NEXT SET OF SLIDES FROM DENNIS FREY S FALL 2011 CMSC313.

Transcription

1 NEXT SET OF SLIDES FROM DENNIS FREY S FALL 2011 CMSC313

2 Programming in C! Characters and Strings!! 1/19/10

3 ASCII! The American Standard Code for Information Interchange (ASCII) character set, has 128 characters designed to encode the Roman alphabet used in English and other Western European languages.! C was designed to work with ASCII and we will only use the ASCII character set in this course. The char data type is used to store ASCII characters in C! ASCII can represent 128 characters and is encoded in one eight bit byte with a leading 0. Seven bits can encode numbers 0 to 127. Since integers in the range of 0 to 127 can be stored in 1 byte of space, the sizeof(char) is 1.! The characters 0 through 31 represent control characters (e.g., line feed, back space), are printable characters, and 127 is delete.! 1/19/10

4 char type! C supports the char data type for storing a single character.! char uses one byte of memory! char constants are enclosed in single quotes! char mygrade = A ;! char yourgrade =? ;!! 1/19/10

5 1/19/10 ASCII Character Chart!

6 Special Characters! The backslash character, \, is used to indicate that the char that follows has special meaning. E.g. for unprintable characters and special characters.! For example! \n is the newline character! \t is the tab character! \ is the double quote (necessary since double quotes are used to enclose strings! \ is the single quote (necessary since single quotes are used to enclose chars! \\ is the backslash (necessary since \ now has special meaning! \a is beep which is unprintable! 1/19/10

7 Special Char Example Code! What is the output from these statements?! printf( \t\tmove over\n\nworld, here I come\n");!!!move over!! World, here I come! printf("i\ ve written \ Hello World\ \n\t many times\n\a ); I ve written Hello World!!many times <beep>! 1/19/10

8 Character Library! There are many functions to handle characters. To use these functions in your code,!!!#include <ctype.h>! Note that the function parameter type is int, not char. Why is this ok?! Note that the return type for some functions is int since ANSI C does not support the bool data type. Recall that zero is false, non-zero is true.! A few of the commonly used functions are listed on the next slide. For a full list of ctype.h functions, type man ctype.h at the unix prompt.! 1/19/10

9 int isdigit (int c);! ctype.h! Determine if c is a decimal digit ( 0-9 )! int isxdigit(int c);! Determines if c is a hexadecimal digit ( 0-9, a - f, or A - F )! int isalpha (int c);! Determines if c is an alphabetic character ( a - z or A- Z )! int isspace (int c);! Determines if c is a whitespace character (space, tab, etc)! int isprint (int c);! Determines if c is a printable character! int tolower (int c);! int toupper (int c);! Returns c changed to lower- or upper-case respectively, if possible! 1/19/10

10 Character Input/Output! Use %c in printf( )and fprintf( )to output a single character.! char yourgrade = A ;! printf( Your grade is %c\n, yourgrade);!! Input char(s) using %c with scanf( ) or fscanf( )!!!char grade, scores[3];! %c inputs the next character, which may be whitespace!scanf( %c, &grade);! %nc inputs the next n characters, which may include whitespace.!!!scanf( %3c, scores);! // note -- no & needed! 1/19/10

11 Array of char! An array of chars may be (partially) initialized. This declaration reserves 20 char (bytes) of memory, but only the first 5 are initialized! char name2 [ 20 ] = { B, o, b, b, y }; You can let the compiler count the chars for you. This declaration allocates and initializes exactly 5 chars (bytes) of memory! char name3 [ ] = { B, o, b, b, y }; An array of chars is NOT a string! 1/19/10

12 Strings in C! In C, a string is an array of characters terminated with the null character ( \0, value = 0, see ASCII chart).! A string may be defined as a char array by initializing the last char to \0! char name4[ 20 ] = { B, o, b, b, y, \0 }; Char arrays are permitted a special initialization using a string constant. Note that the size of the array must account for the \0 character.! char name5[6] = Bobby ; // this is NOT assignment Or let the compiler count the chars and allocate the appropriate array size! char name6[ ] = Bobby ; All string constants are enclosed in double quotes and include the terminating \0 character! 1/19/10

13 String Output! Use %s in printf( ) or fprintf( ) to print a string. All chars will be output until the \0 character is seen.! char name[ ] = Bobby Smith ;! printf( My name is %s\n, name);! As with all conversion specifications, a minimum field width and justification may be specified! char book1[ ] = Flatland ;! char book2[ ] = Brave New World ;!! printf ( My favorite books are %12s and %12s\n, book1, book2);! printf ( My favorite books are %-12s and %-12s\n, book1, book2);!!! 1/19/10

14 Dangerous String Input! The most common and most dangerous method to get string input from the user is to use %s with scanf( ) or fscanf( )! This method interprets the next set of consecutive non-whitespace characters as a string, stores it in the specified char array, and appends a terminating \0 character.! char name[22];! printf( Enter your name: );! scanf( %s, name);! Why is this dangerous?! 1/19/10

15 Safer String Input! A safer method of string input is to use %ns with scanf( ) or fscanf( ) where n is an integer! This will interpret the next set of consecutive nonwhitespace characters up to a maximum of n characters as a string, store it in the specified char array, and append a terminating \0 character.! char name[ 22 ];! printf( Enter your name: );! scanf( %21s, name);!// note 21, not 22! 1/19/10

16 C String Library! C provides a library of string functions.! To use the string functions, include <string.h>. Some of the more common functions are listed here on the next slides.! To see all the string functions, type man string.h at the unix prompt.! 1/19/10

17 C String Library (2)! Commonly used string functions! These functions look for the \0 character to determine the end and size of the string! strlen( const char string[ ] )! Returns the number of characters in the string, not including the null character! strcpy( char s1[ ], const char s2[ ] )! Copies s2 on top of s1.! The order of the parameters mimics the assignment operator! strcmp ( const char s1[ ], const char s2[ ] )! Returns < 0, 0, > 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically! strcat( char s1[ ], const char s2[ ])! Appends (concatenates) s2 to s1! 1/19/10

18 C String Library (3)! Some function in the C String library have an additional size parameter.! strncpy( char s1[ ], const char s2[ ], int n )! Copies at most n characters of s2 on top of s1.! The order of the parameters mimics the assignment operator! strncmp ( const char s1[ ], const char s2[ ], int n )! Compares up to n characters of s1 with s2! Returns < 0, 0, > 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically! strncat( char s1[ ], const char s2[ ], int n)! Appends at most n characters of s2 to s1! 1/19/10

19 String Code! char first[10] = bobby ; char last[15] = smith ; char name[30]; char you[ ] = bobo ; strcpy( name, first ); strcat( name, last ); printf( %d, %s\n, strlen(name), name ); strncpy( name, last, 2 ); printf( %d, %s\n, strlen(name), name ); int result = strcmp( you, first ); result = strncmp( you, first, 3 ); strcat( first, last ); 1/19/10

20 Simple Encryption! char c, msg[] = "this is a secret message"; int i = 0; char code[26] = /* Initialize our encryption code */ {'t','f','h','x','q','j','e','m','u','p','i','d','c', 'k','v','b','a','o','l','r','z','w','g','n','s','y'} ; /* Print the original phrase */ printf ("Original phrase: %s\n", msg); /* Encrypt */ while( msg[i]!= '\0 ){ if( isalpha( msg[ i ] ) ) { c = tolower( msg[ i ] ) ; msg[ i ] = code[ c - a ] ; } ++i; } printf("encrypted: %s\n", msg ) ;

21 Arrays of Strings! Since strings are arrays themselves, using an array of strings can be a little tricky! An initialized array of string constants! char months[ ][ 12 ] = { Jan, Feb,... };! int m;! for ( m = 0; m < 12; m++ )!!printf( %s\n, months[ m ] );! 1/19/10

22 Arrays of Strings (2)! An array of string 12 variables, each 20 chars long! char names[ 12 ] [ 21 ];! int n;! for( n = 0; n < 12; ++n )! {! }!!printf( Please enter your name: );!!scanf( %20s, names[ n ] );! 1/19/10

23 gets( ) to read a line! The gets( ) function is used to read a line of input (including the whitespace) from stdin until the \n character is encountered. The \n character is replaced with the terminating \0 character.!! #include <stdio.h> char mystring[ 101 ]; gets( mystring ); Why is this dangerous?! 1/19/10

24 fgets( ) to read a line! The fgets( ) function is used to read a line of input (including the whitespace) from the specified FILE until the \n character is encountered or until the specified number of chars is read.! 1/19/10

25 fgets( )! #include <stdio.h> #include <stdlib.h> /* exit */ int main ( ) { double x ; FILE *ifp ; char myline[42 ]; /* for terminating \0 */ ifp = fopen("test_data.dat", "r"); if (ifp == NULL) { printf ("Error opening test_data.dat\n"); exit (-1); } fgets(myline, 42, ifp ); /* read up to 41 chars*/ fclose(ifp); /* close the file when finished */ } /* check to see what you read */ printf( myline = %s\n, myline); return 0; 1/19/10

26 Detecting EOF with fgets( )! fgets( ) returns the memory address in which the line was stored (the char array provided). However, when fgets( ) encounters EOF, the special value NULL is returned.! FILE *infile; infile = fopen( myfile, r ); /* check that the file was opened */ char string[120]; while ( fgets(string, 120, infile )!= NULL ) printf( %s\n, string ); fclose( infile );! 1/19/10!

27 Using fgets( ) instead of gets( )! Since fgets( ) can read any file, it can be used in place of gets( ) to get input from the user!! #include <stdio.h> char mystring[ 101 ];!!Instead of! gets( mystring );!Use!!fgets( mystring, 100, stdin ); 1/19/10

28 Big Enough! The owner of a string is responsible for allocating array space which is big enough to store the string (including the null character).! scanf( ), fscanf( ), and gets( ) assume the char array argument is big enough! String functions that do not provide a parameter for the length rely on the \0 character to determine the end of the string.! Most string library functions do not check the size of the string memory. E.g. strcpy 1/19/10

29 What can happen?! int main( ) { char first[10] = "bobby"; char last[15] = "smith"; printf("first contains %d chars: %s\n", strlen(first), first); printf("last contains %d chars: %s\n", strlen(last), last); strcpy(first, " "); /* too big */ printf("first contains %d chars: %s\n", strlen(first), first); printf("last contains %d chars: %s\n", strlen(last), last); } return 0; /* output */ first contains 5 chars: bobby last contains 5 chars: smith first contains 13 chars: last contains 5 chars: smith Segmentation fault 28

30 The Lesson! Avoid scanf( %s, buffer);! Use scanf( %100s, buffer); instead! Avoid gets( );! Use fgets(...,..., stdin); instead! 1/19/10

31 sprintf( )! Sometimes it s necessary to format a string in an array of chars. Something akin to tostring( ) in Java.! sprintf( ) works just like printf( ) or fprintf( ), but puts its output into the specified character array.! As always, the character array must be big enough.!! char message[ 100 ]; int myage = 4; sprintf( message, I am %d years old\n, age); printf( %s\n, message); 1/19/10

32 NEXT SET OF SLIDES FROM DENNIS FREY S FALL 2011 CMSC313

33 Programming in C Structs and Unions

34 Java vs C Suppose you were assigned a write an application about points and straight lines in a coordinate plane. In Java, you d correctly design a Point class and a Line class using composition. What about in C?

35 No Classes in C Because C is not an OOP language, there is no way to combine data and code into a single entity. C does allow us to combine related data into a structure using the keyword struct. All data in a struct variable can be accessed by any code. Think of a struct as an OOP class in which all data members are public, and which has no methods.

36 Struct definition The general form of a structure definition is struct tag Note the semi-colon { member1_declaration; member2_declaration; member3_declaration;... membern_declaration; }; where struct is the keyword, tag names this kind of struct, and member_declarations are variable declarations which define the members.

37 C struct Example Defining a struct to represent a point in a coordinate plane struct point { int x; /* x-coordinate */ int y; /* y-coordinate */ }; Given the declarations struct point p1; struct point p2; point is the struct tag we can access the members of these struct variables: * the x-coordinate of p1 is p1.x * the y-coordinate of p1 is p1.y * the x-coordinate of p2 is p2.x * the y-coordinate of p2 is p2.y

38 Using structs and members The members of a struct are variables just like any other and can be used wherever any other variable of the same type may be used. For example, the members of the struct point can then be used just like any other integer variables.

39 Using struct members int main ( ) { struct point lefeendpt, rightendpt, newendpt; // get x- and y-coordinates of left end point printf( Left end point cooridinates ); scanf( %d %d, &lefeendpt.x, &leftendpt.y); // get x- and y-coordinates of right end point printf( Right end point s x-coordinate: ); scanf( %d %d, &righteendpt.x, &rightendpt.y); // add the endpoints newendpt.x = leftendpt.x + rightendpt.x; newendpt.y = leftendpt.y + rightendpt.y; // print new end point printf( New endpoint (%2d, %2d), newendpt.x, newendpt.); } return 0;

40 Initializing A struct A struct variable may be initialized when it is declared by providing the initial values for each member in the order they appear struct point middle = { 6, -3 }; defines the struct point variable named middle and initializes middle.x = 6 and middle.y = -3

41 struct Variants struct variables may be declared at the same time the struct is defined struct point { int x, y; } endpoint, upperleft; defines the structure named point AND declares the variables endpoint and upperleft to be of this type.

42 struct typedef It s common to use a typedef for the name of a struct to make code more concise. typedef struct point { int x, y; } POINT; defines the structure named point and defines POINT as a typedef (alias) for the struct. We can now declare variables as struct point endpoint; or as POINT upperright;

43 struct assignment The contents of a struct variable may be copied to another struct variable of the same type using the assignment (=) operator After this code is executed struct point p1; struct point p2; p1.x = 42; p1.y = 59; p2 = p1; /* structure assignment copies members */ The values of p2 s members are the same as p1 s members. E.g. p1.x = p2.x = 42 and p1.y = p2.y = 59

44 struct within a struct A data element in a struct may be another struct (similar to composition with classes in Java / C++). This example defines a line in the coordinate plane by specifying its endpoints as POINT structs typedef struct line { POINT leftendpoint; POINT rightendpoint; } LINE; Given the declarations below, how do we access the x- and y-coodinates of each line s endpoints? struct line line1, line2; line1.leftendpoint.x line1.rightendpoint.y line2.leftendpoint.x line2.rightendpoint.y

45 Arrays of struct Since a struct is a variable type, we can create arrays of structs just like we create arrays of primitives. Write the declaration for an array of 5 line structures name lines struct line lines[ 5 ]; or LINE lines[ 5 ]; Write the code to print the x-coordinate of the left end point of the 3rd line in the array printf( %d\n, lines[2].leftendpoint.x); 1/20/10 13

46 Array of struct Code /* assume same point and line struct definitions */ int main( ) { LINE sides[5]; int k; /* write code to initialize all data members to zero */ for (k = 0; k < 5; k++) { sides[k].leftendpoint.x = 0; sides[k].leftendpoint.y = 0; sides[k].rightendpoint.x = 0; sides[k].rightendpoint.y = 0; } /* write the code to print the x-coordinate of ** the left end point of the 3rd line */ } return 0; 14

47 Arrays within a struct Structs may contain arrays as well as primitive types struct month { int nrdays; char name[ ]; }; struct month january = { 31, JAN };

48 A bit more complex struct month allmonths[ 12 ] = { {31, JAN }, {28, FEB }, {31, MAR }, {30, APR }, {31, MAY }, {30, JUN }, {31, JUL }, {31, AUG }, {30, SEP }, {31, OCT }, {30, NOV }, {31, DEC } }; // write the code to print the data for September printf( %s has %d days\n, allmonths[8].name, allmonths[8].nrdays); // what is the value of allmonths[3].name[1] P

49 Size of a struct As with primitive types, we can use sizeof( ) to determine the number of bytes in a struct int pointsize = sizeof( POINT ); int linesize = sizeof (struct line); As we ll see later, the answers may surprise you!

50 Bitfields When saving space in memory or a communications message is of paramount importance, we sometimes need to pack lots of information into a small space. We can use struct syntax to define variables which are as small as 1 bit in size. These variables are known as bit fields. struct weather { }; unsigned int temperature : 5; unsigned int windspeed : 6; unsigned int israining : 1; unsigned int issunny : 1; unsigned int issnowing : 1;

51 Using Bitfields Bit fields are referenced like any other struct member struct weather todaysweather; todaysweather.temperature = 44; todaysweather.issnowing = 0; todaysweather.windspeed = 23; /* etc */ if (todaysweather.israining) printf( %s\n, Take your umbrella ); if (todaysweather.temperature < 32 ) printf( %s\n, Stay home );

52 More on Bit fields Almost everything about bit fields is implementation (machine and compiler) specific. Bit fields may only defined as (unsigned) ints Bit fields do not have addresses, so the & operator cannot be applied to them We ll see more on this later

53 Unions A union is a variable type that may hold different type of members of different sizes, BUT only one type at a time. All members of the union share the same memory. The compiler assigns enough memory for the largest of the member types. The syntax for defining a union and using its members is the same as the syntax for a struct.

54 Formal Union Definition The general form of a union definition is union tag { member1_declaration; member2_declaration; member3_declaration;... membern_declaration; }; where union is the keyword, tag names this kind of union, and member_declarations are variable declarations which define the members. Note that the syntax for defining a union is exactly the same as the syntax for a struct.

55 An application of Unions struct square { int length; }; struct circle { int radius; }; struct rectangle { int width; int height; }; enum shapetype {SQUARE, CIRCLE, RECTANGLE }; union shapes { struct square asquare; struct circle acircle; struct rectangle arectangle; }; struct shape { enum shapetype type; union shapes theshape; };

56 An application of Unions (2) double area( struct shape s) { switch( s.type ) { case SQUARE: return s.theshape.asquare.length * s.theshape.asquare.length; case CIRCLE: return 3.14 * s.theshape.acircle.radius * s.theshape.acircle.radius; case RECTANGLE : return s.theshape.arectangle.height * s.theshape.arectangle.width; } }

57 Union vs. Struct Similarities Definition syntax virtually identical Member access syntax identical Differences Members of a struct each have their own address in memory. The size of a struct is at least as big as the sum of the sizes of the members (more on this later) Members of a union share the same memory. The size of a union is the size of the largest member.