Arduino: Text/Character programming:

[last updated: 2026-06-08]
Arduino programming
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On this page:

• Type String:
• define a String
• get length of a String
• concatenate a String
• convert String to char array
• convert String to integer
• remove a portion of a String
• get a portion of a String
• convert some other dataType to String
• Type char:
• convert char to int
• convert int to char
• Type char array:
• define a char array
• reference an individual element of an array
• get length of an array
• convert array to String
• Compare two arrays (strcmp)

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• Type String:
• Define a String:
  String str01 = "a";
  String str01 = "abc";
  String str02 = "def";
  String str01 = String(52); // using 'String Constructor' to define str01 as "52"
  String str0x = 'a'; // THIS DOES NOT WORK
• Get length of a String:
  int str03_length = str03.length(); (note does not include appended null char)
• Concatenate a String:
  [Always initialize strings before concatenating them]
  str01 = str01 + " plus this";
  str01 += " plus this";
  String str03 = str01 + str02;
  str01.concat(" plus this");
  str01.concat(str02);
  str03 = str01 + 'A'; // adding a constant character
  str03 = str01 + "abc"; // adding a constant string
  str01.concat(1234); // add an integer
  str03 = str01 + 123456789; // adding a constant long interger
  str03 = str01 + millis();
  str03 = str01 + analogRead(A0);
      these work because millis and analogRead return an integer or long int,
      which can be added to a String
• Change a String to an integer:
  inString.toInt()
• Change a String to a char array:
  [stringVarName].toCharArray([charArrayName], [charArrayLength])
  String str01 = "abc";
  char newCharArray[10];
  str01.toCharArray(newCharArray,10);
• String str = "This is my string";
  // Length (with one extra character for the null terminator)
  int str_len = str.length() + 1;
  // Prepare the character array (the buffer)
  char char_array[str_len];
  // Copy it over
  str.toCharArray(char_array, str_len);
• Remove a portion of a String:
  String str01 = "Hello World";
  str01.remove(8); // removes chars starting at index 8 (='r') to end of String,
     so that str01 is now: "Hello Wo"
  OR:
  str01.remove(2, 6); // removes 6 chars starting at index 2
     so that str01 is now: "Herld"
• Get a portion of a String:
  String partOfString = wholeString.substring(from, to);
  where partOfString will include the char at the 'from' index, but not the char at the 'to' index.
  If the 'to' index is omitted, the substring will go to the end of the orig wholeString
  Get a single-char (as type char) portion of a String:
  wholeString.charAt(position)
• Convert some other dataType to String:
  given a non-String variable oldVar,
  first define your new String:
        String newString;
  then convert oldVar:
        newString = String(oldVar);

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• Type Char:
• Convert char to int:
  Suppose you have a char = "3", and you want to convert it to the integer 3:
        char01 = "3";
        int int01 = char01 - '0';
  This works because the ASCII code for '3' is 51 and the ASCII for '0' is 48 so 51 - 48 = 3 (as int)
• convert byte ascii_value = 65:
  char text_char = (char)ascii_value; will store the character 'A' in text_char

• Convert int to char:
  itoa ... ???
  For a single-digit integer, use this:
  int x = 3;
  char xChar = x + '0';

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• Type Char Array:
• Define a char array:
  char var01[10] = "hgdf";
  char var2[] = "uytcf";
  char var3[10];
• Reference an individual element of a char array:
  char var01[5] = "adfg"; // defines the array
  var01[0] is the first element of the array ('a')
  char oneElement = var01[2]; // will return 'f'
  int anotherElement = var01[3]; // returns the ascii value of 'g'
• Get length of an array:
  "length" of an array is ambiguous, and has two possible meanings:
  the length of the memory space reserved for the array
  or the number of elements stored in the array

  if you define your array with: ary1[10];
  then sizeof(ary1) returns 10 (the memory reserved for it as you've defined it)
  and unless you've put something into it after defining it,
  strlen(ary1) will be indeterminate
  further, Serial.print(ary1); will also be indeterminate

  
  if you define the array with: ary2[] = "asdf";
  then sizeof(ary2) returns 5, the total memory space reserved for it, which is the actual number of chars in it, plus the null termination
  also, strlen(ary2) returns 4, the actual number of chars in it
  and Serial.print(ary2) will return "asdf"

  if the array was created with eg. ary3[10] = "asdf";
  then sizeof(ary3) will return 10,
  strlen(ary3) will return 4,

  if you define the array as: ary4[10];
  then sizeof(ary4) will always return 10 as noted above,
  and as noted, if you haven't otherwise added elements to it, then strlen(ary4) will be indeterminate,
  but then if you manually add elements to it, with eg.:
  ary4[0] = 'a';
  ary4[1] = 's';
  then strlen(ary4) will STILL be indeterminate,
  unless you remember to manually add a null termination, eg:
  ary4[2] = '\0';
  in which case strlen(ary4) will return 2 (the 'a' and 's' that you manually added)

• Convert char array to String:
  Suppose you have a char array: oldCharArray
  and you want to convert it to a String: newStringFromCharArray
      String newStringFromCharArray = String(oldCharArray);
        However I could not get this to work on my current IDE 2.3.6...
• Compare two arrays ("strings"):
  char tmp1[] = "some text";
  char tmp2[] = "other text";
  int cmp = strcmp(tmp1, tmp2);
  // if cmp = 0, then strings are equal
  // if cmp < 0, then str1 < str2
  // if cmp > 0, then str1 > str2
  In this example, cmp = 4, meaning tmp1 > tmp2
  Surely has to do with ascii values, but I'm otherwise clueless how to calculate/interpret this.
  If you're dealing with string representations of numbers, the situation is clear and easy to understand.

        However I could not get this to work on my current IDE 2.3.6.
  Cmp value changed each time through loop even though parameters didn't...

• null termination notes:
  • char test1[] = {1,2,3,4,5};
    initializes the elements explicitly, so no terminating null is added. the compiler assumes you know what you're doing.
  • char test1[] = "chars";
    initializes the array with a string constant, ie by definition a string, so
    the compiler will add a null, and will dimension the array as 6 bytes long.
  • The null character is used by some methods to indicate the end of the string.
    eg. strcpy, strlen, etc. need some way to know when the end of the string is reached. the null provides that.
    If you never use such operations, and always access the array explicitly, eg. tmpAry[x], then you don't need the null termination.
• Multi-dimensional char arrays:
  • char mainMenu[3][11] = { {"exit/run? "}, {"program? "}, {"manual? "} };
    then this will retrieve the nth char array:
    menuItemToDisplay = String(mainMenu[n]);

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