string concatenation does not work

Projects Programming
1

Hi,

I just started playing around with Arduino and here is what I have, when trying to concatenate some strings...

#define ARD_LABEL  111  // unique id 





void setup() {


 unique_id = "Id_"+ARD_LABEL; 


 Serial.println("Unique Label is : " + unique_id);


 delay(500);


}

The serial.println prints garbage, not what I expect. In fact in print another log message (have of it) that I have later in my code.

Unique Label is : tering a new user...

when the tering a new user... is the "garbage"

What am I missing?

2

It looks like some of your code is missing.

ignch90:
What am I missing?

Some of your code is missing. This snippet won't compile.

I'll recommend that you show us a complete sketch that illustrates the problem. If you want people to look at the sketch and recommend solutions, you'd be well advised to post a short, minimal sketch that demonstrates the problem that you're trying to solve. If you post a long and complicated sketch, a lot of readers will decline to look at it, and will move on to the next post.

To illustrate the importance of posting a complete sketch, note that we can't tell what kind of object unique_id is. It could be a character pointer, or maybe a String. We need to see its declaration to know what it is.

3

This is the code snippet you need... Anything after that does not affect the program. Anyway, since you mentioned that here is the complete code, but I doubt it makes any difference.

#include <Bridge.h>
#include <Parse.h>

#define ARD_LABEL  111  // unique id for this board

String unique_id ;

void setup() {

  // Initialize digital pin 13 as an output.
pinMode(13, OUTPUT);
 
// Initialize Bridge
Bridge.begin();
 
// Initialize Serial
Serial.begin(9600);
 
while (!Serial); // wait for a serial connection
Serial.println("Parse Starter Project");

// String for unique_id

unique_id = "Id_"+ARD_LABEL;
Serial.println("Unique Label is : " + unique_id);
delay(500);

}

void loop() {
      Serial.println("Looping...");
      delay(2000);
}

I removed everything from my code, and since the Serial.println prints either garbage or nothing...

4

here is the complete code, but I doubt it makes any difference.

Well, your first piece of code had no declaration or type for unique_id , so yeah, it does make a difference.

5

unique_id = "Id_"+ARD_LABEL;

If you want to concatenate two Strings, then the two things you are trying to concatenate need to be both Strings, or have a default conversion to Strings. Do they ?

6

In the programming language Java, the plus sign may be used to concatenate two strings. However:

  1. Both sides of the plus sign had better be strings. ARD_LABEL is not a string.

  2. Most people program Arduinos in C or C++, not Java. C never uses a plus sign to concatenate two strings. C++ could use the plus sign to concatenate two strings, but you had better have the right library - and use it properly.

7

Most Arduino users tend to avoid the String class. True, it brings a lot of easy-to-use stuff to the table, but it eats resources like crazy. Most would use C strings and the family of string processing functions (e.g., strcat(), strcmp(), strtok(), etc.) that are available.

8

ignch90:
... I doubt it makes any difference.

Well, yeah, it does. Now we can see that unique_id is a String.

Here's a quote from the reference page for the String Addition Operator page, saying, essentially, "don't do that:"

int sensorValue = analogRead(A0);

String stringOne = "Sensor value: ";
 String stringThree = stringOne + sensorValue;
 Serial.println(stringThree);



results in "Sensor Value: 402" or whatever the analogRead() result is, but

int sensorValue = analogRead(A0);
 String stringThree = "Sensor value: " + sensorValue;
 Serial.println(stringThree);



gives unpredictable results because stringThree never got an initial value before you started concatenating different data types.

That page will probably clarify the correct coding for you.

I suspect that this code -

 unique_id = "Id_"+ARD_LABEL;

sees "Id_" as a lower-case-"s" string, and adds 111 to its pointer's value and returns the result to unique_id, which is then interpreted as a capital-"S" String, showing everything from that location to the first null character that it encounters. But, I don't know - something else could be happening..

This stripped-down code, without the unnecessary #include directives, seems to work:

#define ARD_LABEL  111  // unique id for this board
String unique_id ;
String tempString;

void setup() {
  // Initialize Serial
  Serial.begin(9600);
  while (!Serial); // wait for a serial connection

  Serial.println("Parse Starter Project");

  // String for unique_id
  tempString = "Id_";
  unique_id = tempString + ARD_LABEL;
  Serial.println("Unique Label is: " + unique_id);
  Serial.println("Done.");
}

void loop() {}

The concatenation in the Serial.println(), concatenating a constant string and a String variable, may not be entirely kosher, based on what I see on the reference page. It seems to work in this case, but it's probably wisest to avoid that construction, and, when you're concatenating Strings, concatenate only Strings.

9

ignch90:
I just started playing around with Arduino and here is what I have, when trying to concatenate some strings...
...

What am I missing?

111 isn't a string, that is what you are missing. So you should say "I am trying to concatenate a string and a number".

Try:

#define ARD_LABEL  111  // unique id for this board
String unique_id ;
#define xstr(s) str(s)
#define str(s) #s
void setup() 
{
  Serial.begin(115200);
  while (!Serial); // wait for a serial connection
  Serial.println("Parse Starter Project");
  unique_id = "Id_" xstr (ARD_LABEL);
  Serial.println("Unique Label is : " + unique_id);
}

void loop() { }

That prints:

Parse Starter Project
Unique Label is : Id_111
10

Somewhat shorter and less confusing is:

#define ARD_LABEL  111  // unique id for this board
void setup() 
{
  Serial.begin(115200);
  while (!Serial); // wait for a serial connection
  Serial.println("Parse Starter Project");
  Serial.print("Unique Label is : ID_");
  Serial.println (ARD_LABEL);
}
void loop() { }
11

Or you could use snprintf:

#define ARD_LABEL  111  // unique id for this board
void setup() 
{
  Serial.begin(115200);
  while (!Serial); // wait for a serial connection
  Serial.println("Parse Starter Project");
  char buf [10];
  snprintf (buf, sizeof buf, "ID_%i", ARD_LABEL);
  Serial.print("Unique Label is : ");
  Serial.println (buf);
}
void loop() { }
12 
unique_id = "Id_"+ARD_LABEL;

"Id_" is a pointer into memory, equal to the place where the compiler has placed that constant string.
You then add 111 to that memory location, resulting in a pointer to string that references some completely unrelated stuff.
You then use the overloaded '=' operator of the C++ string class to set unique_id to whatever was at that location.

13

Absolutely. You may as well have asked:

unique_id = "Id_"+111;

Why does that print garbage?

14

I think the OP wants sprintf.

http://www.cplusplus.com/reference/cstdio/sprintf/

15

Thank you all for the replies, I managed to sort it out.