CSI 121 Structured Programming Language Lecture 21 Character

CSI 121 Structured Programming Language Lecture 21 Character Strings 1

Topics • • • Representation Declaration Index of a char in a string String operations Common mistakes 2

Representation • Recall: Main memory – contiguous array of cells – each cell has an address 0 x 1 FFE 0 x 1 FFF 0 x 2000 0 x 2001 0 x 2002 etc 3

Representation (cont) • Recall: Variable declaration – sets aside a “box” to contain a value Example: char ch; ch = ‘B’; 0 x 1 FFE 0 x 1 FFF 0 x 2000 ‘B’ ch 0 x 2001 0 x 2002 etc 4

Representation (cont) • String declaration – sets aside an array of cells – each cell contains a char – address of first cell in the array Example: char name[5]; Specifies number of cells in the array 5

Representation (cont) • String declaration – sets aside an array of cells – each cell contains a char – address of first cell in the array Example: char name[5]; name is 0 x 2000 0 x 2004 6

Character Arrays vs Character Strings • A character string is a char array • A character string must have the terminating character (’’) • The terminating character allows scanf() and printf() to handle character strings 7

Character Strings Declaration 1: char name[5]; Declaration 2: #define MAXLENGTH 5 char name[MAXLENGTH]; name is 0 x 2000 0 x 2004 8

String Input/Output #include <stdio. h> #define MAXLENGTH 15 No ampersand (&)! int main() { char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; scanf("%s %s", string 1, string 2); printf("%s %sn", string 1, string 2); return 0; } 9

Character String Declaration Terminating Character: • Marks the end of string • Special char: ’’ • aka NUL (single L) Declaration 1: char name[5] = “Ann”; name A n n is 0 x 2000 0 x 2004 10

Character String Declaration 1: char name[5] = “Ann”; Could have defined this as an array: name char name[5] = {’A’, ’n’, ’’}; A n n is 0 x 2000 0 x 2004 11

Character String Declaration (cont) Can store at most 4 letters, because of `’ Declaration 1: char name[5] = “Ann”; name A n n is 0 x 2000 0 x 2004 12

Character String Declaration (cont) Takes up an extra cell for ‘’ Declaration 2: char name[] = “Ann”; name A n n is 0 x 2000 0 x 2003 13

Character String Declaration (cont) Declaration 3: char *name = “Ann”; 0 x 3000 Result is “undefined” if you try to modify this string A n n name 0 x 3000 0 x 3003 14

Character String Declaration (cont) Declaration 4: char name[]; String with arbitrary length? No! Will cause an error “array size missing in `name’” 15

A Char in a String • The size of a character string is fixed • Character at position index: – string[index] – first character has index 0 16

A Char in a String (cont) 0 x 3995 name J o h 0 x 399 C n is 0 x 3995 index 0 index 4 char name[8] = “John”; int i = 2; printf(“Char at index %d is %c. n”, i, name[i]); output: Char at index 2 is h. 17

A Char in a String (cont) 0 x 3995 name J o h X 0 x 399 C n is 0 x 3995 index 2 char name[8] = “John”; name[2] = ‘X’; printf(“Name: %sn”, name); 18

A Char in a String (cont) 0 x 3995 name J o X 0 x 399 C n is 0 x 3995 index 2 char name[8] = “John”; name[2] = ‘X’; printf(“Name: %sn”, name); output: Name: Jo. Xn 19

String Operations • #include <string. h> • Operations: – Assignment: strcpy() – Concatenation: strcat() – Comparison: strcmp() – Length: strlen() • All relay on and maintain the NUL termination of the strings. 20

String Operation: Assignment #include <stdio. h> #include <string. h> #define MAXLENGTH 100 int main() { char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Hello World!”); strcpy(string 2, string 1); } return 0; string 1: <garbage> string 2: <garbage> 21

String Operation: Assignment (cont) #include <stdio. h> #include <string. h> #define MAXLENGTH 100 int main() { char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Hello World!”); strcpy(string 2, string 1); } return 0; string 1: “Hello World!” string 2: <garbage> 22

String Operation: Assignment (cont) #include <stdio. h> #include <string. h> #define MAXLENGTH 100 int main() { char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Hello World!”); strcpy(string 2, string 1); } return 0; string 1: “Hello World!” string 2: “Hello World!” 23

Common Mistake 1: Incompatible types Example: char name 1[5] = “Ann”; name 2[5] = “Dave”; name 2 = name 1; Error: “LValue required. . . ” 24

Common Mistake 2: Not enough space char name[] = “Ann”; strcpy(name, “David”); name is 0 x 2000 A 0 x 2000 n n 0 x 2003 25

Common Mistake 2: Not enough space char name[] = “Ann”; strcpy(name, “David”); name is 0 x 2000 D 0 x 2000 a v i d 0 x 2003 26

Caution 1: Pointer Assignment Example: char *name 1 = “Ann”; *name 2 = “Dave”; name 2 = name 1; 27

Caution 1: Pointer Assignment char 0 x 2000 A n n *name 1 = “Ann”; *name 2 = “Dave”; name 1 0 x 2000 0 x 3990 D 0 x 2003 a v e name 2 0 x 3990 0 x 3994 28

Caution 1: Pointer Assignment name 2 = name 1; 0 x 2000 A n n name 1 0 x 2000 D 0 x 2003 a v e name 2 0 x 3990 0 x 3994 29

Example: strassign. c #include <stdio. h> #include <string. h> #define MAXLENGTH 5 int main() { char name 1[MAXLENGTH] = “Ann”; char name 2[] = “Ann”; char *name 3 = “John”; char name 4[MAXLENGTH]; printf(“n. BEFOREnname 1=%s, name 2=%s, name 3=%s”, name 1, name 2, name 3); strcpy(name 1, ”Fred”); strcpy(name 2, ”Fred”); strcpy(name 4, name 1); name 3 = name 2; printf(“n. AFTERnname 1=%s, name 2=%s, name 3=%s, name 4=%s”, name 1, name 2, name 3, name 4); } strcpy(name 1, ”Jack”); strcpy(name 2, ”Jim”); printf(“n. LASTnname 1=%s, name 2=%s, name 3=%s, name 4=%s”, name 1, name 2, name 3, name 4); return 0; 30

String Operation: Concatenation char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Goodbye”); strcpy(string 2, “, Cruel ”); strcat(string 1, string 2); strcat(string 1, “World!”); string 1: “Goodbye” string 2: “, Cruel “ 31

String Operation: Concatenation (cont) char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Goodbye”); strcpy(string 2, “, Cruel ”); strcat(string 1, string 2); strcat(string 1, “World!”); string 1: “Goodbye, Cruel ” string 2: “, Cruel ” 32

String Operation: Concatenation (cont) char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Goodbye”); strcpy(string 2, “, Cruel ”); strcat(string 1, string 2); strcat(string 1, “World!”); string 1: “Goodbye, Cruel ” string 2: “, Cruel ” 33

String Operation: Concatenation (cont) char string 1[MAXLENGTH]; char string 2[MAXLENGTH]; strcpy(string 1, “Goodbye”); strcpy(string 2, “, Cruel ”); strcat(string 1, string 2); strcat(string 1, “World!”); string 1: “Goodbye, Cruel World!” string 2: “, Cruel ” 34

Common Mistake: Not enough space char name[5]; strcpy(name, “Ann”); strcat(name, “ Smith”); name A n n is 0 x 2000 0 x 2004 35

Common Mistake: Not enough space char name[5]; strcpy(name, “Ann”); strcat(name, “ Smith”); name A n n S m i t h is 0 x 2000 0 x 2004 36

String Operation: Comparison strcpy(string 1, “Apple”); strcpy(string 2, “Wax”); if (strcmp(string 1, string 2) < 0) { printf(“%s %sn”, string 1, string 2); } else Returns: { printf(“%s %sn”, string 2, ifstring 1); negative string 1 < } string 2 zero if string 1 == string 2 positive if string 1 > string 2 37

String Operation: Comparison (cont) strcpy(string 1, “Apple”); strcpy(string 2, “Wax”); if (strcmp(string 1, string 2) < 0) { printf(“%s %sn”, string 1, string 2); } else { printf(“%s %sn”, string 2, string 1); } output: Apple Wax 38

Common Mistake: Wrong Comparison strcpy(string 1, “Apple”); strcpy(string 2, “Wax”); if (string 1 < string 2) { printf(“%s %sn”, string 1, string 2); } else { printf(“%s %sn”, string 2, string 1); } 39

Caution 1: Not a Boolean strcpy(string 1, “Hi Mum”); strcpy(string 2, “Hi Mum”); if ( strcmp(string 1, string 2) ) { printf(“%s and %s are the samen”, string 1, string 2); } Returns zero if the strings are the same. 40

String Operation: Length char string 1[100]; strcpy(string 1, “Apple”); printf(“%dn”, strlen(string 1)); output: 5 Number of char-s before the `’ 41

Common Mistake: Not enough space char name[5]; strcpy(name, “David”); name D a v Don’t forget the ‘’ i d is 0 x 3990 0 x 3994 42

Character Strings as Parameters • Strings as formal parameters are declared as char* or char[] – Examples: void Greet ( char* name ) void Greet ( char name[] ) • They point to the first element of the string (array of chars) • Changes to the string inside the function affect the actual string 43

Example: hello 3. c #include <stdio. h> #include <string. h> #define NAMELEN 50 int main() { char user[NAMELEN]; /* Print a simple greeting to the user */ printf("Who are you? "); scanf("%s", user); Greet(user); printf("%sn", user); void Greet ( char * name ) { strcat(name, "! How are ya? "); } return 0; } user Jake 44

Example: hello 3. c (cont) #include <stdio. h> #include <string. h> #define NAMELEN 50 int main() { char user[NAMELEN]; /* Print a simple greeting to the user */ printf("Who are you? "); scanf("%s", user); Greet(user); printf("%sn", user); void Greet ( char * name ) { strcat(name, "! How are ya? "); } return 0; } name user Jake 45

Example: hello 3. c (cont) #include <stdio. h> #include <string. h> #define NAMELEN 50 int main() { char user[NAMELEN]; /* Print a simple greeting to the user */ printf("Who are you? "); scanf("%s", user); Greet(user); printf("%sn", user); void Greet ( char * name ) { strcat(name, "! How are ya? "); } return 0; } name user Jake! How are ya? 46

Example: hello 3. c (cont) #include <stdio. h> #include <string. h> #define NAMELEN 50 int main() { char user[NAMELEN]; /* Print a simple greeting to the user */ printf("Who are you? "); scanf("%s", user); Greet(user); printf("%sn", user); void Greet ( char * name ) { strcat(name, "! How are ya? "); } return 0; } user Jake! How are ya? 47

More of scanf demystified No ampersand (&) in scanf with strings! int main() { char user[NAMELEN]; printf("Who are you? "); scanf("%s", user); Greet(user); printf("%sn", user); } return 0; 48