Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 1Winter Quarter Strings Lecture 16

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 2Winter Quarter Character Strings Up until now, we have dealt mostly with simple character variables. A variable of type char may hold a single character and is assigned a value using a pair of single quotes: –Example:char onechar = 'z' ; On the other hand, character strings are arrays of simple characters with a special character inserted into the string at the very end. They are assigned values with a pair of double quotes: –Example:char arraychar[6] = "abcde" ;

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 3Winter Quarter Character Strings This array of characters, or string, ends in the special null character ( '\0' ). Strings are normally accessed by a pointer to the first character in the string. This means that the value of a string is the address of it's first character. Thus we say that a string is a pointer, mostly because we often use the string name in manipulating the string. Since the string name is the name of the array of characters, it is a pointer like the name of any other array in C is a pointer.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 4Winter Quarter Character Strings Declaration and initialization: char color [ ] = "scarlet" ; or char *colorPtr = "scarlet" ; or char color [8] = {'s', 'c', 'a', 'r', 'l', 'e', 't', '\0'} ; NOTE: Allowance must always be made for the terminating null character.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 5Winter Quarter String I/O Library Routines #include /* The following are function prototypes for some of the String I/O and Handling Library Routines */ /* Input next character as an integer */ int getchar (void) ; /* Input string into array s until newline */ char *gets (char *s) ;

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 6Winter Quarter String I/O Library Routines #include /* Print character stored in character variable c */ int putchar (int c) ; /* Print character string s followed by \n */ int puts (const char *s) ; /* Performs scanf function on string s */ int sscanf (char *s, const char *format, … ) ;

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 7Winter Quarter String I/O Library Routines #include <notice!!! – from stdio lib !!! /* To input a number of characters from a stream */ char *fgets (char *string1, int n, FILE *stream) ; /* where up to (n - 1) characters are accepted, including newline */ /* To output a string of characters to a stream */ char *fputs (char *string1, FILE *stream) ; /* where all the characters are placed into the I/O stream */

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 8Winter Quarter String Handling Library Routines #include < Notice !!! – from string lib!!! /* To copy string 2 into string 1 */ char *strcpy (char *string1, const char *string2) ; /* To append string 2 to the end of string 1 */ char *strcat (char *string1, const char *string2) ; /* where first character of string 2 replaces the null in string 1 */ /* To find out length of string 1 (# of characters) */ size_t strlen (const char *string1) ; /* where "size_t" is either an "unsigned int" or "unsigned long" */

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 9Winter Quarter String Handling Library Routines #include /* Compare string 1 to string 2. The function returns 0, less than 0, or greater than 0 if string 1 is equal to, less than, or greater than string 2 respectively. */ int strcmp (const char *string1, const char *string2 ) ; /* In this context, less than and greater than depend on the integer values assigned to each of the the individual characters. In the ASCII scheme of things, the integer values (or character codes) are assigned in an order so that we can do things alphabetically, i.e., 'a' is less than 'b' is less than 'c', and so on. */

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 10Winter Quarter Arrays, Strings, and Pointers /* Passing an array to a function */ #include void mysub ( char [ ] ) ; int main ( ) { char name[20] = "Richard J. Freuler" ; mysub (name) ; return 0 ; }

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 11Winter Quarter Arrays, Strings, and Pointers void mysub ( char text[ ] ) { int len, k ; len = strlen (text) ; printf ("%d\n", len ) ; for (k = 0 ; k < len ; k++) printf ("%c", text [k] ) ; } /*Program Output */ 18 RichardJ.Freuler blanks

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 12Winter Quarter Sorting Strings with Pointers A character string is actually just a one- dimensional array of simple character variables. So, an array of character strings is really an array of arrays, that is, a two-dimensional array. It might be declared as follows: char mystrings [ m ] [ n ] ; where m is the number of rows (strings) in the array and n is the maximum number of characters in each string.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 13Winter Quarter Sorting Strings with Pointers Now, C has to know how many characters are actually stored in each string. Recall that a string termination character, or null, ('\0') is inserted after the last character. Note that a string termination character takes one array element to store, therefore the maximum string length that could be stored in one row of the array mystrings is n - 1. Often, a string is stored with a newline character ('\n'), which also takes a character space. Thus, the max string length can be reduced to n - 2.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 14Winter Quarter Sorting Strings with Pointers The address of the beginning of the array mystrings is just the array name mystrings without any subscripts. The address of any string (any row) in the array is mystrings[k] where k is the row number. Since all subscripts start at zero in C, mystrings and mystrings[0] are effectively the same address.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 15Winter Quarter Sorting Strings with Pointers Strings can be sorted using an array of pointers. Since the first element of each string (each row) is the address of the string, that address can be assigned to one of the pointers. When address of each string (or row) is assigned to a pointer, the array of pointers can be sorted and used to retrieve the strings in the new order. The order (location) of the actual strings in memory will NOT be changed.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 16Winter Quarter Sorting Strings with Pointers /* This program sorts strings using array of pointers */ #include #define SIZE 4 int main ( ) { int k, swaps ; char name[SIZE][30], *nptr[SIZE], *temp ; char filename[ ] = "namelist.dat" ; FILE *fptr ; fptr = fopen (filename,"r") ;

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 17Winter Quarter Sorting Strings with Pointers printf ("\n Original list\n") ; for (k = 0 ; k < SIZE ; k++) /* Assign addresses */ { /* to pointers in array */ nptr[k] = name[k] ; fgets (name[k], 30, fptr) ; printf ("%s", name[k]) ; /* Print original list */ }

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 18Winter Quarter Sorting Strings with Pointers … So, the name array might look like: And, assuming name array started at memory location xxx200, the nptr array would look like:

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 19Winter Quarter Sorting Strings with Pointers do /* Sort the pointers in ascending alphabetic */ { /* order of the strings */ swaps = 0 ; for (k = 0 ; k < (SIZE - 1) ; k++) { if (strcmp (nptr[k], nptr[k+1]) > 0) { temp = nptr[k] ; nptr[k] = nptr[k+1] ; nptr[k+1] = temp ; swaps=1 ; } } } while (swaps);

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 20Winter Quarter Sorting Strings with Pointers … So, the name array would still look like: And, the nptr array would end up as:

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 21Winter Quarter Sorting Strings with Pointers printf ("\n New list\n") ; for (k = 0 ; k < SIZE ; k++) printf ("%s", nptr[k]) ; printf ("\n Original list again\n") ; for (k = 0 ; k < SIZE ; k++) printf ("%s", name[k]) ; printf ("\n") ; }

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 22Winter Quarter Sorting Strings with Pointers Original list Smith, Robert A. Sanderson, John T. Alberts, Mary C. Dreese, Edward A. New list Alberts, Mary C. Dreese, Edward A. Sanderson, John T. Smith, Robert A. Original list again Smith, Robert A. Sanderson, John T. Alberts, Mary C. Dreese, Edward A.

Engineering H192 - Computer Programming The Ohio State University Gateway Engineering Education Coalition Lect 16P. 23Winter Quarter Assignment G14 The previous example provides the essential elements for solving G14. Suggestions: –Copy file to your directory –Use ‘more’ to see what is in the file –First write your program to read and print the contents of the file –Then develop your sorting routine for the file.