Arduino - Strings
Arduino - Strings
Used to store text. also can be used to display text on an LCD (Liquid Crystal Display) or in the Arduino IDE Serial Monitor window.
Example:
void setup() {
char arduino_str[6]; // an array for a 6 character string
Serial.begin(9600);
arduino_str[0] = 'H'; // the string consists of 5 characters (HELLO)
arduino_str[1] = 'E';
arduino_str[2] = 'L';
arduino_str[3] = 'L';
arduino_str[4] = 'O';
arduino_str[5] = 0; // 6th array element is a null terminator
Serial.println(arduino_str);
}
void loop() {
}The following example shows what a string is made up of; a character array with printable characters and 0 as the last element of the array to show that this is where the string ends. The string can be printed out to the Arduino IDE Serial Monitor window by using Serial.println() and passing the name of the string.
This same example above can be written in a more convenient way as shown below
Example
void setup() {
char arduino_str[] = "HELLO";
Serial.begin(9600);
Serial.println(arduino_str); //output HELLO
}
void loop() {
}Manipulating String Arrays
A string array can be altered within a sketch as shown below
void setup() {
char like[] = "I like coffee and cake"; // create a string
Serial.begin(9600);
// (1) print the string
Serial.println(like);
// (2) delete part of the string
like[13] = 0;
Serial.println(like);
// (3) substitute a word into the string
like[13] = ' '; // replace the null terminator with a space
like[18] = 't'; // insert the new word
like[19] = 'e';
like[20] = 'a';
like[21] = 0; // terminate the string
Serial.println(like);
}
void loop() {
}Output:
I like coffee and cake I like coffee I like coffee and tea
Creating and Printing the String
In this sketch, a new string is created and then printed for display in the Serial Monitor window (1).
Shortening the String
The string is shortened by replacing the 14th character in the string with a null terminating zero (2). This is element number 13 in the string array counting from 0.
When the string is printed out, all the characters are printed up to the new null terminating zero. The other characters do not disappear – they still exist in memory and the string array is still the same size. The only difference is that any function that works with strings will only see the string up to the first null terminator.
Changing a Word in the String
Finally the sketch replaces the word "cake" with "tea" (3). It first has to replace the null terminator at like[13] with a space so that the string is restored to how it was originally created.
New characters overwrite "cak" of the work "cake" with the word "tea". This is done by overwriting individual characters. The 'e' of "cake" is replaced with a new null terminating character. The result is that the string is actually terminated with two null characters – the original one at the end of the string and the new one that replaces the 'e' in "cake". This makes no difference when the new string is printed out because the function that prints the string stops printing string characters when it encounters the first null terminator.
Functions to Manipulate String Arrays
The previous sketch manipulated the string in a very manual way by accessing individual characters in the string. To make it easier to manipulate string arrays, you could write your own functions to do so, or use some of the string functions from the C language library.
The next sketch uses some C string functions.
void setup() {
char str[] = "This is my string"; // create a string
char out_str[40]; // output from string functions placed here
int num; // general purpose integer
Serial.begin(9600);
// (1) print the string
Serial.println(str);
// (2) get the length of the string (excludes null terminator)
num = strlen(str);
Serial.print("String length is: ");
Serial.println(num);
// (3) get the length of the array (includes null terminator)
num = sizeof(str); // sizeof() is not a C string function
Serial.print("Size of the array: ");
Serial.println(num);
// (4) copy a string
strcpy(out_str, str);
Serial.println(out_str);
// (5) add a string to the end of a string (append)
strcat(out_str, " sketch.");
Serial.println(out_str);
num = strlen(out_str);
Serial.print("String length is: ");
Serial.println(num);
num = sizeof(out_str);
Serial.print("Size of the array out_str[]: ");
Serial.println(num);
}
void loop() {
}
Output
This is my string String length is: 17 Size of the array: 18 This is my string This is my string sketch. String length is: 25 Size of the array out_str[]: 40
Print the String
The newly created string is printed to the Serial Monitor window as done in previous sketches.
Get the Length of the String
The strlen() function is used to get the length of the string. The length of the string is for the printable characters only and does not include the null terminator.
The string contains 17 characters, so we see 17 printed in the Serial Monitor window.
Get the Length of the Array
The operator sizeof() is used to get the length of the array that contains the string. The length includes the null terminator, so the length is one more than the length of the string.
sizeof() looks like a function, but technically is an operator. It is not a part of the C string library, but was used in the sketch to show the difference between the size of the array and the size of the string (or string length).
Copy a String
The strcpy() function is used to copy the str[] string to the out_num[] array. The strcpy() function copies the second string passed to it into the first string. A copy of the string now exists in the out_num[] array, but only takes up 18 elements of the array, so we still have 22 free char elements in the array. These free elements are found after the string in memory.
The string was copied to the array so that we would have some extra space in the array to use in the next part of the sketch, which is adding a string to the end of a string.
Append a String to a String (Concatenate)
The sketch joins one string to another, which is known as concatenation. This is done using the strcat() function. The strcat() function puts the second string passed to it onto the end of the first string passed to it.
After concatenation, the length of the string is printed to show the new string length. The length of the array is then printed to show that we have a 25-character long string in a 40 element long array.
Remember that the 25-character long string actually takes up 26 characters of the array because of the null terminating zero.
Array Bounds
When working with strings and arrays, it is very important to work within the bounds of strings or arrays. In the example sketch, an array was created, which was 40 characters long, in order to allocate the memory that could be used to manipulate strings.
If the array was made too small and we tried to copy a string that is bigger than the array to it, the string would be copied over the end of the array. The memory beyond the end of the array could contain other important data used in the sketch, which would then be overwritten by our string. If the memory beyond the end of the string is overrun, it could crash the sketch or cause unexpected behavior.
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