C Structures (structs)
Structures
Structures (also called structs) are a way to group several related variables into one place. Each variable in the structure is known as a member of the structure.
Unlike an array, a structure can contain many different data types (int, float, char, etc.).
Create a Structure
You can create a structure by using the struct
keyword and declare each of its members inside curly braces:
struct MyStructure { // Structure declaration
int myNum; // Member (int
variable)
char myLetter; // Member (char variable)
};
// End the structure with a semicolon
To access the structure, you must create a variable of it.
Use the struct keyword
inside the main() method, followed by the name
of the structure and then the name of the structure variable:
Create a struct variable with the name "s1":
struct myStructure {
int myNum;
char myLetter;
};
int main()
{
struct myStructure s1;
return 0;
}
Access Structure Members
To access members of a structure, use the dot syntax (.):
Example
// Create a structure called myStructure
struct myStructure {
int myNum;
char myLetter;
};
int main() {
// Create a structure variable
of myStructure called s1
struct myStructure s1;
// Assign values to members of s1
s1.myNum = 13;
s1.myLetter = 'B';
// Print values
printf("My
number: %d\n", s1.myNum);
printf("My letter: %c\n", s1.myLetter);
return 0;
}
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Now you can easily create multiple structure variables with different values, using just one structure:
Example
// Create different struct variables
struct myStructure s1;
struct myStructure s2;
// Assign values to different struct variables
s1.myNum = 13;
s1.myLetter = 'B';
s2.myNum = 20;
s2.myLetter = 'C';
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What About Strings in Structures?
Remember that strings in C are actually an array of characters, and unfortunately, you can't assign a value to an array like this:
Example
struct myStructure {
int myNum;
char myLetter;
char myString[30]; //
String
};
int main() {
struct myStructure s1;
// Trying to assign a value to the string
s1.myString = "Some text";
// Trying to print the value
printf("My string: %s",
s1.myString);
return 0;
}
An error will occur:
prog.c:12:15: error: assignment to expression with array type
However, there is a solution for this! You can use the strcpy()
function and assign the value to s1.myString, like this:
Example
struct myStructure {
int myNum;
char myLetter;
char myString[30]; // String
};
int main() {
struct myStructure s1;
// Assign a value to the string using
the strcpy function
strcpy(s1.myString, "Some text");
// Print the value
printf("My string: %s", s1.myString);
return 0;
}
Result:
My string: Some text
Simpler Syntax
You can also assign values to members of a structure variable at declaration time, in a single line.
Just insert the values in a comma-separated list
inside curly braces {}. Note that you don't
have to use the strcpy() function for string
values with this
technique:
Example
// Create a structure
struct myStructure {
int myNum;
char myLetter;
char myString[30];
};
int main() {
// Create a structure variable and assign values to it
struct myStructure s1 = {13,
'B', "Some text"};
// Print values
printf("%d %c %s", s1.myNum, s1.myLetter,
s1.myString);
return 0;
}
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Note: The order of the inserted values must match the order of the variable types declared in the structure (13 for int, 'B' for char, etc).
Copy Structures
You can also assign one structure to another.
In the following example, the values of s1 are copied to s2:
Example
struct myStructure s1 = {13, 'B', "Some text"};
struct myStructure s2;
s2 = s1;
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Modify Values
If you want to change/modify a value, you can use the dot syntax (.).
And to modify a
string value, the strcpy() function is useful again:
Example
struct myStructure {
int myNum;
char myLetter;
char myString[30];
};
int main() {
// Create a structure variable and assign values to it
struct
myStructure s1 = {13, 'B', "Some text"};
// Modify values
s1.myNum = 30;
s1.myLetter = 'C';
strcpy(s1.myString,
"Something else");
// Print values
printf("%d %c
%s", s1.myNum, s1.myLetter, s1.myString);
return 0;
}
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Modifying values are especially useful when you copy structure values:
Example
// Create a structure variable and assign values to it
struct
myStructure s1 = {13, 'B', "Some text"};
// Create another structure
variable
struct myStructure s2;
// Copy s1 values to s2
s2 =
s1;
// Change s2 values
s2.myNum = 30;
s2.myLetter = 'C';
strcpy(s2.myString,
"Something else");
// Print values
printf("%d %c
%s\n", s1.myNum, s1.myLetter, s1.myString);
printf("%d %c
%s\n", s2.myNum, s2.myLetter, s2.myString);
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Ok, so, how are structures useful?
Imagine you have to write a program to store different information about Cars, such as brand, model, and year. What's great about structures is that you can create a single "Car template" and use it for every cars you make. See below for a real life example.
Real-Life Example
Use a structure to store different information about Cars:
Example
struct Car {
char brand[50];
char model[50];
int
year;
};
int main() {
struct Car car1 = {"BMW", "X5",
1999};
struct Car car2 = {"Ford", "Mustang", 1969};
struct
Car car3 = {"Toyota", "Corolla", 2011};
printf("%s %s %d\n",
car1.brand, car1.model, car1.year);
printf("%s %s %d\n", car2.brand,
car2.model, car2.year);
printf("%s %s %d\n", car3.brand, car3.model,
car3.year);
return 0;
}
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