Arduino subtracting chars from strings

Question

Hello I have an Arduino programme that has a string with the words hello in it. I can add characters and other strings to the end of that one using:

 String test = "hello";
 test+=" jack";

it would then say "hello jack" The problem is I want to get rid of the last character of the string every 2 seconds so it would be "hello jac" then "hello ja" and so on. How can I do this?

Answer 1

You may use remove() method to remove last character.

String s = "1234567890";
while(s.length() > 0) {
 int lastIndex = s.length() - 1;
 s.remove(lastIndex);
 Serial.println(s);
}

• Avamander: Could you please include an example to your answer?
• IOB Toolkit Team: @Avamander Example provided. :)
• Nick Gammon: Two typos there: Sstring and lenght.

Answer 2

I want to get rid of the last character of the string every 2 seconds so it would be "hello jac" then "hello ja" and so on. How can I do this?

This is how you could do that. Avoid using String and putting the string constants in program memory.

// A buffer for the message
const size_t MSG_MAX = 16;
char msg[MSG_MAX];
void setup()
{
 Serial.begin(9600);
 while (!Serial);
}
void loop()
{
 static int len = 0;
 // Check if it is time to create the initial message
 if (len == 0) {
 strcpy_P(msg, (const char*) F("hello"));
 strcat_P(msg, (const char*) F(" jack"));
 len = strlen(msg) - 1;
 }
 // Otherwise shorten the message
 else {
 msg[len--] = 0;
 }
 // And print current message
 Serial.println(msg);
 delay(2000);
}

Answer 3

You are best of learning about C-style strings rather than the String class. The String class can eventually fragment dynamic memory.

To shorten a string a simple method is to move the "string terminator" inwards by overwriting the last byte of the message with zero, like this:

const size_t MSG_MAX = 20;
char msg [MSG_MAX];
void setup ()
 {
 Serial.begin (115200);
 Serial.println ();
 strcpy (msg, "hello"); // initial message
 strcat (msg, " jack"); // concatenate more stuff
 } // end of setup
 void loop ()
 {
 if (strlen (msg) > 0)
 {
 Serial.println (msg); // display current message
 msg [strlen (msg) - 1] = 0; // move null-terminator in
 delay (1000);
 } // end of any message left
 } // end of loop

Results:

hello jack
hello jac
hello ja
hello j
hello 
hello
hell
hel
he
h