Store a Record of Data

struct is just another word for "record"

A data table can be stored as an array of structs.

Return Multiple Values from a Function

Example: Calculate the centroid of a triangle.

def centroid(x_A, y_A, x_B, y_B, x_C, y_C):
    x_G = 1 / 3 * (x_A + x_B + x_C)
    y_G = 1 / 3 * (y_A + y_B + y_C)
    return x_G, y_G

Without structs, returning more than one value is not possible in C.

Declaration

struct struct_name {
   type member1;
   type member2;
   /* declare as many members as desired */
};

Example: Declaration and Definition of a Date

struct date {
   int year;
   int month;
   int day;
};
struct date d = { 2021, 12, 10 };

struct date d = { .year=2021, .month=12, .day=10 };

Typedef

• Used to create an additional name (alias) for another data type
• Particularly useful in combination with structs

typedef type_definition new_identifier;

Example: Declaration of Point in a 2 Dimensional Space

struct point {
   float x;
   float y;
};
struct point p = {1.5, 3.0};

typedef struct point {
   float x;
   float y;
} Point;
Point p = {1.5, 3.0};

Assignment

#include <stdio.h>

typedef struct {
  float x;
  float y;
} Point;

int main() {
  Point p1 = { .x=1.5 , .y=3.0 };
  Point p2 = p1;  // copy assignment
  p2.x = -5;  // does not affect `p1`
  return 0;
}

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Passing Structs to Functions

#include <stdio.h>

typedef struct {
  float x;
  float y;
} Point;

void print_point(Point p) {
  printf("P(%.2f/%.2f)\n", p.x, p.y);
  p.x = 0;  // does not affect p in main
}

int main() {
  Point p = { .x=1.5 , .y=3.0 };
  print_point(p);  // pass by value
  return 0;
}

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Returning Structs from Functions

typedef struct {
  float x;
  float y;
} Point;

Point centroid(Point A, Point B, Point C) {
  float x_G = 1.0 / 3.0 * (A.x + B.x + C.x);
  float y_G = 1.0 / 3.0 * (A.y + B.y + C.y);
  Point G = {x_G, y_G};
  return G;
}

int main() {
  Point A = {1.5, 3.0}, B = {12.0, 3.0}, C = {4.5, 9.0};
  Point G = centroid(A, B, C);  // pass by value
  return 0;
}

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C-Arrays can be Returned within Structs

#include <stdio.h>
typedef struct {
  int id;
  char name[15];
  unsigned int grades[8];
} Student;

Student read_student(int id) {
  // get data from eg. a database
  Student s = { id, "Pat Kaling", { 1, 1, 2, 1, 3, 2, 1, 1 } };  // from a database
  return s;
}
int main() {
  Student s = read_student(123);
  printf("%s has a %u in English.", s.name, s.grades[3]);
  return 0;
}

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Exercise

1. Create a typedef struct Date that holds int year, int month and int day.
2. Define a function print_date(Date d) that uses printf to print a given date to standard output. The date must be printed in ISO 8601 format (YYYY-MM-DD), followed by a newline.
3. Define today's Date in main and call print_date to print today's date.

Solution

#include <stdio.h>

typedef struct {
  int year;
  int month;
  int day;
} Date;

void print_date(Date d) {
  printf("%04d-%02d-%02d\n", d.year, d.month, d.day);
}

int main() {
  Date today = {2021, 12, 9};
  print_date(today);
  return 0;
}

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