Const string and array

[olivierh]

I come back with my array ...

I use MikroC 2.0.0.3.

The following code compile :

Code: Select all

void menu () {
      short i;
      char m1[] = "Calibration";
      char m2[] = "Sauve config.";
      char m3[] = "Quitter";
      short code;
      char scode[5];

      // char * m[] = {m1, m2, m3};
      char * m[4];
      m[0] = m1;
      m[1] = m2;
      m[2] = m3;
      m[3] = '\0';

      for (i = 0 ; i < 3; i++) {
          LCD_Out (1, 1, m[i]);
          Delay_ms (200);
       }

but if I try

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      const char m1[] = "Calibration";
      const char m2[] = "Sauve config.";
      const char m3[] = "Quitter";
      short code;
      char scode[5];

      // char * m[] = {m1, m2, m3};
      char * m[4];
      m[0] = m1;     // <-- Illegal cast
      m[1] = m2;     // <-- Illegal cast
      ....

does not compile.

When I don't use const, th following code does not compile :

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for (i = 0 ; m[i] != '\0'; i++) {

I get an

Operator [] not applicable to this operands []

[gambrose]

you need to declare m as a array of pointers to constants not variables.

[pizon]

You have to use it this way:

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const char m1[] = "Calibration";
const char m2[] = "Sauve config.";
const char m3[] = "Quitter";

short code;
char scode[5];
char msgl;

void main() {
      //const char * m[] = {m1, m2, m3};
      const char * m[4];    // <-- This is how it should be done
      m[0] = m1;            // Now OK
      m[1] = m2;            // Now OK

      msgl = *(m[0] + 3);   // This moves the letter 'i' to the msgl variable
}//~!

The char and const char are different types and typecasting between them is not allowed.

Considering the

for (i = 0 ; m != '\0'; i++)

m is an array of pointers to char. By assigning the '\0' value to it, you created a null pointer, however this time the escape sequence didn't produce the expected result (0x000000), but something different (0x00002C). This could be a bug (and we'll take a look further to it), but the general advice is to use the escape sequences as little as possible, to avoid the ambiguities that might arise. Speaking of which, the sequence '\000' compiles properly to 0x000000. So, in order to compile it properly, you should write

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m[4] = 0 ;  // Explicitly declaring null pointer
...
for (i = 0 ; m[i] != 0; i++)  {...

, or, in your style

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m[4] = '\000';   // This will work also, but is not recommended
...
for (i = 0 ; m[i] != '\000'; i++) {...

For further explanations, please read the "Pointers" and "Escape Sequences" chapters from mikroC's Help.

[olivierh]

[quote="pizon"]You have to use it this way:

Code: Select all

const char m1[] = "Calibration";
const char m2[] = "Sauve config.";
const char m3[] = "Quitter";

short code;
char scode[5];
char msgl;

void main() {
      //const char * m[] = {m1, m2, m3};
      const char * m[4];    // <-- This is how it should be done
      m[0] = m1;            // Now OK
      m[1] = m2;            // Now OK

      msgl = *(m[0] + 3);   // This moves the letter 'i' to the msgl variable
}//~!

The char and const char are different types and typecasting between them is not allowed.


Ok, gambrose is right, I have to declare m as a array of pointers to constants. So if I use

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const char * m[4];

the following code

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LCD_Out (1, 1, m[i]);

give an Illegal cast error.

How can I display a constant string to the LCD ?

[gambrose]

well you can either load the const string into a variable string then call LCD_Out or my personal choice is to write your own function that takes a pointer to a const char and loops through the string calling LCD_Chr_CP.

[olivierh]

well you can either load the const string into a variable string then call LCD_Out

So I have to loops through the string to copy it in a variable, and call LCD_Out which perhaps call LCD_Chr_CP

or my personal choice is to write your own function that takes a pointer to a const char and loops through the string calling LCD_Chr_CP.

I think it's a good choice.
I'll try tomorrow.