C programming---String

String Literals

A string literal is a sequence of characters enclosed within double quotes:“hello world”

Escape Sequence in String Literals“Candy\n Is dandy\n But liquor\n Is quicker.\n

CandyIs dandyBut liquorIs quicker.

“\1234” contains two characters: ‘\123’ and ‘4’“\189” contains three characters: ‘\1’ , ‘8’ and ‘9’“\x0” …. “\xff”: hex escape

Continuing a String Literalprintf(“When you come to a fork in the road, \take it --- Yogi Berra”);

Wrecking the program’s indented structure

printf( “When you come to a fork in the road, ” “take it --- Yogi Berra”);

How String Literals Are Stored

•When we call printf and supply a string literal as an argument, what are we actually passing?•C treats string literals as character arrays.•A string literal of length n will be stored in a n + 1 bytes memory.•The last character in the array is the null character as a marker for the end of the string

How String Literals Are Storednull character: ‘\0’Do not confuse it with ‘0’

Since a string literal is stored as an array, the compiler treatsit as a pointer of type char *.

So when printf(“abc”) is called, the address of “abc” is passed.

Operations on String Literal

char *p = “abc”;

char ch = “abc”[1];

char digit_to_hex_char(int digit){ “0123456789ABCDEF”[digit];}

Operations on String Literal

char *p = “abcd”;

*p = ‘o’;

“a” is different from ‘a’“a” is represented by a pointer to a memory address‘a’ is represented by an integerprintf(“a”);printf(‘a’);

String VariablesJava has a String type, but C does not.

Any one-dimensional array of characters can be used to store a string.

There is no faster way to determine the length of a string than a character-by-character search for ‘\0’

String Variable#define STR_LEN 90……..char str[STR_LEN + 1];

char data1[8] = “June 14”;

char data2[9] = “June 14”;

char data3[7] = “June 14”;

J u n e 1 4 \0

J u n e 1 4 \0 \0

J u n e 1 4

char data4[] = “June 14”

Character Arrays V.S. Character Pointers

char data[] = “june 14”;char *data = “june 14”;

Character Arrays V.S. Character Pointers

char str[STR_LEN + 1], *p;p = str;In this way you can manipulate “string” with pointersp[0] = ‘o’;p[1] = ‘r’;

But char *p;p[0] = ‘a’;p[1] = ‘b’;p[2] = ‘c’;

THIS IS WRONG BECAUSE:?

Reading and Writing Strings• Writing strings with printf and puts printf(“%s”, str); puts(str);• Reading strings with scanf and gets scanf(“%s”, str); //skip white space gets(str); //doesn’t skip white space before starting to read the string

//reads until it finds a new-line character

Reading Strings Character by Character

int read_line(char str[], int n){ int ch, i = 0; while((ch = getchar()) != ‘\n’) { if(i < n) str[i++] = ch; } str[i] = ‘\0’; return i;}

Accessing the Characters in a String

int count_spaces(const char s[]){ int count = 0, i; for(i=0; s[i]!=‘\0’; i++) if(s[i] == ‘ ‘) count++; return count;}

int count_spaces(const char s*){ int count = 0; for(; *s!=‘\0’; s++) if(*s == ‘ ‘) count++; return count;}

Using the C String Library

char str[10], str2[10];str1 = “abc”; // WRONGstr2 = str1; // WRONG

if(str1 == str2) // WRONG

<string.h>

The strcpy(String Copy) Function

char *strcpy(char *s1, const char *s2);str2 = “abcd”;

strcpy(str2, “abcd”);strcpy(str1, str2);

strcpy(str1, strcpy(str2, “abcd”));

It is safer to call strncpy, but it is slower.char *strncpy(char *s1, const char *s2, size_t s);

The strlen(String Length) Functionsize_t strlen(const char *s);

int len = strlen(“abc”);

The strcat (String Concatenation) Function

char *strcat(char *s1, const char *s2);

strcpy(str1, “abc”);strcat(str1, “def”);

char str1[6] = “abc”;strcat(str1, “def”); /*wrong*/

strncat(s1, s2, strlen(s1)-strlen(s2)-1);

The strcmp(String Comparison) Function

int strcmp(const char *s1, const char *2);if(strcmp(s1, s2) < 0)

if(strcmp(s1, s2) <= 0)strcmp compares strings based on their lexicographic ordering, which resembles the way words are arranged in a dictionary:(1)The first i characters of s1 and s2 match, but the (i+1)st character of s1 is less than the (i+1)st character of s2. “abc” < “bcd” “abd” < “abe”(2)All characters of s1 match s2, but s1 is shorter than s2. “abc” < “abcd”

The strcmp(String Comparison) Function

(1) The characters in each of the sequences ‘A’-’Z’, ‘a’-’z’, and ‘0’-’9’ have consecutive codes.

(2) All upper-case letters are less than all lower-case letters: ‘A’-’Z’(65-90) ‘a’-’z’(97-122)

(3) Digits are less than letters: ‘0’-’9’(48-57)(4) Spaces are less than all printing characters: 32 in

ASCII

String Idiomssize_t strlen(const char *s){ size_t n; for(n=0; *s!=‘\0’; s++) n++; return n;}

size_t strlen(const char *s){ size_t n = 0; for(; *s!=‘\0’; s++) n++; return n;}

Searching for the End of a String

size_t strlen(const char *s){ size_t n = 0; for(; *s; s++) n++; return n;}

size_t strlen(const char *s){ size_t n = 0; for(; *s++; ) n++; return n;}

String IdiomSearching for the End of a String

size_t strlen(const char *s){ size_t n = 0; while(*s++) n++; return n;}

size_t strlen(const char *s){ const char *p = s; while(*s) s++; return s - p;}

String Idiom

while(*s) s++;

while(*s++) ;

String IdiomCopying a String

char * strcat(char *s1, const char *s2){ char *p = s1; while(*p) p++; while(*p++ = *s++) // idiom ; return s1;}

Array of Strings

char planets[][8] = { “Mercury”, “Venus”, “Earth”, “Mars”, “Jupiter”, “Saturn”, “Uranus”, “Neptune”, “Pluto”};

char *planets[] = { “Mercury”, “Venus”, “Earth”, “Mars”, “Jupiter”, “Saturn”, “Uranus”, “Neptune”, “Pluto”};

Array of Strings

Mercury\0

Venus\0

Earth\0

Mars\0

Jupiter\0

Command-Line Argumentsargv

Program name

-l\0

remind.c\0

int main(int argc, char *argv[]){ ……….}