07.540

1

The if-statement evaluates 0xC9 & j, and if that comes out as non-zero then it's treated as "true" (LED = 1), otherwise it's "false" (LED = 0).

In C (and many other languages), non-zero means "true" and zero means "false" when used in a boolean context.

edited Jul 05 '16 at 10:49

answered Jul 05 '16 at 10:40

Kusalananda

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Binary AND operator in If statement condition

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