Structures

a tool for grouping elements together.( like an object but just for data types)

Creating a structure

struct struct_name
{
int var1;
int var2;
int var3; 
};

Using a structure

variables can now be declared to be a type of "struct_name"

struct struct_name var1

to set the value of the day in the variable :

struct_name.var1 = value1;

to test the value of the variable:

if ( struct_name.var1 == value )
...

example:

struct date {    // a structure with 3 parameters
int month;
int day;
int year;
};

struct date today;    // a variable of type date

today.month = 5;
today.day = 21;
today.year = 2015;
printf("todays date is %i/%i/%i.\n",today.month, today.day,today.year);

we can also do it this way:

 struct date {   
int month;
int day;
int year;
} today;

we do not have to give a structure a tag name if all of the variables of a particular structure type are defined when the structure is defined the structure name can be omitted

 struct {   
int month;
int day;
int year;
} today;

a disadvantage of this is that we can no longer define further instances of the structure in another statement all the variables of this structure type that you want in your program must be defined in the one statement

Initializing structures

struct date today = { 1,5,2019};

just like an array initialization fewer values might be listed than are contained in the structure:

struct date today = { 1,5}; → no value for year

we can do it this way too:

.member = value
i.e:
struct date date1 = {.month = 12, .day = 23};

Compound literals ( C11 only )

we can assign one or more values to a structure in a single statement using what is known as compound literals

 today = (struct date){9,12,2016}; 

Structure arrays

declaring an array of structures is like declaring any other kind of array:

struct date {
    int day;
    int month;
    int year;
};

struct date myDates[10];

myDates[1].month = 6;
myDates[1].day = 5;
myDates[1].year = 2014;

or

struct  date myDates[5] = {{12,1,1985},{12,5,1984},{11,12,1998}};

also the inner pairs of braces are optional:

struct  date myDates[5] = {12,1,1985,12,5,1984,11,12,1998}; → initialized just the third element of the array to the specified value

struct  date myDates[5] = {[2] = {11,12,1998}}; → sets just the month and day of the second element of the myDates array to 12 and 30

struct  date myDates[5] = {[2].month = 12 };

example:

 struct date {
    int day;
    int month;
    int year;
};
struct date myDates[10] = {[1]=12,2,1998,[2]=19,11,1975,[3]=24,5,1966};
for(int i = 0; i <= 4; ++i)
printf("date: %i/%i/%i\n",myDates[i].day,myDates[i].month,myDates[i].year);

Structures containing arrays

struct month
 {
 int    numberOfDays;
 char name[3];
 };

this sets up a month structure that contains an integer member called numberOfDays and a character member called name member name is actually an array of three characters

struct month aMonth;
 aMonth.numberOfDays = 31;
 aMonth.name[0] = 'j';
 aMonth.name[1] = 'a';
 aMonth.name[2] = 'n';

you can also initialize this variable to the same values:

 struct month aMonth = {31, {'j','a','n'}};

you can setup 12 month structures inside an array to represent each month of the year

 struct month months[12]; 

example:

 struct month {
    int numberOfMonth;
    int numberOfDays;
    char name[3];

};
struct month aMonth;
aMonth.numberOfDays = 31;
aMonth.name[0] = 'j';
aMonth.name[1] = 'a';
aMonth.name[2] = 'n';
aMonth.numberOfMonth = 1;

printf("month name: %i number: %i days: %i",aMonth.name,aMonth.numberOfMonth,aMonth.numberOfDays);

Nested structures

C allows us to define a structure that itself contains other structures as one or more of its members

struct dateAndTime
 {
struct date sdate;
struct time stime;
 };
 
 struct dateAndTme event;
 
 event.sdate.month = 10;
 ++event.stime.secends;
 
 struct dateAndTime event = {{ 2,1,2015}, {3,30,0}};

or

 struct dateAndTime event = {
 {.month = 2 , .day = 1 , .year = 2019},
 {.hour = 3, .minutes = 30, seconds = 0}
 };

we can use members names in initialization

example: an event scheduler with nested structures:

#include <stdio.h>

int main(){

    struct date{
        int day;
        int month;
        int year;
    };

struct time{
    int second;
    int minute;
    int hour;
};


    struct datetime{
        struct date sdate;
        struct time stime;
    };

    struct datetime event;

    printf("enter a second: ");
    scanf("%d",&event.stime.second);
    printf("enter a minute: ");
    scanf("%d",&event.stime.minute);
    printf("enter a hour: ");
    scanf("%d",&event.stime.hour);
    printf("enter a day: ");
    scanf("%d",&event.sdate.day);
    printf("enter a month: ");
    scanf("%d",&event.sdate.month);
    printf("enter a year: ");
    scanf("%d",&event.sdate.year);

    printf("your even scheduled for: %d/%d/%d %d:%d:%d\n",event.sdate.day,event.sdate.month,event.sdate.year,event.stime.second,event.stime.minute,event.stime.hour);

return 0;
};

Array of nested structures

 events[0].stime.hour = 12;
 events[0].stime.minutes = 0;
 events[0].stime.seconds = 2;

Declaring a structure within a structure

struct time
 {
 struct Date
 {
 int day;
 int month;
 int year;
 
 } dob;   // a variable
 
 int hour;
 int minutes;
 int seconds;
 };

the declaration is enclosed within the scope of the time structure definition it doesn't exist outside it it becomes impossible to declare a date variable external to the time structure

Structures and pointers

declaring a struct as a pointer

 struct date *datePtr;
 
 datePtr = &todaysDate;
 
 (*datePtr).day=21;

using structs as pointers

test the value of month stored in the date struct pointed to by datePtr:

 if ((*date*Ptr).month ==12 )

a special operator exists for pointers to structures:

 (*x).y

to be more clearly expressed as:

 x->y

example:

struct date{
    int month;
    int day;
    int year;
};

struct date today, *datePtr;

datePtr = &today;

datePtr->month = 12;
datePtr->day = 4;
datePtr->year = 1998;

printf("today is : %i/%i/%i\n",datePtr->day,datePtr->month,datePtr->year);

Structures containing pointers

example:

struct intPtrs
 {
 int *p1;
 int *p2;
 };

 struct intPtrs pointers;
int i1 = 100, i2;

pointers.p1 = &i1;
pointers.p2=&i2;
*pointers.p2 = -97;

printf("i1 = %i, *pointers.p1=%i\n",i1,*pointers.p1);
printf("i2 = %i, *pointers.p2=%i\n",i2,*pointers.p2);

Structures as arguments to functions

 struct family{
    char name[20];
    int age;
    char father[20];
    char mother[20];
};

bool siblings(struct family member1, struct family member2){
    if (strcmp(member1.mother,member2.mother) == 0)
        return true;
    else
        return false;
}

Returning a structure from a function

the function prototype has to indicate this return value in the normal way

struct Date my_fun(void);

this is a prototype for a function taking no arguments that returns a structureof type Date

its more convenient to return a pointer to a structure.

example:

#include <stdio.h>

struct date{
    int day;
    int month;
    int year;
};

struct time{
    int second;
    int minute;
    int hour;
};


struct datetime{
    struct date sdate;
    struct time stime;
};

char scheduler(struct datetime event){
    printf("your event scheduled for: %d/%d/%d %d:%d:%d\n",event.sdate.day,event.sdate.month,event.sdate.year,event.stime.second,event.stime.minute,event.stime.hour);
};


int main(){
    struct datetime event;
    printf("enter a second: ");
    scanf("%d",&event.stime.second);
    printf("enter a minute: ");
    scanf("%d",&event.stime.minute);
    printf("enter a hour: ");
    scanf("%d",&event.stime.hour);
    printf("enter a day: ");
    scanf("%d",&event.sdate.day);
    printf("enter a month: ");
    scanf("%d",&event.sdate.month);
    printf("enter a year: ");
    scanf("%d",&event.sdate.year);
    scheduler(event);
return 0;
};

example:

#include <stdio.h>

struct funds{
char bank[1000000];
double bankfund;
char save[1000000];
double savefund;
};

double sum(struct funds moolah){
    return(moolah.bankfund + moolah.savefund);
}


int main() {
struct funds stan = {
        "garlic-melon bank",
        47432.34,
        "luckys savings and loan",
        854.435
};

printf("stan has a total of $%.2f.\n",sum(stan));
return 0;
}