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I have an Arduino Uno project which is recording student attendance using fingerprint sensor. So when sensor found a match, the serial monitor will be printing out Student ID and current time like this:

    DateTime now;
    void setup()
      {
          ...
      }
        
    void loop()
      {
          .....
          //found a match
          Serial.print("Found ID #");
          Serial.println(finger.fingerID);
          Serial.print("Time->");
          Serial.print(now.hour(), DEC);
          Serial.print(':');
          Serial.print(now.minute(), DEC);
          Serial.print(':');
          Serial.print(now.second(), DEC);
      }

Sample result will be:

Found ID#1
Time-> 18:11:54

I would like to store the time into a variable and print it out instead of printing hour, minute, and second separately. How can I do that???

I have tried to do like this as below but I'm getting an error, I would like to store it as a string. How can I do that?

string time = now.hour() + ':' + now.minute() + ':' + now.second();
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  • It's going to be more efficient to print piece by piece. Because it constructs the text straight into the output buffer instead of needing to copy it over. Commented Nov 16, 2018 at 10:41
  • 1
    Hint: If you want to do some calculations or store a timestamp the data type time_t is a better choice. It is defined as seconds since midnight Jan 1 1970 (aka unix time). For more details see github.com/PaulStoffregen/Time/blob/master/TimeLib.h#L21 Commented Nov 16, 2018 at 11:52

1 Answer 1

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You want to use snprintf for this:

char timeString[9];

snprintf(timeString, 9, "%02d:%02d:%02d", now.hour(), now.minute(), now.second());
Serial.println(timeString);

By using a format string like this you are taking into account time portions < 10 (like 7 minutes and 23 seconds past 3 am, which would otherwise be 3:7:23) by padding the time with leading zeroes where needed.

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  • Damn, you were three minutes quicker with the same solution. Replacing 9 with sizeof timeString could be useful in case he ever wants to include more data into his time stamp. Commented Nov 16, 2018 at 10:45
  • Could be - as long as timeString is in the same (or greater) scope and not a pointer passed to a function :) Commented Nov 16, 2018 at 10:46
  • @Majenko Do you familiar with python or firebase? Cause I would like to send the output to the python code, for the python code itself will catch the output and push it to firebase real time database. I refer to this website , I made the output like this Serial.println("{'SID':"+ String(finger.fingerID) +",'time':"+ String(timeString) +"}"); but I don't know how to catch it in my python Commented Nov 16, 2018 at 15:47
  • I loathe python with a passion, and I have never heard of firebase. Commented Nov 16, 2018 at 16:08
  • @Majenko It's ok. I do the same thing to date. But I can display date like this2018/11/. Day cannot display, what happened? Commented Nov 18, 2018 at 9:28

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