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Until now, every variable has represented a single entity, whether that entity be a real number, integer, logical value, character, etc. These types are known as simple types. There are times, however, when structured types are required. Two common structured types are the array and the record.

A variable is a named storage location that contains the value of the variable.

This definition of a variable includes the following.

• Individual variables such as a0, a1, a2, etc.
• Array variables such as a[0], a[1], a[2], etc.
• Array record variables such as p[0].a, p[1].a, p[2].a, etc.

These forms are now covered in more detail. We will now look at arrays and records.

Let us first look at the problem to be solved.

Consider a C program that uses a counter loop to read in a series of exactly 4 numbers and then print them in reverse order. Using individual variables, we cannot use a for or while loop as we need to remember each and every number so we can print them in reverse order. (That is the reason for arrays).

We need 4 variables for 4 numbers. Here are the declarations for individual variables.
int a0; int a1; int a2; int a3;
Here is the C code.
#include <stdio.h> int main() { int a0; int a1; int a2; int a3; scanf("%d",&a0); scanf("%d",&a1); scanf("%d",&a2); scanf("%d",&a3); printf("%d\n",a3); printf("%d\n",a2); printf("%d\n",a1); printf("%d\n",a0); return 0; }

Here are some examples of input and output for the above program code.

Here is an example input.
11 22 33 44
For the above example input, here is the expected output.
44 33 22 11
We need a better way as if there were 100 or 1000 or more numbers, this method of a variable for each number does not scale well and is not easy to code (takes a lot of code).

We need a way to clone variables.

An array is a structured data type consisting of elements of the same type that are accessed by an index expression.

Since an array variable is a variable, it needs to be declared. Here is the array declaration in C.
const int N = 4; int a[N];
Note the use of the const (constant, named literal) for the known count. By convention, constants are written in all uppercase names.

This declaration statement declares variable a to be an array of 4 elements of type int indexed from 0 to 3.

Here is the C code.
#include <stdio.h> int main() { const int N = 4; int a[N]; for (int i=0; i < N; i++) { scanf("%d",&a[i]); } for (int i=N-1; i >= 0; i--) { printf("%d\n",a[i]); } return 0; }
This solution using arrays now easily scales up to sizes of 100, 1000, etc.

• The constant is N.
• The literal is 4.

Here are some examples of input and output for the above program code.

Here is an example input.
11 22 33 44
For the above example input, here is the expected output.
44 33 22 11

Here is the above program unrolled to remove the for loops (and the constant N).

Here is the C code.
#include <stdio.h> int main() { const int N = 4; int a[N]; scanf("%d",&a[0]); scanf("%d",&a[1]); scanf("%d",&a[2]); scanf("%d",&a[3]); printf("%d\n",a[3]); printf("%d\n",a[2]); printf("%d\n",a[1]); printf("%d\n",a[0]); return 0; }

Here are some examples of input and output for the above program code.

Here is an example input.
11 22 33 44
For the above example input, here is the expected output.
44 33 22 11

A record is a structured data type consisting of elements called fields of varying data types that are accessed by name.

This declaration statement declares variable p to be an array of 4 records of type Item with a named variable field called a.
const int N = 4; struct Item { int a; }; struct Item p[N];

Here is a solution using records. Note that there is only one named field x in the record structure Item. Often, there is more than one field in a record structure.

Here is the C code.
#include <stdio.h> int main() { const int N = 4; struct Item { int a; }; struct Item p[N]; for (int i=0; i < N; i++) { scanf("%d",&p[i].a); } for (int i=N-1; i >= 0; i--) { printf("%d\n",p[i].a); } return 0; }

Here are some examples of input and output for the above program code.

Here is an example input.
11 22 33 44
For the above example input, here is the expected output.
44 33 22 11

Here is an enumerated solution using records.

Here is the C code.
#include <stdio.h> int main() { const int N = 4; struct Item { int a; }; struct Item p[N]; scanf("%d",&p[0].a); scanf("%d",&p[1].a); scanf("%d",&p[2].a); scanf("%d",&p[3].a); printf("%d\n",p[3].a); printf("%d\n",p[2].a); printf("%d\n",p[1].a); printf("%d\n",p[0].a); return 0; }

Here are some examples of input and output for the above program code.

Here is an example input.
11 22 33 44
For the above example input, here is the expected output.
44 33 22 11

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