How can i concatenate integers as a single string?

int out1, out2, out3 = 0;

    out1 = digitalRead(r1);
    out2 = digitalRead(r2);
    out3 = digitalRead(r3);

I tried like so, between others, with no luck.

Serial.write(out1 + "" + out2 + "" + out3);

For example the expected result when all outputs are HIGH should be 111, while when only the r3 output is HIGH it should be 001

All i want, is reporting to the sender of the serial command, the pins state

  • Since you only use 1s and 0s, you could also convert it to a three digit number, and print that Serial.write(out1*100 + out2*10 + out3);
    – Gerben
    May 17, 2018 at 18:04
  • @Gerben: You mean Serial.println(). May 17, 2018 at 18:41
  • 1
    Correction: Serial.println(out1*100 + out2*10 + out3); (thanks @EdgarBonet)
    – Gerben
    May 17, 2018 at 18:43

3 Answers 3


If you're just outputting to serial you don't need to concatenate - just print each bit separately, such as:


You should avoid using String as it's not good on devices with small amounts of memory like Arduinos. Instead if you really need to concatenate numbers you should use a C string and maybe snprintf to format the numbers into it:

char out[30];
snprintf(out, 30, "%d,%d,%d", out1, out2, out3);

I second Majenko's points about just printing the bits separately if possible, and avoiding String objects.

However, if you do need to build such a string (not String object, just plain C string), you don't need sprintf(), which is quite a big function: you can build the string character by character. For example:

char out[4];  // 3 + 1 char for the termination character
out[0] = out1==HIGH ? '1' : '0';
out[1] = out2==HIGH ? '1' : '0';
out[2] = out3==HIGH ? '1' : '0';
out[3] = '\0';  // add NUL termination

This is using the ternary operator, which here means “if out1 is HIGH, then use the character '1', otherwise use '0'”.

Now, it just happens that in Arduino HIGH means 1 and LOW means 0. And single digit numbers can be converted into character by just adding the numeric code of character “0”, which is 48 but can written as '0' in C++. Thus you can write:

out[0] = out1 + '0';  // convert number to character

And you can put all together as:

char out[4];  // 3 + 1 char for the termination character
out[0] = digitalRead(r1) + '0';
out[1] = digitalRead(r2) + '0';
out[2] = digitalRead(r3) + '0';
out[3] = '\0';  // add NUL termination

Serial.write(b) prints one byte not a string. Use print() to print strings and numbers.


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