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Nested structures in C

mikkel250 profile image mikkel250 ・2 min read
Nested structures

C also allows you to define a structure that itself contains other structures as one or more of its members. If the program you're working on required a timestamp, a convenient way to associate both together would be to define a new structure that contains both time and date as its member elements:

struct dateAndTime
  struct date sDate;
  struct time sTime;
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You could now create an instance of dateAndTime that contains both date and time structures:

struct dateAndTime timestamp;

// to access the members, dot notation is used as usual:

// to reference a particular member inside one of these structures, the same syntax is used:
timestamp.sDate.month = 10;

// increments the seconds inside the sTime member of dateAndTime

// initialization of both date and time inside timestamp
// sets the date to February 1st, 2018 at 3:30:00
struct dateAndTime timestamp = { {2, 1, 2018}, {3, 30, 0} };

// just like other structures, the dot notation can be used to initialize specific member's variables
// and, though it is more typing, is more readable. It is order independent:
struct dateAndTime timestamp = {
  {.month = 2, .day = 1, .year = 2018},
  {.hour = 3, .minutes = 30, .seconds = 0}
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It is also possible to create an array of dateAndTime structures. The array index along with dot notation are used, in the same way as with a non-nested structure within the array:

struct dateAndTime timestamp[100];

timestamp[0].sTime.hour = 3;
timestamp[0].sTime.minutes = 30;
timestamp[0].sTime.seconds = 0;
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It is possible declare a structure within a structure within the actual definition, it follows the same syntax as declaring a structure variable during the definition, but IMO this is not as readable as the longer ways of declaring them, but as with most things, is a matter of personal preference:

struct time
  struct date
    int day;
    int month;
    int year;
  } dateOfBirth;

  int hour;
  int minute;
  int seconds;
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The declaration above is enclosed inside the scope of the time structure definition, and does not exist outside of it. You would not be able to declare a date variable external to the time structure.

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