This is part of the book COMPUTER PROGRAMMING THE C LANGUAGE. by Adriana ALBU. Conspress Publisher, Bucureşti, România, 2013 ISBN:

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1 This is part of the book COMPUTER PROGRAMMING THE C LANGUAGE by Adriana ALBU Conspress Publisher, Bucureşti, România, 2013 ISBN:

2 Chapter 11 User-defined types Besides the standard data types (char, int, float, double), C programming language provides several mechanisms for creating customized data types (user-defined types). Two of these mechanisms are the subject of this chapter: the structure an entity that gathers several variables that are related from a logical point of view, under a single name; the enumeration a list of named integer constants The structure A structure is a collection of variables referred through a single name. Structures are successfully used when complex data must be organized because they allow gathering different variables (that have a certain relation in a given context) and handling them as a unit not as separated entities [BK03]. The variables that belong to a structure are called members or fields and can have different data types. Each member has a name of its own, used in order to select it within the structure Definition and use The keyword struct is used to define a structure. The instruction that performs this action has the following general form [BH92]: struct structure_name{ type1 var11, var12; type2 var2; typen varn; list_of_variables; where: structure_name is an optional label; the new data type can be used to declare variables through this label; var11, var12, var2,, varn are the members of the structure;

3 Computer Programming The C Language type1, type2,, typen represent the types of the structure s members; the variables that have the same type can be defined in a single instruction, separated by comma (,); the variables that don t have the same type are defined in different instructions, separated by semicolon (;); the members are declared within a block enclosed by curly brackets ({); list_of_variables is also optional and can contain one or more variables; the structure s definition ends with semicolon (;), because it is an instruction, on the whole; though both structure_name and list_of_variables are optional, they cannot be omitted in the same time; at least one of them should be present, otherwise the structure s definition has no sense (since neither variables are declared, nor the structure receives a name to be used later). A first example of a structure, quite simply, is represented by the coordinates of a point in a plane. The structure can be defined as: struct point{ int x; int y; ; or as: struct point{ int x, y; ; The members of a structure can have different types, as shown in the following example that defines several features of a student: struct student{ int id; float mark; 2

4 11 User-defined types ; The definition of a structure doesn t allocate memory space. It only describes a new data type that can be considered a pattern. The variables of a structure type can be declared right within the instruction that defines the type. The following example declares two variables A and B of type point: struct point{ int x; int y; A,B; If the structure received a name, then the variables can be declared later in the program, wherever the structure is visible. The name of the structure, preceded by the keyword struct is used when a variable is declared. The following instruction declares a variable using the type student previously declared: struct student stud1; In C++ programming language a variable can also be declared without the keyword struct, the name of the type being enough. Therefore, the previous variable can be declared as: student stud1;. The members of a structure variable can receive initial values right when the variable is declared. The following instruction declares and initializes two points: A with the coordinates (3,4) and B with the coordinates (10,4): struct point A={3,4, B={10,4; Each member of a structure can also be used as an independent variable, but its name is a little bit longer, because it also includes the name of the structure variable that contains the member. The operator., placed between the name of the structure variable and the name of the member variable is used to access the members of a structure: A.x=3; A.y=4; stud1.mark=9.75; strcpy(stud1.name,"ritchie"); printf("%s %.2f",stud1.name,stud1.mark); 3

Computer Programming The C Language The name of a member variable can be identical with the name of a usual variable (non-member) without creating conflicts; the difference between the two variables

5 Computer Programming The C Language The name of a member variable can be identical with the name of a usual variable (non-member) without creating conflicts; the difference between the two variables is made by the context where they are accessed [BK03] (the member variable is always preceded by the name of the structure variable that contains it). Based on the same reasoning, a member of a structure can have the same name with a member of another structure (this is not recommended; it could be used only if the elements have a very close meaning for instance, the radius can be a variable that belongs to a structure that describes a circle and also to one that describes an arc). Program is a complete example where the structure student is used. The structure is global (it is defined outside any function) in order to extend its visibility on the entire program (because this program contains a single function main() the structure could also be defined within this function). A variable s of type student is defined and some of its features are read (id, name, exam mark, laboratory mark). The final mark is calculated and displayed. The program is very simple, being only an exercise of defining and using a structure. Program #include <stdio.h> struct student{ int id; float exam, lab, final; ; struct student s; printf("id: "); scanf("%d",&s.id); printf("the name: "); scanf("%s",s.name); printf("the exam mark: "); scanf("%f", &s.exam); printf("the laboratory mark: "); scanf("%f", &s.lab); 4

6 11 User-defined types s.final=(2*s.exam+s.lab)/3; printf("%s has the final mark %.2f",s.name,s.final); The entire information contained in a structure variable can be copied into another variable of the same type through a single assignment (it is not necessary to perform separated assignments for each member variable). The structure point is used in program in order to illustrate this aspect. Two variables A and B are defined using this structure. The coordinates x and y of the variable A are initialized with the values 3 and 4, respectively. Then, the variable A is assigned to the variable B, which is displayed; B(3,4) is printed on the screen. Program #include <stdio.h> struct point{ int x; int y; ; struct point A, B; A.x=3; A.y=4; B=A; printf("b(%d,%d)",b.x,b.y); The members of a structure can have standard types or user-defined types. Therefore, within a structure, variables that have a structure type can be defined. For instance (program ), it can be imagined a structure called circle that contains a member of type point, representing the coordinates of the circle s center, and a member of type float, representing the radius of the circle: struct circle{ struct point center; 5

7 Computer Programming The C Language float radius; c; Three names are required to access the coordinates of the circle s center: the coordinate s name (x), the center s name (center) and the circle s name (c): c.center.x=10; Program #include <stdio.h> struct point{ int x; int y; ; struct circle{ struct point center; float radius; ; struct circle c; c.center.x=10; c.center.y=20; c.radius=5.37; printf("center: (%d,%d)",c.center.x,c.center.y); printf("\nradius: %.2f",c.radius); A structure can also contain an array (with one or more dimensions): struct vector{ int v[10]; int max; v_str; 6

11 User-defined types An element of the array (for instance, that with the index 3) is naturally accessed this way: v_str.v[3]; Program 11.1.4 uses this structure that contains a vector of integers and a variable that stores the maximum value of the array.

8 11 User-defined types An element of the array (for instance, that with the index 3) is naturally accessed this way: v_str.v[3]; Program uses this structure that contains a vector of integers and a variable that stores the maximum value of the array. The functions of the program (read(), write() and maximum()) operate on a global variable, v_st, of a structure type. Program #include <stdio.h> const n=10; struct vector{ int v[n]; int max; v_st; void read(){ int i; for(i=0;i<n;i++){ printf("v[%d]=",i); scanf("%d",&v_st.v[i]); void write(void){ int i; for(i=0;i<n;i++) printf("%d ",v_st.v[i]); void maximum(void){ int i; v_st.max=v_st.v[0]; for(i=1;i<n;i++) if(v_st.v[i]>v_st.max) v_st.max=v_st.v[i]; 7

9 Computer Programming The C Language read(); write(); maximum(); printf("\nmax: %d", v_st.max); Structures and functions As any other variables, the structure variables can be passed as arguments to functions (by value or by address) or returned from functions. In these cases the structure must have a global declaration in order to be available to many elements of the program. Program illustrates how a structure variable can be returned from a function. The features of a student are read within the function; they are stored in a structure variable that is returned to main() function. This reading is called for two variables of type student. Program #include <stdio.h> struct student{ int id; float mark; ; struct student read(void){ struct student stud; printf("id: "); scanf("%d",&stud.id); printf("name: "); scanf("%s",stud.name); printf("mark: "); 8

10 11 User-defined types scanf("%f", &stud.mark); return stud; struct student s1,s2; s1=read(); s2=read(); printf("%s: %.2f\n",s1.name,s1.mark); printf("%s: %.2f",s2.name,s2.mark); Program is an example of passing by value some structure variables. Two variables A and B of type point are declared and initialized within main(). Then the variables are passed to a function that calculates and returns the distance between these two points (d x B x A y B y A ). Program #include <stdio.h> #include <math.h> struct point{ int x; int y; ; float distance(struct point a, struct point b){ return sqrt(pow((b.x-a.x),2)+pow((b.y-a.y),2)); struct point A={3,4, B={10,4; printf("ab=%.2f", distance(a,b)); 9

11 Computer Programming The C Language Program passes by address a structure variable. A point A is declared and initialized within main(). This point must be moved to other coordinates. In order to have a real movement, the point must be passed by address to the function that performs this movement. The new coordinates are passed by value. On return from the function the new location of the point is displayed to verify that the movement was made. It is found that the values of the structure s variables were changed. Program #include <stdio.h> struct point{ int x; int y; ; void move(struct point *a, int new_x, int new_y){ (*a).x=new_x; (*a).y=new_y; struct point A={3,4; move(&a,10,14); printf("a(%d,%d)", A.x,A.y); Arrays of structures The structure types can be used to create arrays, each element of the array being in fact a structure (a group of variables). Obviously, the structure should be defined before the array. If the number of occurrences of each vowel (a, e, i, o, u) in a text should be counted, then an array of 5 elements with the following structure can be used: struct count_vowels{ 10

12 11 User-defined types char vowel; int counter; ; Program solves this problem. First, the structure count_vowels is defined and then an array v of 5 elements of the previously defined type. The elements of the array are also initialized at declaration: the members vowel with a vowel and the members counter with 0. A text declared as string is read within main() and then it is passed to the function count(). Within this function the vowels are traversed and each vowel v[i].vowel is searched into the text, comparing each letter of the text (text[j]) with the searched vowel. If they are identical, then the counter connected to that vowel (v[i].counter) is incremented. In order to ignore the difference between uppercases and lowercases, each letter of the text is transformed to lowercase (tolower(text[j])), as it is stored into the array of vowels. At the end of the program, a function that displays the vowels and their counters is called. Program #include <stdio.h> #include <string.h> #include <ctype.h> struct count_vowels{ char vowel; int counter; ; struct count_vowels v[5]={ {'a',0, {'e',0, {'i',0, {'o',0, {'u',0; void count(char *text){ int i, j; for(i=0;i<5;i++) for(j=0;j<strlen(text);j++) if(tolower(text[j])==v[i].vowel) 11

13 Computer Programming The C Language v[i].counter++; void display(void){ int i; for(i=0;i<5;i++) printf("\n%c: %d",v[i].vowel,v[i].counter); char text[50]; printf("enter the text: "); gets(text); count(text); display(); Pointers to structures C programming language allows the declaration of pointers to structures as any other pointers are declared. Considering the structure student, created at the beginning of this chapter, the following instruction declares a pointer: struct student *p; The address of a variable of the same structure type can be assigned to a pointer: p=&stud; where stud is a variable of type student. There are two ways of accessing the members of a structure through a pointer: the classic version, through the dereferenced pointer (*p) and using the operator. : (*p).mark;; this version was illustrated when a structure was passed by address; the special version, using the operator -> : p->mark;; program emphasizes this way of accessing a structure. A variable stud and a pointer p are declared using the structure student; the address of the variable 12

14 11 User-defined types stud is assigned to the pointer. Then, the members of the structure receive values through the pointer. At the end, the variable stud is displayed in order to verify if it was correctly accessed. Program #include <stdio.h> #include <string.h> struct student{ int id; float mark; ; struct student stud, *p; p=&stud; p->id=1; strcpy(p->name,"ritchie"); p->mark=10; printf("%s: %.2f",stud.name, stud.mark); 11.2 The enumeration An enumeration is a set of named integer constants that specifies all the legal values that can be taken by a variable of that type [HS98]. The general form of an instruction that defines an enumeration is [BH92]: enum enumeration_name{ const1_name=val1; const2_name=val2; constn_name=valn; list_of_variables; 13

15 Computer Programming The C Language where: enum is the key word used to define the enumeration; enumeration_name is an optional label associated to the enumeration; const1_name, const2_name,, constn_name are the names of the constants that form the enumeration; val1, val2,, valn are the values associated to the constants; these values are optional; if they are present, they should be integers; if they are omitted, then it is considered that the value associated to a constant is the value associated to the previous constant plus 1 and the value 0 is associated by default to the first constant (if nothing else is specified); list_of_variables is an optional way of declaring variables using the enumeration that has just been defined. As example, an enumeration that contains the days of a week is considered: enum day {Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, Saturday; and two variables of this type: enum day today, tomorrow; The variable of enumeration type can be used in instructions as: today=sunday; if(today==sunday) tomorrow=monday; else tomorrow=today+1; printf("%d", today); //a number between 0 and 6 It is important to emphasize that each constant of an enumeration is associated (by default or not) to an integer value. Therefore, these constants can be used wherever an integer is necessary. The explicitly association of values to the constants of an enumeration can be made as in the following example: enum month {Jan=31, Feb=28, Mar=31, Apr=30, May=31, Jun=30, Jul=31, Aug=31, Sep=30, Oct=31, Nov=30, Dec=31; 14

16 11 User-defined types An enumeration where only some constants have explicitly associated values is also correct. Each of the rest will have a value with a unit greater than the previous constant. In the following example, the constant fifth will have by default the value 5 because it follows the constant fourth that has explicitly associated the value 4: enum perfect_intervals{unison=1, fourth=4, fifth, octave=8; Program defines, within an enumeration, the three colors of the traffic light. The values 0, 1 and 2 are, by default, associated to these colors. An integer number (0, 1 or 2) is randomly generated in main() and then it is passed to the function traffic_light().when submitting, the number is identified with one of the three constants of the enumeration. The function only generates a command (simulated by a text message) according to the color of the traffic light received as argument. The command is returned and displayed by main() function. Program #include <stdio.h> #include <stdlib.h> #include <time.h> #include <string.h> enum color{red,yellow,green; void traffic_light(enum color tl_color){ char command[30]; switch(tl_color){ case RED: strcpy(command,"stop!"); break; case YELLOW: strcpy(command,"stop if possible!"); break; case GREEN: strcpy(command,"pass!"); break; 15

17 Computer Programming The C Language printf("%s",command); randomize(); traffic_light(random(3)); 11.3 Questions and exercises A. Find the error. 1. struct person{ int age; 2. struct circle{ int x; int y; float radius; acircle; ; 3. struct float salary; anemployee; 4. struct{ float salary; ; 5. struct employee{ float salary; ; struct employee anemployee; salary=2500; 16

18 11 User-defined types 6. struct employee{ float salary; ; struct employee anemployee, *apointer; apointer=&anemployee; apointer.salary=2500; 7. enum cardinal_point{east,west,north,south; enum cardinal_point point=east; printf("%s", point); B. Considering the following programs, specify what will be printed on the screen once these programs are executed. 8. #include <stdio.h> struct product{ float quantity, price; ; struct product prod1={"bread",20,1.5, prod2; prod2=prod1; printf("%.2f",prod2.price); 9. #include <stdio.h> #include <string.h> struct polygon{ int sides; ; void modify(struct polygon pol){ strcpy(pol.name,"heptagon"); pol.sides=7; struct polygon p={"square",4; 17

19 Computer Programming The C Language modify(p); printf("the %s has %d sides", p.name, p.sides); 10. #include <stdio.h> #include <string.h> struct polygon{ int sides; ; void modify(struct polygon *pol){ strcpy((*pol).name,"heptagon"); (*pol).sides=7; struct polygon p={"square",4; modify(&p); printf("the %s has %d sides", p.name, p.sides); 11. #include <stdio.h> enum boolean{false,true; enum boolean f; float nota=7.25; if(nota<5) f=false; else f=true; printf("%d",f); C. Choose the correct answer (one only). 12. A structure contains: 18

20 11 User-defined types a) a single variable; b) several variables of the same type; c) several variables gathered under the same name; d) several variables with the same initial value; e) several constants. 13. A structure is defined by the keyword: a) user; b) defined; c) type; d) struct; e) structure. 14. Which of the following operators can be used to access the members of a structure: a).; b) &; c) *; d) #; e) \. 15. In the context of a structure, a member is: a) the data type of the structure; b) a variable of the structure; c) a pointer to the structure; d) an array of structures; e) all answers are wrong. 16. Which of the following instructions accesses in a correct manner a member m of a structure variable s? a) s.m; b) s&m; c) s(m); d) s[m]; e) none of them. 17. Which of the following instructions accesses in a correct manner a member m through a pointer p to a structure? a) p.m; b) p[m]; c) p&m; d) p*m; e) p->m. 18. The enumeration is: a) a list of integer constants; b) a list of variables of the same type; c) a waiting list; d) a list of variables that can have different types; e) all answers are wrong. 19. What will be printed on the screen once the following program is executed? #include <stdio.h> enum season{spring=1,summer,autumn,winter; enum season now=autumn; printf("%d",now); a) autumn; b) 3; c) 2; d) 4; e) the program contains an error. 19

21 Computer Programming The C Language D. Write programs to solve the following problems. 20. A bouquet of flowers must be created. It may contain several types of flowers and several flowers of each type (for example: 5 roses, 3 fern leaves, and a bamboo). Simulate the content of a bouquet. Suggestion: the bouquet can be an array of structure variables, the structure containing two fields: the name of a flower and the number of flowers with that name used for the bouquet. 21. Calculate, using Heron s formula, the surface of a triangle that is described by three points whose coordinates are read from the keyboard. Suggestion: there will be used a structure of type point (with the two coordinates); then structures of type segment (two points) or triangle (three points) can be used. The lengths of the three segments that form the triangle are calculated. Then the surface can be found with Heron s formula A s s a s b s c, where a, b and c are the lengths of the sides and s is the semi-perimeter. 22. Define a structure that describes a date through three fields (year, month, and day). Write a function that compares two dates received as argument. The function will have three versions: a. it returns 0 if the dates are equals and 1 otherwise; b. it returns 0 if the dates are equals, -1 if the first date is less than the second and 1 if the first date is greater than the second; c. it returns the number of years, months and days between the two dates. 23. Imagine a card game with two players. There will be used an enumeration containing the four kinds of cards (club, diamond, heart, spade), each one having associated a value set by the programmer. Each player receives a card that is randomly generated and the player that has the card with a greater value wins Answers to questions and exercises A. 1. The semicolon separator is required after the closing curly bracket. 2. Variable acircle is misplaced; it should be declared between the closing curly bracket and the semicolon separator. 3. The opening curly bracket after the struct is missing. The name of the structure is also missing, but this is not an error, because the instruction that defines the structure also declares a 20

22 11 User-defined types variable of this type, which is enough. 4. Though both the structure s name and the list of variables are optional, they cannot be omitted in the same time. In order to obtain a correct definition for the structure, it is necessary to have either a label for the structure or one or more variables declared at the end of the instruction that defines the structure. 5. Variable salary has no meaning by itself. It is a member of a structure and should be accessed through variable anemployee: anemployee.salary=2500;. 6. The operator -> should be used to access a member of a structure through a pointer; therefore, the correct instruction is: apointer->salary=2500;. 7. The value associated to a constant of an enumeration is an integer, therefore the constant should be printed with the descriptor "%d", not "%s". B. 8. The entire content of variable prod1 was copied to variable prod2, therefore 1.50 is printed on the screen. 9. The polygon is passed by value, therefore the changes that are made within function modify() are not visible in main(); The square has 4 sides is displayed on the screen. 10. This time the polygon is passed by address to function modify(), therefore the changes are visible in main(); The heptagon has 7 sides is displayed on the screen. 11. The value 1 is printed on the screen because the enumeration variable f indicates to the constant true that is associated to the value 1. C. 12. c). 13. d). 14. a). 15. b). 16. a). 17. e). 18. a). 19. b). 21

Darshan Institute of Engineering & Technology for Diploma Studies Unit 5

Darshan Institute of Engineering & Technology for Diploma Studies Unit 5 1 What is structure? How to declare a Structure? Explain with Example Structure is a collection of logically related data items of different data types grouped together under a single name. Structure is