1 Introduction to Computing Lecture 11 Character Strings Assist.Prof.Dr. Nükhet ÖZBEK Ege University Department of Electrical&Electronics Engineering

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

3 Introduction Grouping of characters is called a string As C implements the strings using arrays of type char, we could not explore string until we had dealt with arrays Strings are important, because we also have to manipulate textual data as well as numerical data

4 On the Nature of Strings Recall: Main memory –contiguous array of cells –each cell has an address 0x1FFF0x20000x20010x20020x1FFE etc.

5 ch On the Nature of Strings (cont.) Recall: Variable declaration –sets aside a “box” to contain a value Example: char ch; ch = ‘B’; 0x1FFF0x20000x20010x20020x1FFE etc. ‘B’

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

7 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 name is 0x2000

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

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

10 String Input/Output (Alternate functions) #include #define MAXLENGTH 15 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; gets(string1); gets(string2); puts(string1); puts(string2); return 0; }

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

12 Character Strings Initialization: char name[5] = “Ali”; Can store at most 4 letters, because of `\0’ Ali\0 0x2000 0x2004 name is 0x2000

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

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

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

16 Arrays of Strings One string is an array of characters, so an array of strings is a two-dimensional array of characters in which each row is a string Example –char days[7][10] = {“Monday”, “Tuesday”, “Wednesday”, “Thursday”, “Friday”, “Saturday”, “Sunday”};

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

18 char name[8] = “Aise”; int i = 2; printf(“Char at index %d is %c.\n”, i, name[i]); A Char in a String output: Char at index 2 is s. index 0index 4 Aise\0 0x39950x399C name is 0x3995

19 A Char in a String index 2 Aise\0 0x39950x399C name is 0x3995 char name[8] = “Aise”; name[2] = ‘X’; printf(“Name: %s\n”, name);

20 AiXe\0 0x39950x399C name is 0x3995 output: Name: AiXe index 2 char name[8] = “Aise”; name[2] = ‘X’; printf(“Name: %s\n”, name); A Char in a String

21 String Operations #include Operations: –Assignment: strcpy() –Concatenation: strcat() –Comparison: strcmp() –Length: strlen()

22 #include #define MAXLENGTH 100 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Hello World!”); strcpy(string2, string1); return 0; } String Operation: Assignment string1: string2:

23 String Operation: Assignment #include #define MAXLENGTH 100 int main() { char string1[MAXLENGTH]; char string2[MAXLENGTH]; strcpy(string1, “Hello World!”); strcpy(string2, string1); return 0; } string1: “Hello World!” string2:

24 String Operation: Assignment #include #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!”

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

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

27 Common Mistake: Bad Assignment Sami\0 0x39900x3994 Ali\0 0x20000x2003 0x2000 name1 0x3990 name2 char *name1 = “Ali”; char *name2 = “Sami”;

28 Common Mistake: Bad Assignment Sami\0 0x39900x3994 Ali\0 0x20000x2003 0x2000 name1 0x2000 name2 name2 = name1;

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

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

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

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

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

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

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

36 Common Mistake: char name[5]; strcpy(name, “Ali”); strcat(name, “ Osman”); Not enough space AliOsma 0x2000 0x2004 n\0 name is 0x2000

37 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 String Operation: Comparison

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

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

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

41 Common Mistake: char name[5]; strcpy(name, “Osman”); Not enough space Don’t forget the ‘\0’. Osman\0 0x39900x3994 name is 0x3990

42 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.

43 Example: hello3.c #include #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 Ali

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

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

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

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

48 sscanf function sscanf function works exactly like scanf except that instead of taking the data values for its output parameters from the standard input device, it takes data from the string that is its first argument Example sscanf(“ hello”,”%d%lf%s”,&n,&val, word);

49 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 and finding lengths of strings