How to check if a char array is blank (or at least 20 ?)<< SOLVED>>

Projects Programming
1

This is a small test sketch :
I only want to print these lines if they contain anything:
Would anyone please assist me. Many thanks in advance.

char line0 [19];
char line1 [19];
char line2 [19];
char line3 [19];

void setup() {
  
  Serial.begin(9600);  
  while (!Serial);     // wait for serial connection
  Serial.println("Starting... ");
  sprintf(line0, "this line is not blank");
}

void loop() {
// I only want to print these lines if they are not blank
  Serial.println(line0);
  Serial.println(line1);
  Serial.println(line2);
  Serial.println(line3);
}
2

strlen()

3 
  if(strlen(line0) !=0) Serial.println(line0);
is this what you mean - it does not appear to work..
4

Love guessing games! If you did not initialize the lines to anything, they can contain "anything".
What are you trying to do?

5

I only want the lines to print if they contain other than " ".

6

I need to know how not to print line1 Because it only contains blanks.

7

Then you need to set them to blanks when you set them up.

8

Global variables are initialised with zeroes :wink: So they don't contain anything.

Yes, it does work. Your problem is that you are overflowing your line0 with the sprintf so you're writing to memory that is not yours and after that all bets are off. Your lineX can only contain 18 characters taking into account that a c-string is terminated with a '\0'; count the number of characters that you're trying to put in line0.

9

Ok, but I thought the OP was looking for blanks in the line, not nulls.

10

Attempting to write too-long strings into a too-short buffer is a really good use case for snprintf():

snprintf(line0, 18, "this line is not blank")

11

To avoid running off the end of your character array:
snprintf(line0, sizeof line0, "this line is not blank");

bool isAllBlank(const char *str)
{
  for (size_t i = 0; i < strlen(str); i++)
  {
    if (!isblank(str[i]))
      return false;
  }
  return true;
}
12

Thank you - can you explain for me the difference between sprintf and snprintf.
Please

13

sprintf assumes the buffer you provide is large enough to hold all of your output. If your buffer isn't large enough you get undefined results (probably by writing over other variables).

snprintf won't write characters beyond the number of characters you say are available in the buffer. You won't get all of your output if the buffer isn't large enough but at least you won't be writing on other variables.

14

might as well check if 1st byte is 0 for speed

15

not sure I understand - can you elaborate please.

16

Thanks again -

17

OK got it - thanks for this

18

strlen counts bytes until it stumbles on terminating 0. but you don’t need length you need to know if string is empty, that would mean that terminating 0 is the 1st byte. to make absolutely sure it is the 1st byte in newly created array, create it like this

char myarray[n] = {0};

19

That is a little different from:

I took that to mean "empty or only containing space characters" which I extended to mean "empty or only containing whitespace characters: space, tab, newline, or return". (I think that is what "isblank()" tests for)

20

in my book containing anything also implies containing white space