How to concatenate a part of a string and convert then to a int?

Question

Lets say I have

String myString="AR0236";

I wish to extract the number 236 and assign it to an int. How can this be done?

P.S: I'me new to programming and I tried searching the forums for similar problems but surprising none of them worked for me!

• jsotola: note: concatenate means join.... it is opposite of what you are trying to do, which is split
• MichaelT: see stackoverflow.com/questions/40538792/…?

Answer 1

There's multiple ways.

The String class (which TBH you should avoid) has some useful methods:

• substring - extract part of a string as a new string
• toInt - convert a string to an integer

So you can join them together:

int val = myString.substring(3).toInt();

That's the easy way. Grab the string content from the 4th character (it's zero-based, so 3 is the 4th character) onwards, and convert it to an integer.

If you are avoiding String, as you should, then you are working with pointers. For instance:

char myString[10]; // This is where your data gets stored
strcpy(myString, "AR0236"); // Let's put in some dummy data
int val = atoi(myString + 3);

myString + 3 gives you a new string pointer to a location that is 3 bytes further up in memory from myString - which equates to where the start of your number is. Then convert that portion of the string to an integer.

The "easy" way consumes and makes a mess of your heap, since it's having to make a new String object and copy data into it. The "better" (second) way uses no more memory. It just converts a subset of the existing string directly.

• Razib Sarker: Thank you for your response. The first method works although with a bit of a delay. I wish to use the second and more efficient method but doesnt work if i pass a variable in the second argument of the strcpy. The string I want to convert is from the serial so I the the string is unknown. How would you get around this?
• Majenko: Start by reading the serial properly. Start here: majenko.co.uk/blog/reading-serial-arduino
• Razib Sarker: Thank you! Your link was really useful and my circuit works as intended to :)