CS 108 Computing Fundamentals March 26, 2015

Class Notes Last day to withdraw from a class is Monday, April 6 Next week I sent s to a small number of folks I think should consider withdrawing Let's start today's class by finishing "Array Boot Camp"

Fundamentals of Characters and Strings Characters in C consist of any printable or nonprintable character in the computer's character set including lowercase letters, uppercase letters, decimal digits, special characters and escape sequences. A character is usually stored in the computer as an 8-bits (1 byte) unsigned integer. The integer value stored for a character depends on the character set used by the computer on which the program is running.

There are two commonly used character sets: ­ ASCII (American Standard Code for Information Interchange) ­ EBCDIC (Extended Binary Coded Decimal Interchange Code) Fundamentals of Characters and Strings

There are two commonly used character sets: ­ ASCII (American Standard Code for Information Interchange) ­ EBCDIC (Extended Binary Coded Decimal Interchange Code) Fundamentals of Characters and Strings

Example: ASCII character #include // 1a.c int main(void) { char demo_A = 'A'; char demo_Z = 'Z'; char demo_a = 'a'; char demo_z = 'z'; printf("\n ASCII integer value for A is %d", demo_A ) ; printf("\n ASCII integer value for Z is %d", demo_Z ) ; printf("\n ASCII integer value for a is %d", demo_a ) ; printf("\n ASCII integer value for z is %d", demo_z ) ; printf("\n\n\n"); printf("\n 65 in ASCII represents %c", 65 ); printf("\n 90 in ASCII represents %c", 90 ); printf("\n 97 in ASCII represents %c", 97 ); printf("\n 122 in ASCII represents %c \n\n", 122 ); return 0 ; }

Example (continued) #include //2a.c int main(void) { char ch; printf("\n\nEnter a character: "); scanf("%c", &ch); if ( ch >= 'A' && ch <= 'Z' ) { printf("\nCapital letter.\n"); } return 0 ; } #include //3a.c int main(void) { char ch; printf("\n\nEnter a character: "); scanf("%c", &ch); if ( ch >= 65 && ch <= (65+26) ) { printf("\nCapital letter.\n"); } return 0 ; } equivalent to

Character Type: char In C, characters are indicated by single quotes: char input_char = 'P'; printf("input_char is %c\n", input_char );

Strings In C a string is really an array of characters that ends with a special character: the null character ( '\0' ) … we use double quotes: "this is a string" // " " means the \0 is added But you can't use an assignment operator to assign a string to a character array char demo[80]; demo = "this is a string"; // Unacceptable assignment

Strings A null ( '\0' ) character is placed to mark the end of each string String functions use '\0' to locate end of string (so you don't need to pass a string's length as argument to a string function) tsisihas\0gnirt

Fundamentals of Characters and Strings A string in C is an array of characters ending with the null character ( '\0' ). It is written inside a double quotation mark ( " " ) A string may be assigned (in a declaration) to either a char array or to a char pointer: char color[ ] = "green";

In C Strings are Character Arrays (logically) Strings in C, in a logical sense, are arrays of characters which terminate with a delimiter (\0 or ASCII NULL character). char str_1 [ ] = {'H', 'e', 'l', 'l', 'o', '\0'}; char str_2 [10] = {'\0'}; str_2 is a ten-element array that can hold a up to nine characters + \0 Strings in C need the delimeter to determine where the string ends (remember: the length of a string is dynamic… it isn't determined before runtime) It's the delimiter position (not the size of the array) that determines the length of the string

Accessing String Characters Like any array, the first element of a string in C is at index 0. The second is at index 1, and so on … char str_2 [10] ; str_2[0] = 'P'; str_2[1] = 'u'; str_2[2] = 'l'; str_2[3] = 'p'; str_2[4] = '\0'; str_2 now contains the string "Pulp" + \0 + 5 unused (at this time) "blanks"

String Literals String literals are given as a string inside double quotes. You've used string literals many times already printf("Enter an inter value:"); String literals are often used to initialize a string array/pointer variable char str_1[10] = "Roswell"; char str_1 [ ] = "Roswell"; Remember: the NULL is added automatically to the end (if there is space available)

Entering Strings with scanf ( ) Use %s format specifier: ­ %s scans up to but does not include next white space ­ %ns scans the next n characters or up to the next white space, whichever is first ­ You only need to provide the starting address of the string with %s Example: scanf ("%s", str_1); scanf ("%2s", str_1); No ampersand (&) necessary when inputting strings (%s) into character arrays! But you an use the & if you like.

#include /* 4a.c */ int main (void) { char str_1[ ] = "Barber"; int index = 0; printf(" \n The pointer variable str_1 holds the value: %p \n ", str_1 ) ; printf(" \n The pointer variable str_1 points to string: %s \n ", str_1) ; printf(" \n Using array notation, str_1 element 0 is at address: %p \n ", &str_1[0] ) ; printf(" \n Using char format specifier and *, str_1 points to: %c \n\n\n\n", *str_1) ; printf("\n Using character format specifier and *, str_1 ele 6 holds the value: %c \n", *(str_1 + 6) ) ; printf("\n Using character format specifier and array notation, str_1 ele 6 holds value: %c \n", str_1[6] ) ; printf("\n Using int format specifier, str_1 ele 6 holds the value: %d \n", *(str_1 +6) ) ; printf("\n Using int format specifier and array notation, str_1 ele 6 holds the value: %d \n\n\n\n", str_1[6] ) ; printf("\n Using char format specifier and *, str_1 element 2 holds the value: %c \n", *(str_1 + 2) ) ; printf("\n Using char format specifier and array notation, str_1 ele 2 holds value: %c \n", str_1[2] ) ; printf("\n Using int format specifier, str_1 element 2 holds the value: %d \n", *(str_1 + 2 ) ) ; printf("\n Using int format specifier and array notation, str_1 element 2 holds the value: %d \n\n\n\n", str_1[2] ) ; return (0) ; }

Reading and Printing a String #include /* 5a.c */ int main ( void ) { char name [ 15 ] = {'\0'}; printf( " \n Enter your first name: " ); scanf( " %s ", name); printf( " \n Your first name is: %s \n\n ", name); printf( " \n Enter your whole name: " ); scanf( " %s ", name); printf( " \n Your whole name is: %s \n\n ", name); return (0); }

Reading and Printing a String #include /* 6a.c … it's safer to limit the characters input*/ int main ( void ) { char name [ 15 ] = {'\0'}; printf( " \n Enter your first name: " ); scanf( " %14s ", name); printf( " \n Your first name is: %s \n\n ", name); printf( " \n Enter your whole name: " ); scanf( " %14s ", name); printf( " \n Your whole name is: %s \n\n ", name); return (0); }

Strings in C We can change parts of a string variable char str_1[6] = "hello"; str_1[3] = 'p'; /* str_1 now contains "helpo" */ str_1[4] = '\0'; /* str_1 now contains "help" */ We must retain the delimiter… by replacing str_1[5] in the original string with a character other than '\0', this makes a string that has no end, which means it is not a string (it is an array of characters) We must stay within limits of array char str_1[6] = "Hello"; /* this is fine */ char str_2[5] = "Hello"; /* this is not fine… no space for the \0)

Strings in C %s format specifier is used to read a string in a scanf( ) call ­ %s ignores all leading white space ­ %s reads all characters until next white space is found ­ %s stores \0 (NULL) after last character ­ %s reads into an array (no & necessary because an array is a pointer constant) Example: char str_1[15]; scanf("%s", str_1); Remember: too many characters for array causes problems

Strings in C Use a width value in with the format specifier to limit the number of characters read: char str_1[15]; scanf("%14s", str_1); Important Point: you always need one space for the \0 so the "effective" width is always one less than the size of the array Strings shorter than the field specification are read normally (as The Terminator might say, "no problemo")

Strings in C %s format specifier used to print a string in printf( ) call: ­ characters in a string printed until \0 encountered char str_1[15] = "Santa"; printf("%s", str_1); Output: Santa Use width value to print string with spaces: printf("%14s", str_1); Output: Santa Use - flag to print string as "left justified": printf("%-14s", str_1); Output: Santa

String Functions  Pretty straightforward  The text is pretty clear  Practice questions at the end of the chapter are useful  Play with your food  We'll cover a variety of these functions today

String Functions #1: strcpy #include // 7a.c #include int main(void) { char your_state[15] ; char my_state[15]; printf("What state are you from? "); gets( your_state ) ; my_state = your_state ; // this won't work… we'll get a compiler error printf("I am from %s and you are from %s, too!\n", my_state, your_state); return(0); }

String Functions #1 continued: strcpy #include // 8a.c #include int main(void) { char your_state[15] ; char my_state[15]; printf("\nWhat state are you from? "); gets( your_state ) ; // compiler will produce a warning… we will discuss this strcpy( my_state, your_state ) ; printf("\nI am from %s and you are from %s, too!\n\n", my_state, your_state ); return(0); }

String Functions #2: strcmp #include // 9a.c #include int main(void) { char magic_word[ ] = "goosfraba"; char guessed_word [ 25 ] ; int result; printf("Try to guess the magic word: "); gets( guessed_word ) ; result = strcmp( magic_word, guessed_word ) ; if(result == 0) puts("That's the correct word!"); else puts("Sorry, that is not the correct word!"); return(0); }

String Functions #3: strcasecmp #include // 10a.c #include int main(void) { char magic_word[ ] = "goosfraba"; char guessed_word [ 25 ] ; int result; printf("Try to guess the magic word:"); gets( guessed_word ) ; result = strcasecmp( magic_word, guessed_word ) ; if(result == 0) puts("That's the correct word!"); else puts("Sorry, that is not the correct word!"); return(0); }

String Functions #4: strcat #include // 11a.c #include int main(void) { char fake_fang_prompt [ ] = ; char snappy_retort [ ] = " I don't do " ; char user_input [100] ; char response [100] ; while (strcasecmp ( user_input, "quit" ) ) { printf("%s", fake_fang_prompt) ; scanf("%s", &user_input) ; strcpy( response, snappy_retort ) ; strcat( response, user_input ) ; puts(response) ; } printf("OK, just for you I'll quit.\n") ; return(0); }

String Functions #5: strlen #include //12a.c #include int main(void) { char user_input[100]; int size = 0, index = 0; printf("\n\nEnter a word: ") ; scanf("%s", user_input ); printf("%s backwards is ", user_input ); size = strlen(user_input) ; for( index = size - 1 ; index >= 0 ; index = index - 1) putchar( user_input [ index ] ) ; printf("\n\n") ; return(0); }

String Functions #6: isspace/isalpha #include // 13a.c #include int main() { char input[200]; int subscript = 0, spaces = 0, letters = 0 ; printf("Enter a sentence: "); gets(input); while( input [ subscript ] ) { if( isspace( input [ subscript ] ) ) spaces = spaces + 1 ; if( isalpha( input [ subscript ] ) ) letters = letters + 1 ; subscript = subscript + 1 ; } printf("Your sentence has %d spaces and %d letters.\n", spaces, letters); return(0); }

String Functions #7: toupper/tolower #include //14a.c #include int main(void) { char input[200]; int index = 0 ; char c ; printf("Enter a sentence: "); gets(input); do { input[index] = toupper( input[index] ) ; index = index + 1; } while(input[index]); puts(input); return(0); }

Playing with String #include //15a.c int main(void) /* a program that counts the number of characters in a string */ { char sentence[ ] = "I love SUNY Poly."; int i, count = 0; for ( i = 0 ; sentence[i] != '\0' ; i++ ) { count++; } printf(" %s: %d characters including the whitespace.\n\n", sentence, count); return 0 ; }

Arrays of Strings (1) Since strings are arrays themselves, using an array of strings can be a little tricky An initialized array of string constants char months[ ][ 10 ] = { "Jan", "Feb", "March", "April", "May", "June", "July", "Aug", "Sept", "Oct", "Nov", "Dec" }; int m; for ( m = 0; m < 12; m++ ) printf( "%s\n", months[ m ] );

Arrays of Strings (2) An array of 12 strings each 20 chars long (remember that we need space for the \0) char names[ 12 ] [ 21 ]; int n; for( n = 0; n < 12; ++n ) { printf( "Please enter your name: " ); scanf( "%20s", names[ n ] ); }

scanf and Strings scanf( ) reads up to the first white space… it ignores the anything typed in after that. Be careful using it. char demo[10]; printf("Enter a string: "); scanf("%9s", demo); // Remember: no & necessary printf("You entered %s\n\n", demo)

scanf and Strings: Safer Approach 1 Use %widths to copy only up to some number number of characters. char demo[10]; printf("Enter a string: "); scanf("%9s", demo); printf("You entered %s\n\n", demo)

scanf and Strings: Safer Approach 2 #include //16a.c #include int main(void) { char user_input[11]; int size = 0, index = 0; printf("\n\nEnter a word: ") ; scanf("%10s", user_input ); printf("%s backwards is ", user_input ); size = strlen(user_input) ; for( index = size - 1 ; index >= 0 ; index = index - 1) putchar( user_input [ index ] ) ; printf("\n\n") ; return(0); }

getchar getchar ( ) will fetch one (1) character from the input stream char demo ; printf("Enter a character: "); demo = getchar( ); printf("You entered %c \n\n", demo);

fgets char * fgets( charArray, lengthLimit, file_pointer of stdin ) fetches a whole line, up to the size limit or when it reads a new line character '\0' added at the end of string Example: char demo[80]; fgets( demo, 79, stdin ) ; // fetch from keyboard Returns a NULL if something went wrong, otherwise a pointer to the array

#include //17.c #include int main(void) { char user_input[100]; int size = 0, index = 0; printf("\n\nEnter a word: ") ; fgets(user_input, 99, stdin ); printf("%s backwards is ", user_input ); size = strlen(user_input) ; for( index = size - 1 ; index >= 0 ; index = index - 1) putchar( user_input [ index ] ) ; printf("\n\n") ; return(0); }

String Copy Thoughts (strcpy) Consider this: char your_state[15] = "Alaska" ; char my_state[7]; printf("What state are you from? "); fgets( your_state, 14, stdin) ; strcpy( my_state, your_state ) ; There is no error checking! ­ If the destination array is shorter than source array, strange things could happen… no compile errors and often no runtime errors… could be a source of a security breach

Safer Approach: strncpy Copies at most N characters from source to destination (or up to '\0') If length of the source is greater than N, it copies only the first N characters If length of the source is less than N, it pads the remaining elements in destination with '\0'

strncpy in Action Consider this: char your_state[15] = "Alaska" ; char my_state[7]; printf("What state are you from? "); fgets( your_state, 14, stdin) ; strncpy( my_state, your_state, 7 ) ;