Taking a single number from a string and converting it into an integer

Projects Programming
1

I have a string of single digit numbers String numstring = "123456789" then I want to take a character from that string and convert it into an integer.

I Have tried combinations of .charAt() and .toInt but I cannot seem to hit the correct method. I couldn't find an explanation to such a simple thing from a search on google either.

I'm sure it is simple, can anyone explain a method?

2

My approach is to avoid Strings, because they cause memory problems and program crashes on MCUs, and use C-strings (zero terminated character arrays) instead.

char numstring[]="12345";
int num = atoi(numstring);
// or for larger numbers (greater than the maximum for int)
long num2 = atol(numstring);
3 
int num = numstring.charAt(3) - '0';
1 Like
4

So you just want a single digit converted?

String numstring = "123456789";

void setup()
{
  Serial.begin(115200);  
  int number = numstring.charAt(3) - '0'; // At index 3
  Serial.println(number);
}


void loop()
{}

Edit... @Whandall beat me to it :slight_smile:

1 Like
5

You are confusing Strings, strings and chars

charAt(), not surprisingly, returns a char

Try this

String numstring = "123456789";

void setup()
{
  Serial.begin(115200);
  char theChar = numstring.charAt(2);
  int theInt = theChar - '0';
  Serial.println(theInt);
  Serial.println(theInt * 9); //prove it is an int
 
}

void loop()
{
}
6

It is very possible that this string of digits will start with a zero. I was avoiding storing the entire number as one number incase the leading zeroes would be affected.

I'm not sure I was clear with my question, I want to extract a single digit number from the String and convert that single digit number into an int.

7

See posts #3 #4 #5.

8 
void setup()
{
  Serial.begin(115200);
  char numstring[] = {"123456789"};
  Serial.print(numstring[3]); // this will print "4"
}
void loop(){}

oops... I forgot to "subtract the character zero" from "the character three"

  Serial.print(numstring[3]-'0'); // this will print "4"
9

Thanks for the help. I thought there might be a way to do it without defining a new variable.

11 
String numstring = "123456789";

void setup()
{
  Serial.begin(115200);
  Serial.println(numstring.substring(2, 3).toInt() * 9); //prove it is an int
}

void loop()
{
}
1 Like
12

Then just say "single digit", because a number or C-string is usually composed of more than one digit.

C-strings are great because you can so easily manipulate them, including changing single digits, and unlike Strings, they don't cause memory problems if used correctly.

As for leading zero digits, that does not matter with the function atoi(). It DOES matter in a C/C++ program, because a leading 0 indicates octal representation. 010 octal = decimal 8.

Try this example

void setup() {
  Serial.begin(115200);
  while (!Serial); //wait for connection

  char numstring[] = "12345";
  int num = atoi(numstring);  //convert number
  Serial.print(numstring);
  Serial.print(" = ");
  Serial.println(num);

  numstring[0] = '0'; //replace the digit '1' with '0'
  Serial.print(numstring);
  Serial.print(" = ");
  Serial.println(atoi(numstring));
}

void loop() {}
13

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