Struggling with arrays of character strings,

Projects Programming
1

I am trying to understand how to pass arrays of character strings to a function. I know I can probably do it easier with String objects, but the project I am working on is probobly going th be pretty big so memory may be an issue..

So, if I define some arrays like this ...

char *array1[] = {“one”,”two”,”three”,”\0”}
char *array2[] = {“alpha”,”Baker”,”charlie”,”delta”,”\0”}

I would like to pass them to a function like...

doSomethingWith(array2);

void doSomethingWith(char *ar) {
Serial.println( ar[1]). // would be “two” if passed array1 or “Baker” if passed array2
}

I know the above is not a functional syntax, but not sure what is. I think there is also a problem with passing an array of indetermanent length (hence the “\0” to find the length.) In most of the languages I’ve used there was a way to handle this, but I am not familiar enough with c++ to figure it out. Any suggestions? I’ve googled the crap out of this, but still haven’t found a relivent solution that I could implement.

My guess there might be some cludge that would work. Still looking.

Two things I don’t do well: brain surgery and explaining my problem)

2

If you want to pass variable length arrays, then either you need to pass the length, or the array needs a terminator. For arrays of pointers to char (which is what an array of C strings is), a good choice is to use NULL as a terminator rather than using a zero-length string. In conditional expressions, a NULL is treated as a logical false, and this creates compact code that other C programmer can understand.

The type of these paramters is [nobbc]char **x[/nobbc], or equivalently [nobbc]char *x[][/nobbc]

char *array1[] = {"one","two","three",NULL};
char *array2[] = {"alpha","Baker","charlie","delta",NULL};

void foo(char **s) {
  while(*s) { 
    Serial.print(*s);
    s++;
  }
}

void bar() {
  foo(array1);
  foo(array2);
}

Although the name 'array1' is a constant, the name 's' in the function 'foo' is not.

To do the equivalent the way you are doing it - using a pointer to an empty string as the terminator, you would:

char *array1[] = {"one","two","three",""};
char *array2[] = {"alpha","Baker","charlie","delta",""};

void foo(char **s) {
  while(**s) { // DETECT IF THE INDEXED STRING IS OF ZERO LENGTH
    Serial.print(*s);
    s++;
  }
}

void bar() {
  foo(array1);
  foo(array2);
}

Using "\0" is unnecessary - the double quotes already get a nul-terminated string. Using "\0" crates an array of char of length 2.

3

Throwing in some 'const' declarations will stop the compiler warnings:

#include "Arduino.h"

void doSomethingWith(char const **);

void setup() {
	Serial.begin(115200);
	delay(2000);
	char const *array2[] = { "alpha", "Baker", "charlie", "delta", nullptr };
	doSomethingWith(array2);
}

void loop() {
}

void doSomethingWith(char const **ar) {
	while (**ar) {
		Serial.println(*ar);
		ar++;
	}
}
4 
  while(**s) { // DETECT IF THE INDEXED STRING IS OF ZERO LENGTH

That is NOT what that code does.

5

Ok, you'all got me much closer. Using the actual array's I am trying to use, here is what I came up with...

#include "Arduino.h"

char const *menu1[] = {"Options","Start","Movement","Shutter",NULL};
char const *menu1_1[] = {"Step Size","Settle Time","Pause Time","Frames","Distance","Exit",NULL};

void displayMenu(char const **);

void setup() {
    Serial.begin (9600);
    }


void loop(){
    displayMenu(char const **menu1);   // <---- error highlights this line.
    }

//for now, I simply want to print the number of elements.
void displayMenu(char const **a) {
    int ln = 0
    while (**a) {
          a++;
          ln++;
          Serial.println(ln);
          }

this results in the following error: "expected primary-expression before 'char'
highlighted line is displayMenu(char const **menu1);

not really sure what its telling me.

6

  displayMenu(char const **menu1); That almost looks like a function prototype.

7

TolpuddleSartre:
That almost looks like a function prototype.

hmmmm... well, the intention was to call the function. I have a prototype up above the line that is throwing the error. Maybe you could give me a clue of what I am doing wrong. Me and C++ are kind of new acquaintances. char string arrays and function prototypes are things that are relatively new to me.

8

See my Reply #2. Function call should just be:

displayMenu(menu1);
9

gfvalvo:
See my Reply #2. Function call should just be:

displayMenu(menu1);

Opps..your right. I missed that. Works exactly as I need it to now. Thank you all very much. Once again, I've learned a lot.