Representing Strings and String I/O

1. Representing Strings and String I/O

2. Introduction • A string is a sequence of characters and is treated as a single data item. • A string constant, also termed a string literal, is anything enclosed in double quotation marks. printf(“Hello world”); • To use a double quotation mark within a string, precede the quotation mark with a backslash (\) printf(“\”Run, Spot, run!\” exclaimed Dick.\n”);

3. Introduction • C standard library provides many functions specifically designed to work with strings. • Reading and writing strings. • Combining strings together. • Copying one string to another. • Comparing strings for equality. • Extracting a portion of a string

4. Declaring and initializing string variables C does not support strings as a data type The only support for strings in the C is that the compiler will translate a quoted string constant into a null-terminated string, which is stored in static memory. The general form of declaration of a string variable is charstring_name[size]; Examples: charcity[10]; char name[30]; A string is a char array terminated with a null character (\0). char city[9] = ”Seoul”; char city[9] = {‘S’, ‘e’, ‘o’, ‘u’, ‘l’, ‘\0’}; string length [] array notation

5. Declaring and initializing string variables • C permits us to initialize a character array without specifying the number of elements. char string [] = {‘G’, ‘O’, ‘O’, ‘D’, ‘\0’}; • The string can be declared with much larger size then the string size In the initializer. charstr[10] = “GOOD”. Set to zeros

6. Reading strings from terminal • The familiar input function scanffunction can be used with %s format specifier. char city[10]; scanf(“%s”, city); • scanf()terminates its input on the first white space it finds. • white space include: • blanks, • tabs • new lines. • If the following line of text is typed in at the terminal, NEW YORK Then only “NEW” will be read into the array address. The ampersand (&) is not required before the variable name. Because city is already address in memory where string is stored.

7. Writing strings to screen • The format %s in the printffunction can be used to display an array of characters that is terminated by the null character. printf(“%s”, name); • We can specify the precision with which the array is displayed. printf(“%10.4s”, name); or printf(“%-10.4s”, name); %10.4 indicates that the first four characters are to be printed in a field width of 10 columns. %-10.4, the string will be printed left-justified

8. Example • The printfsupport feature that allows for variable field width. printf(“%*.*s\n”, w, d, string) • printfthe first d characters of the string in the field width of w.

9. Alternative to printf() • We can use putchar() to output the values of a string. charch = ‘A’; putchar (ch). • We can use this function repeatedly to print out a string. char name [6] = “PARIS”; for (I = 0; I < 5; i++) putchar(name[i]); putchar(‘\n’) • Another convenient way of printing string values is to use the function puts. puts (str);

10. Example

11. Each string has an address Very important example! • The %s format should print the string We. • The %p format produces an address. So if the phrase "are" is an address, then %p should print the address of the first character in the string. Character located at the address “space travellers” Address of “are”

12. Missing Operators • There is no string assignment operators. • There are not string comparison operators. • There are not string combination operators. • However, there are built-in functions to do this common tasks. String functions prototypes defined in <string.h>

13. Built-in String Functions • String assignment. There is no ‘=‘ for string but there isstrcpy( destination, source )char name[ 25 ]; /* contains nothing */strcpy( name, “Hilton” ); /* name now contains “Hilton” */ • String comparison. There is no != for string but there isstrcmp( strA, strB ); • If strA comes after strB, the function returns a positive number. • Is strB comes last, the function returns a negative number. • If strA and strB are the same thing, the function returns a zero. result = strcmp( “CMSC”, “IFSM” ); /* negative */ result = strcmp( “IFSM”, “CMSC” ); /* positive */ result = strcmp( “CMSC”, “CMSC” ); /* zero */

14. Built-in String Functions (cont’d) • String combination:strcat( destination, source ) • The source is not changed. • The destination contain exactly what it had before plus what was in the source. Nothing else is added. NOTE: If you are combining a first name and last name for a full name, you must use another strcat to add the space between them:strcpy( fullName, firstName);strcat( fullName, “ “ );strcat( fullName, lastName );

15. Built-in String Functions (cont’d) • Extracting words (tokens) from a string: /* get the first token (delimited by a blank) */ printf( "%s\n", strtok( str, " " ) ); /* This is more useful after you learn to use pointers. */

16. Built-in String Functions (cont’d) • What if I want to get a menu choice, that is the numbers 1 to 4 or the char ‘q’? Use getchar( ) to get the menu choice, check for ‘q’ and if it is not, then convert it to a number. /* convert a string (ASCII) to an integer */ printf( "%d\n", atoi( "1234" ) ); /* convert a string (ASCII) to a float */ printf( "%f\n", atof( "1234.5678" ) ); • How long is the data in the string (not counting the null terminator)? stringSize = strlen( strA );

17. String Libraries #include files: #include <stdlib.h> /* needed by atoi( ) and atof( ) */ #include <string.h> /* needed by str...( ) functions */

18. Sample Program

19. Sample Program

20. Sample Program Output string a is >Excellence< After strcpy(b, a), string b is now >Excellence< ============= String b = >Excellence< and is 10 characters long After strcat(b, " "), string b = >Excellence < and is 11 characters long After strcat(b, a), string b = >Excellence Excellence< and is 21 characters long strtok( b, " " ) gives Excellence

21. Sample Program Output (cont’d) ============= string a = Excellence string c = Failure strcmp( a, c ) gives -1 strcmp( c, a ) gives 1 strcmp( a, "Excellence" gives 0 After strcmp( "CMSC", "IFSM" ), result is -1 After strcmp( "IFSM", "CMSC" ), result is 1 After strcmp( "CMSC", "CMSC" ), result is 0

22. Sample Program Output (cont’d) ============= atoi( "1234”) gives 1234 atof( "1234.5678" ) gives 1234.567800

23. Other String Functions The statement copies first n characters of the source string s2 into the target string s1. • Since the first ncharacters may not include the terminating null character, we have to place it explicitly in the 6th position of s2 s1[n+1] = ‘\0’; This compares the left-most n characters of s1 to s2 and returns. • 0 if the are equal; • Negative number if s1 sub-string is less than s2; and • Positive number , otherwise.

24. Other String Functions • Concatenate the left-most n characters of s2 to the end of s1. s1 s2 after(s1, s2, 6) • s1 is a string to search within. • s2 is the substring that you want to find.

25. Array Versus Pointer* You can use pointer notation to set up a string. const char *m3 = "\nEnough about me -- what's your name?"; This declaration is very nearly the same as this one:char m3[] = "\nEnough about me -- what's your name?" In short, initializing the array copies a string from static storage to the array, whereas initializing the pointer merely copies the address of the string. char heart[] = "I love Tillie!"; char *head = "I love Millie!"; The difference is that the array name heart is a constant, but the pointer head is a variable. head = heart; /* head now points to the array heart */ heart = head; /* illegal construction */

26. Example: reverse characters in an input string

27. Passing parameter to the main function