Exercise 11 Dynamic allocation and structures

Dynamic Allocation Array variables have fixed size, used to store a fixed and known amount of variables – known at the time of compilation This size can’t be changed after compilation However, we don’t always know in advance how much space we would need for an array or a variable We would like to be able to dynamically allocate memory

The malloc function void * malloc(unsigned int nBytes); The function malloc is used to dynamically allocate nBytes in memory malloc returns a pointer to the allocated area on success, NULL on failure You should always check whether memory was successfully allocated Remember to #include

Example dynamic_reverse_array.c

Why casting? The casting in y = (int *)malloc(n*sizeof(int)); is needed because malloc returns void * : void * malloc(unsigned int nbytes); The type ( void * ) specifies a general pointer, which can be cast to any pointer type.

What is this ‘ sizeof ’ ? The sizeof operator gets a variable or a type as an input and outputs its size in bytes: double x; s1=sizeof(x); /* s1 is 8 */ s2=sizeof(int) /* s2 is 4 */

Free the allocated memory segment void free(void *ptr); We use free(p) to free the allocated memory pointed to by p If p doesn’t point to an area allocated by malloc, a run-time error occurs Always remember to free the allocated memory once you don’t need it anymore Otherwise, you may run out of memory before you know it!

Another example another_strcpy.c

Exercise Implement the function my_strcat :  Input – two strings, s1 and s2  Output – a pointer to a dynamically allocated concatenation (‘shirshur’)  For example: The concatenation of “hello_” and “world!” is the string “hello_world!” Write a program that accepts two strings from the user and prints their concatenation  Assume input strings are no longer than a 100 chars

Solution my_strcat.c (my_strcat2.c)

What’s wrong with this? char *my_strcat(char *str1, char *str2) { int len; char result[500]; /* Let’s assume this is large enough */ len = strlen(str1); strcpy(result, str1); strcpy(result+len, str2); return result; }

Structures Often we want to be able to manipulate ‘logical entities’ as a whole  For example, complex numbers, dates, student records, etc’  Each of these must be composed of more than one variable, but are logically units

Structures A struct (short for structure) is a collection of variables of different types, gathered into one super-variable It is used to define more complex data types Variables in a struct are called members or fields

Example – complex numbers. The following is the definition of a new ‘variable’ of type complex number: struct complex { int real; int img; }; Once we define a structure, we can treat it as any type.  In a program, we can then write: struct complex num1, num2, num3;

Access structure members If A is a variable of some structure type with a member named x, then A.x is that member of A  struct complex C; C.real = 0; If pA is a pointer to a structure with a member x, then pA->x is that member of the variable pointed by pA.  This is simply shorthand for (*pA).x struct complex *pc = &C; pc->real = 1;

A more convenient usage with typedef An alternative definition: typedef struct complex_t { int real; int img; } complex; Now the program has a new variable type - “ complex ”. This way we don’t have to write “ struct complex ” every time! For example, we can define two complex numbers in the following line: complex num1, num2;

Examples AddComplex.c IncDate.c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; …… realimg a …… realimg b …… realimg c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; …… realimg a …… realimg b …… realimg c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; realimg a …… realimg b …… realimg c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; realimg a …… realimg b …… realimg c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; realimg a realimg b …… realimg c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; realimg a realimg b …… realimg c

AddComplex – step by step complex AddComp(complex x, complex y) { complex z; z.real = x.real + y.real; z.img = x.img + y.img; return z; } realimg x realimg y …… realimg z

AddComplex – step by step complex AddComp(complex x, complex y) { complex z; z.real = x.real + y.real; z.img = x.img + y.img; return z; } realimg x realimg y …6.0 realimg z

AddComplex – step by step complex AddComp(complex x, complex y) { complex z; z.real = x.real + y.real; z.img = x.img + y.img; return z; } realimg x realimg y realimg z

AddComplex – step by step complex AddComp(complex x, complex y) { complex z; z.real = x.real + y.real; z.img = x.img + y.img; return z; } realimg x realimg y realimg z

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; realimg a realimg b realimg c

AddComplex – step by step complex a, b, c; printf(“…"); scanf("%lf%lf",&(a.real),&(a.img)); printf(“…"); scanf("%lf%lf",&(b.real),&(b.img)); c = AddComp(a,b); printf(“result = %g+%gi\n",c.real,c.img); return 0; realimg a realimg b realimg c

Miscellaneous structure trivia Structure members may be ordinary variable types, but also other structures and even arrays! Structures can therefore be rather large and take up a lot of space Many times we prefer to pass structures to functions by address, and not by value  Thus a new copy of the structure is not created – just a pointer to the existing structure

More trivia Structures cannot be compared using the == operator  They must be compared member by member  Usually this will be done in a separate function Structures can be copied using the = operator  Member-wise copy

Example Is_In_Circle.c

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); …… yx d (dot)c (circle) …… yx center (dot) radius …

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); …… yx d (dot)c (circle) …… yx center (dot) radius …

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) …… yx center (dot) radius …

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) …… yx center (dot) radius …

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) 0.0 yx center (dot) radius …

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) 0.0 yx center (dot) radius …

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) 0.0 yx center (dot) radius 5

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) 0.0 yx center (dot) radius 5

Is_in_circle – step by step int IsInCircle(dot *p_dot, circle *p_circle) { double x_dist,y_dist; x_dist = p_dot->x - p_circle->center.x; y_dist = p_dot->y - p_circle->center.y; if (x_dist*x_dist + y_dist*y_dist radius*p_circle->radius) return 1; return 0; } yx (dot)(circle) 0.0 yx center (dot) radius 5 x_disty_dist …… p_circlep_dot

Is_in_circle – step by step int IsInCircle(dot *p_dot, circle *p_circle) { double x_dist,y_dist; x_dist = p_dot->x - p_circle->center.x; y_dist = p_dot->y - p_circle->center.y; if (x_dist*x_dist + y_dist*y_dist radius*p_circle->radius) return 1; return 0; } yx (dot)(circle) 0.0 yx center (dot) radius 5 x_disty_dist 1.0… p_circlep_dot

Is_in_circle – step by step int IsInCircle(dot *p_dot, circle *p_circle) { double x_dist,y_dist; x_dist = p_dot->x - p_circle->center.x; y_dist = p_dot->y - p_circle->center.y; if (x_dist*x_dist + y_dist*y_dist radius*p_circle->radius) return 1; return 0; } yx (dot)(circle) 0.0 yx center (dot) radius 5 x_disty_dist p_circlep_dot

Is_in_circle – step by step int IsInCircle(dot *p_dot, circle *p_circle) { double x_dist,y_dist; x_dist = p_dot->x - p_circle->center.x; y_dist = p_dot->y - p_circle->center.y; if (x_dist*x_dist + y_dist*y_dist radius*p_circle->radius) return 1; return 0; } yx (dot)(circle) 0.0 yx center (dot) radius 5 x_disty_dist p_circlep_dot

Is_in_circle – step by step int IsInCircle(dot *p_dot, circle *p_circle) { double x_dist,y_dist; x_dist = p_dot->x - p_circle->center.x; y_dist = p_dot->y - p_circle->center.y; if (x_dist*x_dist + y_dist*y_dist radius*p_circle->radius) return 1; return 0; } yx (dot)(circle) 0.0 yx center (dot) radius 5 x_disty_dist p_circlep_dot

Is_in_circle – step by step printf(“Enter dot\n"); scanf("%lf%lf",&d.x,&d.y); printf("Enter circle center\n"); scanf("%lf%lf",&c.center.x,&c.center.y); printf("Enter circle radius\n"); scanf("%lf",&c.radius); if (IsInCircle(&d, &c)) printf("dot is in circle\n"); else printf("dot is out of circle\n"); yx d (dot)c (circle) 0.0 yx center (dot) radius 5

Exercise Implement the MultComplex function:  Input – two complex numbers  Output – their multiplication  Note: if x=a+ib and y=c+id then: z = xy = (ac-bd)+i(ad+bc) Write a program that uses the above function to multiply two complex numbers given by the user

Solution MultiplyComplex.c

Exiting the program void exit(int status); Sometimes an error occurs and we want the program to immediately exit The exit function closes all open files, frees all allocated memory, and exits the program Equivalent to calling ‘return’ within main Remember to #include See strcpy_with_exit.c

Structures containing arrays A structure member that is an array does not ‘behave’ like an ordinary array When copying a structure that contains a member which is an array, the array is copied element by element  Not just the address gets copied  For example - array_member.c Reminder – ordinary arrays can’t be copied simply by using the ‘=‘ operator  They must be copied using a loop

Structures containing arrays When passing the structure to a function by value  Changing the array field inside the function won’t change it in the calling function Reminder – when passing an ordinary array to a function, all that gets passed is the address of its first element  Hence every change to the array within the function, changes the array in the calling function

Pointers are another matter If the member is a pointer, for example to a dynamically allocated array, all that gets copied is the pointer (the address) itself  For example, pointer_member.c Hence, we should take extra care when manipulating structures that contain pointers