Using an Arduino Mega 2560 and PN532 NFC/RFID module, I want to be able to scan a card and see if its value is (0xF0, 0x21, 0x5D, 0xA7). It should print "accepted" in the serial monitor.

However during compilation it keeps telling me:

iso14443a_uid:99:11: error: expected primary-expression before ']' token

if (uid[] = {0xF0, 0x21, 0x5D, 0xA7}){  

exit status 1 expected primary-expression before ']' token

This is my code:

/**************************************************************************/ /*! This example will attempt to connect to an ISO14443A card or tag and retrieve some basic information about it that can be used to determine what type of card it is.

    Note that you need the baud rate to be 115200 because we need to print
    out the data and read from the card at the same time!

    To enable debug message, define DEBUG in PN532/PN532_debug.h


/* When the number after #if set as 1, it will be switch to SPI Mode*/
#if 0
  #include <SPI.h>
  #include <PN532_SPI.h>
  #include "PN532.h"

  PN532_SPI pn532spi(SPI, 10);
  PN532 nfc(pn532spi);

/* When the number after #elif set as 1, it will be switch to HSU Mode*/
#elif 0
  #include <PN532_HSU.h>
  #include <PN532.h>

  PN532_HSU pn532hsu(Serial1);
  PN532 nfc(pn532hsu);

/* When the number after #if & #elif set as 0, it will be switch to I2C Mode*/
  #include <Wire.h>
  #include <PN532_I2C.h>
  #include <PN532.h>
  #include <NfcAdapter.h>

  PN532_I2C pn532i2c(Wire);
  PN532 nfc(pn532i2c);

void setup(void) {


  uint32_t versiondata = nfc.getFirmwareVersion();
  if (! versiondata) {
    Serial.print("Didn't find PN53x board");
    while (1); // halt

  // Got ok data, print it out!
  Serial.print("Found chip PN5"); Serial.println((versiondata>>24) & 0xFF, HEX); 
  Serial.print("Firmware ver. "); Serial.print((versiondata>>16) & 0xFF, DEC); 
  Serial.print('.'); Serial.println((versiondata>>8) & 0xFF, DEC);

  // Set the max number of retry attempts to read from a card
  // This prevents us from waiting forever for a card, which is
  // the default behaviour of the PN532.

  // configure board to read RFID tags

  Serial.println("Waiting for an ISO14443A card");

void loop(void) {
  boolean success;
  uint8_t uid[] = { 0, 0, 0, 0, 0, 0, 0 };  // Buffer to store the returned UID
  uint8_t uidLength;                        // Length of the UID (4 or 7 bytes depending on ISO14443A card type)

  // Wait for an ISO14443A type cards (Mifare, etc.).  When one is found
  // 'uid' will be populated with the UID, and uidLength will indicate
  // if the uid is 4 bytes (Mifare Classic) or 7 bytes (Mifare Ultralight)
  success = nfc.readPassiveTargetID(PN532_MIFARE_ISO14443A, &uid[0], &uidLength);

  if (success) {
    Serial.println("Found a card!");
    Serial.print("UID Length: ");Serial.print(uidLength, DEC);Serial.println(" bytes");
    Serial.print("UID Value: ");
    for (uint8_t i=0; i < uidLength; i++) 
      Serial.print(" 0x");Serial.print(uid[i], HEX); 
    // Wait 1 second before continuing
    // PN532 probably timed out waiting for a card
    Serial.println("Timed out waiting for a card");
  if (uid[] = {0xF0, 0x21, 0x5D, 0xA7}){
  • 1
    = is assignment operator. you can't use constant initialization to assign a value to a variable. In if condition usually a comparison operator is '==' is used and it can't be used to compare arrays
    – Juraj
    Dec 7, 2018 at 19:41
  • well, it same if i use == and if i can't compare arrays so what is the right way to make that happens like if the card serial / uid is what i want to give access and print success is there a way to do that? Dec 7, 2018 at 19:45
  • You have to compare each value in the array once at a time. Like if( uid[0]== 0xF0 && uid[1]==0x21 && ...)
    – chrisl
    Dec 7, 2018 at 20:02
  • Can you recommend a course for me that can get me to that as fast as i can, I've less than 3 months to get that project done, I've no idea about arduino or c++ my college put me in this situation i didn't even studied anything about electronics or even c++ Dec 7, 2018 at 20:11
  • @AbdalrahmanGamal, lol, i am very curious .... you used the statement I wanna in your post .... i have seen a lot of people use it .... i am wondering, how did you learn the word wanna? .... it not a word in the English language, by the way ... the statement should be I want to
    – jsotola
    Dec 8, 2018 at 2:40

1 Answer 1


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) && (uid[1] == 0x21) && (uid[2] == 0x5D) && 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.

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