avr c: creating blink / beep sound based on characters in string - c

Question

I am trying to create beep sound from characters in the string.
Here is the code:
/*
* Buzzer connected to Arduino uno digital pin 13
* Switch connected to digital pin 2
*/
#include <avr/io.h>
#include <util/delay.h>
const int TBEEP = 1000;
const int TBEEEEP = 3500;
const int TGAP = 500;
const int TGAPLETTER = 2000;
int portb = 0x20;
void beep() {
PORTB = ~portb; _delay_ms(TGAP);
PORTB = portb; _delay_ms(TBEEP);
PORTB = ~portb; _delay_ms(TGAP);
}
void beeeep() {
PORTB = ~portb; _delay_ms(TGAP);
PORTB = portb; _delay_ms(TBEEEEP);
PORTB = ~portb; _delay_ms(TGAP);
}
void gapLetter() {
PORTB = ~portb; _delay_ms(TGAPLETTER);
}
void morse_S() {
beep(); beep(); beep();
gapLetter();
}
void morse_M() {
beeeep(); beeeep();
gapLetter();
}
void morse_SMS() {
morse_S(); morse_M(); morse_S();
}
void morse(char theString[]) {
for (int i = 0; theString[i] != '\0'; i++)
{
if(&theString[i] == "S")
morse_S();
else if(&theString[i] == "M")
morse_M();
}
}
int main (void)
{
DDRB = 0xFF;
DDRD = 0x00;
PORTD = 0x04;
while (1) {
if (PIND & 0x04) {
PORTB = ~0x20;
} else {
//morse_SMS(); // it works
morse("SMS"); // this one doesnt work like morse_SMS() PLEASE HELP!
}
}
return 0;
}
In function void morse(char theString[]) {...}, I want to produce beep sound from every character in the string "SMS". Unfortunately, only the last character can make it.
I am using Atmel Studio 6. When I build solution (F7) there is no error but warning which I dont understand (sorry for being such a total noob)
comparison with string literal results in unspecified behavior [-Waddress]
How to force every character to beep one after another?

Answer 1

First of all,
const int TBEEP = 1000;
const int TBEEEEP = 3500;
These made my day. :)
Apart from that, you should really get a good beginner C book. You can't compare strings using the == operator, because that compares pointers, not contents. If you want to compare strings, use the strcmp() function from the C standard library.
But: in your case, you don't want to compare strings. You want to compare characters. And that can be done with ==, just dereference the character pointer to obtain the actual character, and compare it with a character literal, not with a string literal:
if (theString[i] == 'S')
morse_S();
else if (theString[i] == 'M')
morse_M();
Oh, and probably you want to avoid that enormous chained if...else if...else monster. Assuming UTF-8 or at least ASCII encoding, where the character codes of English letters are in alphabetical order:
void (*morse_funcs[26])();
morse_funcs[0] = morse_A;
morse_funcs[1] = morse_B;
// ...
morse_funcs[25] = morse_Z;
// ...
void morse(const char *s)
{
while (*s)
morse_funcs[tolower(*s++) - 'a']();
}
Also, notice how I changed char * into const char *. If you don't modify a string, tell the compiler that you don't intend to modify it, so you can safely pass in string literals as well.
Even better: don't use a table of function pointers, but a table of Morse codes. Like this:
const char *mcodes[26] = {
".-", // 'a'
"-...", // 'b'
// etc...
"--.." // 'z'
};
void do_morse(const char *code)
{
while (*code)
if (*code++ == '-')
beeeep();
else
beep();
}
void morse(const char *s)
{
while (*s)
do_morse(mcodes[tolower(*s++) - 'a']);
}

Answer 2

Try this:
void morse(char theString[]) {
for (int i = 0; theString[i] != '\0'; i++)
{
if(theString[i] == 'S')
morse_S();
else if(theString[i] == 'M')
morse_M();
}
}