Arduino String class

Using Arduino Programming Questions
1

I'm exploring the Arduino String class and I've written the following two cases. I want to know which case is efficient and not causing memory fragmentation at all?
Thanks in Advance!
The variables l, m, n, o are constant for the following two cases. However, for my project these variables won't remain constant, they will change as per the user input in the webpage and I'm a bit worried about the memory fragmentation issue.
Case 1:

float l = 7.568, m = 99.7864;
byte n[] = "ESP32 ESP8266", o[] = "Arduino Mega";
void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.println( pc(F("a01")) );
  Serial.println( pc(F("a02")) );
  Serial.println( pc(F("a03")) );
  Serial.println( pc(F("a04")) );
  Serial.println();
  delay(1000);
}

String pc(const String& var) {
  if (var == F("a01")) {
    return String(l, 3);
  }
  else if (var == F("a02")) {
    return String(m, 4);
  }
  else if (var == F("a03")) {
    return String((char *)n);
  }
  else if (var == F("a04")) {
    return String((char *)o);
  }
  return String();
}

Case 2:

float l = 7.568, m = 99.7864;
byte n[] = "ESP32 ESP8266", o[] = "Arduino Mega";
String str((char *)0);
void setup() {
  // put your setup code here, to run once:
 Serial.begin(115200);
 str.reserve(100);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.println( pc(F("a01")) );
  Serial.println( pc(F("a02")) );
  Serial.println( pc(F("a03")) );
  Serial.println( pc(F("a04")) );
  Serial.println();
  delay(1000);
}

String pc(const String& var) {
  str.clear(); //str = "";
  if (var == F("a01")) {
    return str += String(l, 3);
  }
  else if (var == F("a02")) {
    return str +=  String(m, 4);
  }
  else if (var == F("a03")) {
    return str +=  String((char *)n);
  }
  else if (var == F("a04")) {
    return str +=  String((char *)o);
  }
  return String();
}
2

I think the first solution will be the best. Operation with stings use a lot of time and resources.
Check safeString library, it's a better version of String default library

3

why use String?
consider using c-strings and dtostrf()

float l = 7.568, m = 99.7864;
byte n[] = "ESP32 ESP8266";
byte o[] = "Arduino Mega";

void
disp (void)
{
    char s  [80];
    char s0 [10];
    char s1 [10];

    dtostrf (l, 8, 4, s0);
    dtostrf (m, 8, 4, s1);

    sprintf (s, " %s %s %s %s", s0, s1, n, o);
    Serial.println (s);
}

void
loop (void)
{
    disp ();
    delay (1000);
}

void
setup (void)
{
    Serial.begin (115200);
}

   7.5680  99.7864 ESP32 ESP8266 Arduino Mega
   7.5680  99.7864 ESP32 ESP8266 Arduino Mega
   7.5680  99.7864 ESP32 ESP8266 Arduino Mega
   7.5680  99.7864 ESP32 ESP8266 Arduino Mega
   7.5680  99.7864 ESP32 ESP8266 Arduino Mega
4

You should not use the "String" class at all when working on AVR based MCU's. You do not need to copy a substring if you only need the end of it:

char* is_a23(char* val)
{
  if ((val[0] == 'a') && (val[1] == '2') && (val[2] == '3') && (val[3] == 0)) return &val[1];
  return (char*)0;
}

char str1[] = "a23";
char str2[] = "a24";

char *res = is_a23(str1);
if (res) Serial.println(res); //Printed

res = is_a23(str2);
if (res) Serial.println(res); //Not printed

It should be more efficient than working with "String".

5

No, it is a more bloated version of the String library. Don't use either on memory starved MCUs.

2 Likes
6

How is PString-Arduino-lib Library?

7

Just another wrapper around using strings (null terminated arrays of char)

8

Then using any of the mentioned libraries rather than using either String class or revamping the code from the beginning (c-strings or array of char) , isn't the best choice?
And what about using Arduino's built-in String library for the robust microcontrollers like ESP8266 or ESP32?

9

Once you are off AVR MCUs, the String class is used everywhere, including many libraries. It is a matter of available SRAM.

10

I was wondering, Are ESP series microcontrollers, AVR microcontrollers? because they also follow open-source RISC Architecture.

11

No, AVR is an Atmel thing. It is not only the available memory that differs, AVR uses a simple bootloader to launch the code directly on the hardware, whereas ESP's are running an operating system (RTOS) which then handles code execution. AVR uses a very crude memory management compared to ESP's which is more advanced.

12

Ohh! thanks for the information. Unnecessarily I was worrying about, using String class for ESP32!

13

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