How can I pass a char array as the parameter to a function?

Question

I am creating a menu for adjusting system variables.

The menu is made up of pointers like so:

char* options4[] = {"hMin1", "hMax1", "refr1", "fSpeed1"};

I want to pass the selected string as the parameter for an 'adjust' function for use in some conditional logic and for display on a HCMAX7219 7 segment display. Here's what I've got:

int option = 0;
float variable = 82.0;
if(Serial.available() > 0) {
 key = Serial.read();
 if(key == back) {
 return;
 } else if(key == fwd) {
 float newVar = adjust(String(options4[option%4]), variable);
 } else if(key == down) {
 option++;
 } else if(key == up) {
 option--;
 }
}
float adjust(char* str, float var) {
 ...do some stuff...
 display.print7Seg(str, 8);
 EDIT: forgot this call..
 display.print7Seg(var, 1, 4);
 ...do stuff to var...
 return var;
}

The library function takes (char[] TextString, unsigned int Offset) as it's parameters.

EDIT: the library also has functions print7Seg(long number, byte decimalPlace, unsigned int Offset) and print7Seg(long number, unsigned int Offset)

Now I'm new to pointers, but I can see that I am passing a pointer to my adjust function, and expecting it to dig out a char array from memory. I can't see any problems with that, though I may be missing something. When I try to compile, the IDE just crashes with exit status 84; so no debug info.

Am I going about this the wrong way?

Answer 1

There are one or two things missing in your code. Where is setup() and loop()? Or is it just a snippet?

But answer to the question first. This is not right:

newVar = adjust(String(options4[option%4]), variable);

Should be:

newVar = adjust(options4[option%4], variable);

You want to index the vector of strings and pass one of the elements to the function.

Cheers!

• xeuari: It is just a snippet yeah. The full code is about 300 lines so thought I'd try to keep it brief. There are 12 variables to adjust so a generic function is what I'm aiming for. You're absolutely right about the typecasting on the function call, I didn't spot that. I've commented that line out for compilation, as I'm yet to think of a way of assigning the variable, so that's not the cause. Thanks for the spot though.
• Mikael Patel: Do you mean the cause of the IDE crash?
• xeuari: Yeah, the cause of the IDE crash could be linked to a problem in the code.
• Mikael Patel: Then you will have to post all the code and tell us about the Arduino IDE version, board, etc. Have you turned on the "Show verbose output during:" in the IDE Preferences? Collect as much info as possible before the crash of the IDE.
• Mikael Patel: And I would also suggest changing the title of this post??

Answer 2

Ah .. ok I've just figured it out..

This is a legitimate way to pass char arrays to functions.

The original char* options4[] array is just an array of pointers to char arrays in memory, so passing one of these pointers to a function works fine.

The culprite was actually my float parameter, I was trying to make a library function call with a float to this function: print7Seg(long number, byte decimalPlace, unsigned int Offset).

This is what was causing the IDE to crash.

I got around it by having an int version of each of the variables I need to set. Then typecasting and dividing by a float to get variables that will be used in the main program. like so:

int hMin1Int = 820;
float hMin1 = (float)hMin1Int / 10.0;
int hMax1Int = 880;
float hMax1 = (float)hMax1Int / 10.0;
int refr1 = 30; //minutes
int fSpeed1 = 128;

I was then able to change my function to accept an int, the library function, so far, seems to be ok treating an int as a long, but if not I can just make all my ints longs:

int adjust(char* str, int var) {
 ...do some stuff...
 display.print7Seg(str, 8);
 display.print7Seg(var, 1, 4);
 ...do stuff to var...
 return var;
}

I was asking the wrong question!

Still I hope this will be useful to someone, so I will leave it up. Thanks for the response Mikael Patel.

Moral of the story, know your library.