Struct

Advanced Programming Language

Struct

• Struct allows us to create a new data type

• We can now declare “struct Point” as a new data type and can use it as another type of data:

struct Point {int x;int y;

};

struct Point P1, P2;

P1.x = 10; P1.y = 20;P2.x = 100; P2.y = 150;

printf(“P1.x = %d - P1.y = %d\n”, P1.x, P1.y);printf(“P2.x = %d - P2.y = %d\n”, P2.x, P2.y);

User Defined Data Type

• Sometimes, we need to declare new data type, which is not available in c

• For example, we want to keep the record of student mid-term scores– We need to keep student_id, student_name, and score.

• Struct can be used to declare a new data type to support this need.

Defining a Struct

• struct std_score consists of three elements: – std_id (type of string)– std_name (type of string)– score (type of float)

• Thus, we can now create a new data type called: “struct std_score”

struct std_score {char std_id[15];char std_name[50];float score;

};

Declaring a Struct Variable

• Andy and Bill are now type of “struct std_score” and contain all information as defined previously

• The instances of both variables contain std_id, std_name, and score

struct std_score Andy, Bill;

Assigning Values to Struct

• Each element of Andy can be assigned just like a normal data type as follows

• Note: we cannot use = operator to assign a string to the variable directly (See next slide, how we can do it)

strcpy(Andy.std_id, “53-2074-044-5”);strcpy(Andy.std_name, “Andy Williams”);Andy.score = 78;

strcpy(Bill.std_id, “53-2075-042-2”);strcpy(Bill.std_name, “Bill Gates”);Bill.score = 78;

strcopy()

• strcpy() copies string pointed to by src, to buffer pointed to by dest.

• strncpy() is similar to strcpy() but it copies at most n bytes for src to dest

• Keep in mind: we must have enough space for dest buffer

char *strcpy(char *dest, const char *src)

and

char *strncpy(char *dest, const char *src, size_t n)

Example of String Copy

char buf[20];char *str = “This is a book.”;

strncpy(buf, str, 10);printf(“%s\n”, buf);

strcpy(buf, str);printf(“%s\n”, buf);

This is a This is a book.

The result is:

Retrieving Values from Struct

• We can read the value of each element from the variable just as we do it for a normal data type.

printf(“ID:\t %s\n”, Andy.std_id);printf(“Name:\t %s\n”, Andy.std_name);printf(“Score:\t %.2f\n”, Andy.score);

printf(“ID:\t %s\n”, Bill.std_id);printf(“Name:\t %s\n”, Bill.std_name);printf(“Score:\t %.2f\n”, Bill.score);

Array of Struct

• Struct can also be declared as an array of data type

struct std_score TCT1RA[38], TCT1TA[38];

strcpy(TCT1RA[0].std_id, “53-2074-001-1”);strcpy(TCT1RA[0].std_name, “Somchai”);TCT1RA[0].score = 82;

strcpy(TCT1TA[10].std_id, “53-2075-011-1”);strcpy(TCT1TA[10].std_name, “Tongchai”);TCT1TA[10].score = 74;

printf(“ID:\t %s\n”, TCT1RA[8].std_id);printf(“Name:\t %s\n”, TCT1RA[8].std_name);printf(“Score:\t %.2f\n”, TCT1RA[8].score);

typedef

• typedef allows us to introduce synonyms for types which could have been declared some other way

• The new name becomes equivalent to the type that we wanted– typedef int SIZE

• We can then use SIZE as if we use int– SIZE x; // is now equivalent to int x;

Using typedef for struct

typedef struct {int x;int y;

} POINT;

POINT p1, p2;

p1.x = 5;p1.y = 10;

p2.x = 20;p2.y = 40;

We can now use POINT as if it is a new type of data:

Using typedef for struct

typedef struct {int width;int height;

} RECT;

RECT rect;

rect.width = 5;rect.height = 10;

printf(“Area of rect is %d\n”, rect.width * rect.height);

Question?

• How can you sort the array of struct by the score, by name, or by ID ?

• Hint: – we need to be able to swap two struct– We have to swap element by element to make sure that the

entire struct are moved

• Can you use struct with pointer?– You may need calloc() or malloc() to dynamically allocate

spaces of those records (struct).

Union

Union

• How we use Union is the same as how we use struct

• The different is that all elements of union share the same memory space

charint

double

union un_test {int x;double y;char c;

};

Size of Union

• The size of the union is size of the largest element contained in union

union un_test {int x;double y;unsigned char byte;

} un;

printf(“Size of un is %d byte.\n”, sizeof(un));

Size of un is 8 bytes.

Using union in struct

• Wind_chill is calculated when it is cold• Heat_index is calculated when it is hot• Thus, we know that we don’t need wind_chill and

heat_index at the same time• It is a good idea to save some memory by declaring both

variable as a union

struct conditions {

float temp;union feels_like {

float wind_chill;float heat_index;

}} today;

Another Example of Union

struct var_type{ int type_in_union; union{

float un_float; char un_char; int un_int;

} vt_un;} var_type;

Struct & Union Definition

#include <stdio.h>#include <stdlib.h>

/* code for types in union */#define FLOAT_TYPE 1#define CHAR_TYPE 2#define INT_TYPE 3

struct var_type{ int type_in_union; union{

float un_float; char un_char; int un_int;

} vt_un;} var_type;

Print function

void print_vt(void){

switch(var_type.type_in_union){

case FLOAT_TYPE: printf("%f\n", var_type.vt_un.un_float);

break;case CHAR_TYPE:

printf("%c\n", var_type.vt_un.un_char);

break; case INT_TYPE:

printf("%d\n", var_type.vt_un.un_int);break;

default: printf("Unknown type in union\n");

}}

Main Function

int main(){

var_type.type_in_union = FLOAT_TYPE; var_type.vt_un.un_float = 3.5;

print_vt();

var_type.type_in_union = CHAR_TYPE; var_type.vt_un.un_char = 'a';

print_vt();

exit(EXIT_SUCCESS);}

Question?

• Can we modify the print_vt() so that it can receive any instance of the same type?