Today's lecture: characters and strings

1. The h/w have primitives to represent ints of various sizes and floats.
How to represent text characters?

ASCII  (a convention that maps each character to a one-byte number)
Developed in 60s, based on the English Alphabet
Includes a-z, A-Z, 0-9, some symbols (,.-^><=)
Encodes 128 characters, so the MSB is always zero.

0 (null)
0x30 ('0')
...
0x39 ('9')
0x61 ('a')
0x62 ('b')
...
0x41 ('A')
0x42 ('B')
..

Unicode is the modern standard 
includes 120,000 characters, Chinese, Korean, etc.
UTF-8 is the common implementation of Unicode.
Variable lengths: 1 to 4 bytes.
ASCII bytes are valid UTF-8-encoding (1 byte).

2. Strings 
char *h = "hello";

String is represented an array of characters.
But C does not encode array length, how do we know when is the end?

an array of characters terminated by the null character '\0'
char *h = "hello";
printf("string is %s\n", h);
<draw memory representation>

int 
length(char *s)
{
	int i = 0;
	while (s[i] != '\0') {
		i++;
	}
	return i;
}

Another example:
How to change a string of characters to only lower case?
void ToLower(char *s)
{
	int i = 0;
	while (s[i]!=0) {
		if ((s[i] >= 'A') && (s[i] <= 'Z')) {
			printf("s[i] %c %d\n", s[i], i);
			s[i] = s[i] + 'a' - 'A';
		}			
		i++;
	}
}
int main() {
	char x[20] = "heLLo World";
	ToLower(x);
	printf("%s\n", x);
}

How to implement atoi()?
int Atoi(char *s)
{
	int i = 0;
	int result = 0;
	while (s[i]!=0) {
		if (s[i] >= '0' && s[i] <= '9') {
			result = result * 10 + (s[i] -'0');
		}else{
			break;
		}
	}
	return result
}


3. Strings and pointers

char *w;
w = h; //does it make another copy of hello? 


int 
copy(char *src, char *dst)
{
	int i = 0;
	while (src[i]!= '\0') {
		dst[i] = src[i];
		i++;
	}
}

copy(h, w); //correct?
//change char *w to char w[100];


//the null termination is by convention, not enforced
h[5] = 'a';
printf("string is %s\n", h); //??? undefined
length(h); //???  undefined

4. array of pointers, pointer to pointers
char* names[3] = { "alice", "bob", "clark"};

<draw picture>
names: |_____|_____|_____|
       
       |a l i c e \0|
       |b o b \0|
       |c l a r k \0|


char **namep;

namep =  names; //namep = &names[0];

The most common array of pointers
int
main(int argc, char *argv[]) 
	//alternativesly, char **argv
{
	int i;
	for (i = 0; i < argc; i++) {
		printf("%s ", argv[i]);
	}
	printf("\n");
}

$./a.out 1 2 3
./a.out 1 2 3