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Introduction to Computing : Lecture 16 Character Strings

Introduction to Computing : Lecture 16 Character Strings. Dr. Bekir KARLIK Yasar University Department of Computer Engineering bekir. karlik@ yasar.edu.tr. Topics . Strings Representation Declaration Functions Common mistakes Index of a char in a string. On the Nature of Strings.

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Introduction to Computing : Lecture 16 Character Strings

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  1. Introduction to Computing: Lecture 16Character Strings • Dr. Bekir KARLIK • Yasar University • Department of Computer Engineering • bekir.karlik@yasar.edu.tr

  2. Topics • Strings • Representation • Declaration • Functions • Common mistakes • Index of a char in a string

  3. On the Nature of Strings • Recall: Main memory • contiguous array of cells • each cell has an address 0x1FFE 0x1FFF 0x2000 0x2001 0x2002 etc.

  4. ch On the Nature of Strings (cont.) • Recall:Variable declaration • sets aside a “box” to contain a value Example: char ch; ch = ‘B’; 0x1FFE 0x1FFF 0x2000 0x2001 0x2002 etc. ‘B’

  5. On the Nature of Strings (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

  6. name is 0x2000 On the Nature of Strings (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]; 0x2000 0x2004

  7. Character Strings Declaration 1: char name[5]; Declaration 2: #define MAXLENGTH 5 char name[MAXLENGTH]; name is 0x2000 0x2000 0x2004

  8. String Input/Output No ampersand (&)! #include <stdio.h> #define MAXLENGTH 15 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; scanf("%s %s", string1, string2); printf("%s %s\n", string1, string2); return 0; }

  9. Character Strings • Terminating Character: • Marks the end of string • Special char:’\0’ • aka NUL (single L) Initialization: char name[5] = “Ali”; name is 0x2000 A l i \0 0x2000 0x2004

  10. Character Strings Can store at most4 letters, because of `\0’ Initialization: char name[5] = “Ali”; name is 0x2000 A l i \0 0x2000 0x2004

  11. Character Strings Takes up an extra cell for ‘\0’ Declaration 3: char name[] = “Ali”; name is 0x2000 A l i \0 0x2000 0x2003

  12. Character Strings Result is “undefined” if you try to modify this string. Declaration 4: char *name = “Ali”; 0x3000 A l i \0 name 0x3000 0x3003

  13. Character Strings Declaration 5: char name[]; String with arbitrary length? No! Will cause an error.

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

  15. A Char in a String 0x3995 0x399C name is 0x3995 A i s e \0 index 0 index 4 char name[8] = “Aise”; int i = 2; printf(“Char at index %d is %c.\n”, i, name[i]); output:Char at index 2 is h.

  16. A Char in a String 0x3995 0x399C name is 0x3995 A i s e \0 index 2 char name[8] = “Aise”; name[2] = ‘X’; printf(“Name: %s\n”, name);

  17. A Char in a String 0x3995 0x399C name is 0x3995 A i X e \0 index 2 char name[8] = “Aise”; name[2] = ‘X’; printf(“Name: %s\n”, name); output:Name: AiXe

  18. String Operations • #include <string.h> • Operations: • Assignment:strcpy() • Concatenation: strcat() • Comparison: strcmp() • Length:strlen()

  19. String Operation: Assignment #include <stdio.h> #include <string.h> #define MAXLENGTH 100 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Hello World!”); strcpy(string2, string1); return 0; } string1:<garbage> string2:<garbage>

  20. String Operation: Assignment #include <stdio.h> #include <string.h> #define MAXLENGTH 100 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Hello World!”); strcpy(string2, string1); return 0; } string1:“Hello World!” string2:<garbage>

  21. String Operation: Assignment #include <stdio.h> #include <string.h> #define MAXLENGTH 100 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Hello World!”); strcpy(string2, string1); return 0; } string1:“Hello World!” string2:“Hello World!”

  22. Common Mistake: Wrong Assignment Example 1: char name1[5] = “Ali”; char name2[5] = “Sami”; name2 = name1; Error:“LValue required....”

  23. Common Mistake: Bad Assignment Example 2: char *name1 = “Ali”; char *name2 = “Sami”; name2 = name1; Better avoid initialising strings this way. (Usually, no error message.)

  24. 0x2000 A l i \0 name1 0x2000 0x2003 0x3990 S a m i \0 name2 0x3990 0x3994 Common Mistake: Bad Assignment char *name1 = “Ali”; char *name2 = “Sami”;

  25. Common Mistake: Bad Assignment name2 = name1; 0x2000 A l i \0 name1 0x2000 0x2003 0x2000 S a m i \0 name2 0x3990 0x3994

  26. Common Mistake: Not enough space char name[] = “Ali”; strcpy(name, “Samir”); name is 0x2000 A l i \0 0x2000 0x2003

  27. Common Mistake: Not enough space Requires caution. char name[] = “Ali”; strcpy(name, “Samir”); name is 0x2000 S a m i r \0 0x2000 0x2003

  28. String Operation: Concatenation char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Goodbye”); strcpy(string2, “, Cruel ”); strcat(string1, string2); strcat(string1, string2); strcat(string1, “World!”); string1:“Goodbye” string2:“, Cruel “

  29. String Operation: Concatenation char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Goodbye”); strcpy(string2, “, Cruel ”); strcat(string1, string2); strcat(string1, string2); strcat(string1, “World!”); string1:“Goodbye, Cruel ” string2:“, Cruel “

  30. String Operation: Concatenation char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Goodbye”); strcpy(string2, “, Cruel ”); strcat(string1, string2); strcat(string1, string2); strcat(string1, “World!”); string1:“Goodbye, Cruel , Cruel ” string2:“, Cruel “

  31. String Operation: Concatenation char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Goodbye”); strcpy(string2, “, Cruel ”); strcat(string1, string2); strcat(string1, string2); strcat(string1, “World!”); string1:“Goodbye, Cruel , Cruel World!” string2:“, Cruel “

  32. Common Mistake: Not enough space char name[5]; strcpy(name, “Ali”); strcat(name, “ Osman”); name is 0x2000 A l i \0 0x2000 0x2004

  33. Common Mistake: Not enough space char name[5]; strcpy(name, “Ali”); strcat(name, “ Osman”); name is 0x2000 A l i O s m a n \0 0x2000 0x2004

  34. String Operation: Comparison strcpy(string1, “Apple”); strcpy(string2, “Wax”); if (strcmp(string1, string2) < 0) { printf(“%s %s\n”, string1, string2); } else { printf(“%s %s\n”, string2, string1); } output:Apple Wax

  35. String Operation: Comparison strcpy(string1, “Apple”); strcpy(string2, “Wax”); if (strcmp(string1, string2) < 0) { printf(“%s %s\n”, string1, string2); } else { printf(“%s %s\n”, string2, string1); } Returns: negative if string1 < string2 zero if string1 == string2 positive if string1 > string2

  36. Common Mistake: Wrong Comparison strcpy(string1, “Apple”); strcpy(string2, “Wax”); if (string1 < string2) { printf(“%s %s\n”, string1, string2); } else { printf(“%s %s\n”, string2, string1); }

  37. Number of char’s before the `\0’. String Operation: Length char string1[100]; strcpy(string1, “Apple”); printf(“%d\n”, strlen(string1)); output:5

  38. name is 0x3990 B e k i r \0 0x3990 0x3994 Common Mistake: Not enough space char name[5]; strcpy(name, “Bekir”); Don’t forget the ‘\0’.

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

  40. user Bekir\0 Example: hello3.c #include <stdio.h> #include <string.h> #define NAMELEN 50 /* Print a simple greeting to the user. */ void Greet ( char * name ) { strcat(name,"! How are you?"); } int main() { char user[NAMELEN]; printf("Who are you? "); scanf("%s", user); Greet(user); printf("%s\n", user); return 0; }

  41. Example: hello3.c #include <stdio.h> #include <string.h> #define NAMELEN 50 /* Print a simple greeting to the user. */ void Greet ( char * name ) { strcat(name,"! How are you?"); } int main() { char user[NAMELEN]; printf("Who are you? "); scanf("%s", user); Greet(user); printf("%s\n", user); return 0; } name user Bekir\0

  42. Example: hello3.c #include <stdio.h> #include <string.h> #define NAMELEN 50 /* Print a simple greeting to the user. */ void Greet ( char * name ) { strcat(name,"! How are you?"); } int main() { char user[NAMELEN]; printf("Who are you? "); scanf("%s", user); Greet(user); printf("%s\n", user); return 0; } name user Bekir! How are you?\0

  43. Example: hello3.c #include <stdio.h> #include <string.h> #define NAMELEN 50 /* Print a simple greeting to the user. */ void Greet ( char * name ) { strcat(name,"! How are you?"); } int main() { char user[NAMELEN]; printf("Who are you? "); scanf("%s", user); Greet(user); printf("%s\n", user); return 0; } user Bekir! How are you?\0

  44. 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("%s\n", user); return 0; }

  45. Summary • A string is a contiguous array of chars • The string identifier is the address of the first char in the string • Individual chars are accessed using the str[index] notation • There are C library functions for copying, concatenating and comparing strings.

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