strcat char concatenation trouble

Question

Still don't understand about char concatenation.

void setup() {

  testCounter(3);

}

void loop() {
  // put your main code here, to run repeatedly:
}

char testCounter(int countx){
  char a[10];
  strcpy(a, "COUNTER: ");
  strcat(a, printDigits(countx));
  return a;
}

char printDigits(int digits){
  char a[3];
  if(digits < 10) {
    strcpy(a, "0");
    sprintf(a,"%d",digits);
  } else {
    sprintf(a,"%d",digits);
  }
  return a;
}

Error: invalid conversion from 'char' to 'const char' [-fpermissive]* in testCounter line:

strcat(a, printDigits(countx));

Don't understand this part of error.

printDigits function return char, so... strcat should be work. But why still error?

Please help...

Answer 1

char printDigits(int digits){

You are telling the compiler you will return one character.

 char a[3];

a is three characters. Thus the declaration doesn't match.

---

You could change the declaration to:

char * printDigits(int digits){

But even that would be bad because a would go out of scope after the function returns.

---

 char a[3];

That's a little short for putting an integer into. An integer might be 5 characters, plus possibly a minus sign, and you need a trailing "null" at the end. Seven bytes would be safer.

---

The simplest way of doing what you seem to be wanting is:

void setup() {
  Serial.begin (115200);
  testCounter(3);
}

void loop() {
}

char testCounter(int countx){
  char a[15];
  sprintf (a, "COUNTER: %02d", countx);
  Serial.println (a);
}

---

Also, here:

  if(digits < 10) {
    strcpy(a, "0");
    sprintf(a,"%d",digits);
  } else {

That won't work because you are putting "0" into a and then overwriting it with sprintf.

---

printDigits function return char, so... strcat should be work. But why still error?

Look at the prototype for strcat:

http://www.cplusplus.com/reference/cstring/strcat/

It expects char * not char. They are different things. One is a pointer, one isn't.