How do I split an incoming string?

Question

I am sending a list of servo positions via the serial connection to the arduino in the following format

1:90&2:80&3:180

Which would be parsed as:

servoId : Position & servoId : Position & servoId : Position

How would I split these values up, and convert them to an integer?

score 98 · Accepted Answer · edited Feb 17 '16 at 06:26

Contrarily to other answers, I'd rather stay away from String for the following reasons:

dynamic memory usage (that may quickly lead to heap fragmentation and memory exhaustion)
quite slow due to construction/destruction/assignment operators

In an embedded environment like Arduino (even for a Mega that has more SRAM), I'd rather use standard C functions:

strchr(): search for a character in a C string (i.e. char *)
strtok(): splits a C string into substrings, based on a separator character
atoi(): converts a C string to an int

That would lead to the following code sample:

// Calculate based on max input size expected for one command
#define INPUT_SIZE 30
...

// Get next command from Serial (add 1 for final 0)
char input[INPUT_SIZE + 1];
byte size = Serial.readBytes(input, INPUT_SIZE);
// Add the final 0 to end the C string
input[size] = 0;

// Read each command pair 
char* command = strtok(input, "&");
while (command != 0)
{
    // Split the command in two values
    char* separator = strchr(command, ':');
    if (separator != 0)
    {
        // Actually split the string in 2: replace ':' with 0
        *separator = 0;
        int servoId = atoi(command);
        ++separator;
        int position = atoi(separator);

        // Do something with servoId and position
    }
    // Find the next command in input string
    command = strtok(0, "&");
}

The advantage here is that no dynamic memory allocation takes place; you can even declare input as a local variable inside a function that would read the commands and execute them; once the function is returned the size occupied by input (in the stack) is recovered.

score 45 · Answer 2 · edited Feb 06 '24 at 01:08

This function can be used to separate a string into pieces based on what the separating character is.

String xval = getValue(myString, ':', 0);
String yval = getValue(myString, ':', 1);
Serial.println("Y:" + yval);
Serial.print("X:" + xval);

Convert String to int

int xvalue = xval.toInt();
int yvalue = yval.toInt();

This Chunk of code takes a string and separates it based on a given character and returns The item between the separating character

String getValue(String data, char separator, int index)
{
    int found = 0;
    int strIndex[] = { 0, -1 };
    int maxIndex = data.length() - 1;
for (int i = 0; i &lt;= maxIndex &amp;&amp; found &lt;= index; i++) {
    if (data.charAt(i) == separator || i == maxIndex) {
        found++;
        strIndex[0] = strIndex[1] + 1;
        strIndex[1] = (i == maxIndex) ? i+1 : i;
    }
}
return found &gt; index ? data.substring(strIndex[0], strIndex[1]) : &quot;&quot;;

}

score 13 · Answer 3 · edited Feb 10 '24 at 20:57

You could do something like the following, but please take into account several things:

If you use readStringUntil(), it will wait until it receives the character or timeouts. Thus, with your current string, the last position will last a little longer, as it has to wait. You can add a trailing & to avoid this timout. You can easily check this behavior in your monitor, try to send the string with and without the extra & and you will see such timeout delay.

You actually do not need the servo index, you can just send your string of positions, and get the servo index by the value position in the string, something like: 90&80&180&.

If you use the servo index, maybe you want to check it (convert to int, and then match the loop index i) to ensure that nothing went wrong with your message.

You have to check that the returning string from readStringUntil is not empty. If the function timeouts, you didn't receive enough data, and thus any attempt to extract your int values will produce strange results.

void setup() {
  Serial.begin(9600);
}
void loop() {
  for (int i=1; i<=3; i++) {
    String servo = Serial.readStringUntil(':');
    if (servo != "") {
      // here you could check the servo number
      String pos = Serial.readStringUntil('&');
      int int_pos=pos.toInt();
  Serial.println(&quot;Pos&quot;);
  Serial.println(int_pos);
}

}
}

score 9 · Answer 4 · answered Mar 31 '14 at 07:36

9

You can use Stream.readStringUntil(terminator) passing a different terminator for each part.

On each part you then call String.toInt

answered Mar 31 '14 at 07:36

Federico Fissore

1,281
7
10

score 7 · Answer 5 · answered Feb 18 '16 at 14:18

Simplest solution is to use sscanf().

  int id1, id2, id3;
  int pos1, pos2, pos3;
  char* buf = "1:90&2:80&3:180";
  int n = sscanf(buf, "%d:%d&%d:%d&%d:%d", &id1, &pos1, &id2, &pos2, &id3, &pos3);
  Serial.print(F("n="));
  Serial.println(n);
  Serial.print(F("id1="));
  Serial.print(id1);
  Serial.print(F(", pos1="));
  Serial.println(pos1);
  Serial.print(F("id2="));
  Serial.print(id2);
  Serial.print(F(", pos2="));
  Serial.println(pos2);
  Serial.print(F("id3="));
  Serial.print(id3);
  Serial.print(F(", pos3="));
  Serial.println(pos3);

This give the following output:

n=6
id1=1, pos1=90
id2=2, pos2=80
id3=3, pos3=180

Cheers!

score 4 · Answer 6 · edited Jul 19 '16 at 18:16

String getValue(String data, char separator, int index)
{
    int maxIndex = data.length() - 1;
    int j = 0;
    String chunkVal = "";

    for (int i = 0; i <= maxIndex && j <= index; i++)
    {
        chunkVal.concat(data[i]);

        if (data[i] == separator)
        {
            j++;

            if (j > index)
            {
                chunkVal.trim();
                return chunkVal;
            }

            chunkVal = "";
        }
        else if ((i == maxIndex) && (j < index)) {
            chunkVal = "";
            return chunkVal;
        }
    }   
}

score 4 · Answer 7 · edited Jan 17 '17 at 17:16

See example at: https://github.com/BenTommyE/Arduino_getStringPartByNr

// splitting a string and return the part nr index split by separator
String getStringPartByNr(String data, char separator, int index) {
    int stringData = 0;        //variable to count data part nr 
    String dataPart = "";      //variable to hole the return text

    for(int i = 0; i<data.length()-1; i++) {    //Walk through the text one letter at a time
        if(data[i]==separator) {
            //Count the number of times separator character appears in the text
            stringData++;
        } else if(stringData==index) {
            //get the text when separator is the rignt one
            dataPart.concat(data[i]);
        } else if(stringData>index) {
            //return text and stop if the next separator appears - to save CPU-time
            return dataPart;
            break;
        }
    }
    //return text if this is the last part
    return dataPart;
}

score 3 · Answer 8 · answered Mar 14 '18 at 21:16

jfpoilpret provided great answer for parsing serial command on Arduino. However Attiny85 doesn't have bidirectional serial - SoftwareSerial has to be used. This is how you port same code for Attiny85

#include <SoftwareSerial.h>

// Calculate based on max input size expected for one command
#define INPUT_SIZE 30

// Initialize SoftwareSerial
SoftwareSerial mySerial(3, 4); // RX=PB3, TX=PB4

// Parameter for receiving Serial command (add 1 for final 0)
char input[INPUT_SIZE + 1];

void setup() {
  mySerial.begin(9600);
}

void loop() {
  // We need this counter to simulate Serial.readBytes which SoftwareSerial lacks
  int key = 0;

  // Start receiving command from Serial
  while (mySerial.available()) {
    delay(3);  // Delay to allow buffer to fill, code gets unstable on Attiny85 without this for some reason
    // Don't read more characters than defined
    if (key < INPUT_SIZE && mySerial.available()) {
      input[key] = mySerial.read();
      key += 1;
    }
  }

  if (key > 0) {
    // Add the final 0 to end the C string
    input[key] = 0;

    // Read each command pair
    char* command = strtok(input, "&");
    while (command != 0)
    {
      // Split the command in two values
      char* separator = strchr(command, ':');
      if (separator != 0)
      {
        // Actually split the string in 2: replace ':' with 0
        *separator = 0;
        int servoId = atoi(command);
        ++separator;
        int position = atoi(separator);
      }
      // Find the next command in input string
      command = strtok(0, "&");
    }
  }
}

Attiny85 schematics for pin numbers

Sketch compiles into:

Sketch uses 2244 bytes (27%) of program storage space. Maximum is 8192 bytes.
Global variables use 161 bytes (31%) of dynamic memory, leaving 351 bytes for local variables. Maximum is 512 bytes.

So there is plenty of space and memory for the rest of code

score 1 · Answer 9 · edited Apr 13 '17 at 12:50

Here is Arduino method to split a String as answer to the question "How to split a string in substring?" declared as a duplicate of the present question.

The objective of the solution is to parse a series of GPS positions logged into a SD card file. Instead of having a String received from Serial, the String is read from file.

The function is StringSplit() parse a String sLine = "1.12345,4.56789,hello" to 3 Strings sParams[0]="1.12345", sParams[1]="4.56789" & sParams[2]="hello".

String sInput: the input lines to be parsed,
char cDelim: the delimiter character between parameters,
String sParams[]: the output array of parameters,
int iMaxParams: the maximum number of parameters,
Output int: the number of parsed parameters,

The function is based on String::indexOf() and String::substring() :

int StringSplit(String sInput, char cDelim, String sParams[], int iMaxParams)
{
    int iParamCount = 0;
    int iPosDelim, iPosStart = 0;

    do {
        // Searching the delimiter using indexOf()
        iPosDelim = sInput.indexOf(cDelim,iPosStart);
        if (iPosDelim > (iPosStart+1)) {
            // Adding a new parameter using substring() 
            sParams[iParamCount] = sInput.substring(iPosStart,iPosDelim-1);
            iParamCount++;
            // Checking the number of parameters
            if (iParamCount >= iMaxParams) {
                return (iParamCount);
            }
            iPosStart = iPosDelim + 1;
        }
    } while (iPosDelim >= 0);
    if (iParamCount < iMaxParams) {
        // Adding the last parameter as the end of the line
        sParams[iParamCount] = sInput.substring(iPosStart);
        iParamCount++;
    }

    return (iParamCount);
}

And the usage is really simple:

String sParams[3];
int iCount, i;
String sLine;

// reading the line from file
sLine = readLine();
// parse only if exists
if (sLine.length() > 0) {
    // parse the line
    iCount = StringSplit(sLine,',',sParams,3);
    // print the extracted paramters
    for(i=0;i<iCount;i++) {
        Serial.print(sParams[i]);
    }
    Serial.println("");
}

score 1 · Answer 10 · answered Jan 22 '18 at 23:34

char str[] = "1:90&2:80&3:180";     // test sample serial input from servo
int servoId;
int position;

char* p = str;
while (sscanf(p, "%d:%d", &servoId, &position) == 2)
{
    // process servoId, position here
    //
    while (*p && *p++ != '&');   // to next id/pos pair
}

score 0 · Answer 11 · answered Jun 20 '19 at 11:56

0

void setup() {
Serial.begin(9600);
char str[] ="1:90&2:80";
char * pch;
pch = strtok(str,"&");
printf ("%s\n",pch);

pch = strtok(NULL,"&"); //pch=next value
printf ("%s\n",pch);
}
void loop(){}

answered Jun 20 '19 at 11:56

Vitalicus

101
1

score 0 · Answer 12 · answered Nov 14 '19 at 17:57

It's not an answer to your question but it may be useful for someone. If your string has a specific prefix, then you can use simply startsWith and substring. E.g.

void loop ()
  if(Serial.available()){
    command = Serial.readStringUntil('\n');
    Serial.println("Received command: " + command);
    if(command.startsWith("height")) {
      Serial.println(command.substring(7));
    }
  }
}

And then send height 10 what will extract 10.

How do I split an incoming string?

12 Answers12

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