How to add if condition in RFID example?
As has been noted in the comments you can't compare arrays like that. Instead you have to iterate through each entry and compare the individual elements. 

If it's just a few entries, then you can do it manually:

if ((uid[0] == 0xF0) &amp;&amp; (uid[1] == 0x21) &amp;&amp; (uid[2] == 0x5D) &amp;&amp; uid[3] == 0xA7) {
    // ...
}


If your values are both in arrays then you can use the memcmp() function to compare them:

uint8_t compare[] = {0xf0, 0x21, 0x5d, 0xa7};

if (memcmp(uid, compare, 4) == 0) {
    // ...
}


memcmp returns 0 if the two arrays match up to the number of bytes given (4 in this case), or a negative or positive number if the one is "less than" or "more than" the other.