I need to build a char array from two integers

Question

My C is rusty, so please forgive this simple problem.

This line works:

Tft.drawString("Test",200,200,2,WHITE);

But, this has compile errors:

String yStr = "Test";  
Tft.drawString(yStr,200,200,2,WHITE);  

The function in the library:

void drawString(char *string,unsigned int poX, unsigned int poY,unsigned int size,unsigned int fgcolor);

The error is "no known conversion for argument 1 from 'String' to 'char*'".

Conclusion - I need a character array. Here's what I am trying to accomplish.

I have two integers that I would like to display on my Tft screen. For example, 225 and 250 to be displayed as "225,250".

I haven't figured out a way to build the char array for the drawString() function.

Assistance would be appreciated.

---

Thanks for the quick reply Ignacio.
I tried that, but the compiler still doesn't like it.

Here's how I built the string:

string stringXY ;
stringXY += xStr;
stringXY += ",";
stringXY += yStr;
Tft.drawString(stringXY.c_str(),200,200,2,RED);  

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

I am apparently using c_str() wrong?

Answer 1

Put it in a String and then use the c_str() method to get the associated char*.

Answer 2

Many thanks to Ignacio.

I changed the function in the library to:

void drawString(const char *string,unsigned int poX, unsigned int poY,unsigned int size,unsigned int fgcolor);

This fixed the problem and gave me more confidence in my vague understanding of C.

Answer 3

Another option in your situation is to not use String at all (which is preferrable for reasons I won't explain here) and exclusively use plain old C strings and their associated functions:

// Define a string buffer to hold the result, ensure it is long enough
// to hold "N1,N2". An additional char must be added to terminate the string with 0
char result[15+1];
unsigned int N1 = ...;
unsigned int N2 = ...;
// Format output as desired
sprintf(result, "%u,%u", N1, N2);
// Now you can use result with drawString
Tft.drawString(result, 200, 200, 2, RED);

sprintf is a very powerful formatting function that offers you plenty of options.

Note that I took it for granted that your numbers are unsigned int; if these were normal integer (possibly negative), you should replace "%u,%u" with "%d,%d".