Basics of C Programming: Data Types, Constants, Variables, and Arrays

Introduction

Understanding the basics of C programming is essential for any aspiring programmer. In this blog post, we will explore the concept of data types and constants in C programming, taking a hands-on approach to deepen our understanding. We will compile and run the code snippets to observe the results firsthand. Let's get started!

Code Snippet

Creating a Directory

To begin, let's create a file named basics.c using the command line. Follow the instructions below based on your operating system:

• Open the command prompt or PowerShell.

• Run the following commands:
  

cd basic-programming/c-programming

• Open the file in your favorite text editor.

code basics.c

or

vim basics.c # installed by default on macOS

or

emacs -Q -nw basics.c

or

nano basics.c # installed by default on Linux

Note Make sure you save changes in the file.

Entry Point of a Program

In C programming, int main() or int main(void) is the entry point of a C program. It is the function where the execution of the program starts. Here's an explanation of each:

int main(void) {
  // Code goes here    
}

• int main():

  • This declaration of main specifies that the function takes an unspecified number of arguments.
  • In C, if no arguments are specified in the parentheses, it is assumed that the function can accept any number of arguments.
  • While this declaration is allowed by the C language, it is generally considered bad practice because it can lead to potential issues related to argument handling and type-checking.

• int main(void):

  • This declaration of main specifies that the function takes no arguments.
  • By explicitly stating (void) in the parentheses, it conveys that the main function does not expect any input parameters.
  • This declaration is considered good practice and is recommended for clarity and to avoid potential issues with argument handling.

Using int main(void) is generally considered better practice because it clearly indicates that the main function does not accept any arguments. It helps in avoiding accidental mistakes related to argument passing and makes the code more readable and self-explanatory.

Exit Point of a Program

In C programming, the return 0; statement at the end of the main function is not required, but it is commonly used. Here's an explanation:

int main(void) {
  // Code goes here 

  return 0;   
}

• The return statement is used to indicate the exit status of the program to the operating system or the program's caller.

• By convention, a return value of 0 typically indicates successful execution or termination of the program.

• If the return statement is omitted in the main function, the C language automatically inserts an implicit return 0; at the end of the function.

• Including an explicit return 0; at the end of the main function is considered good practice for the following reasons:

  • It makes the code more readable and explicit, clearly indicating the intended termination of the program.
  • It allows for consistency with other functions that may have non-zero return values to indicate specific error conditions.
  • It ensures compatibility with some programming environments that may expect a return value from the main function.

While return 0; is not strictly required, it is widely used and considered a best practice. It helps in writing clear and maintainable code, especially when working on larger projects or collaborating with other developers.

Data Types: Defining the Nature of Data

Data types are used to define the type of data that a variable can hold. C provides several built-in data types, including integers, characters, booleans, and arrays. Let's explore these data types by examining the code snippet provided.

/* Data Types */

const int numb = 89;
const char *str = "Bello Osagie";
const bool isTrue = 5 > 3;

In the above code, we can identify three different data types being used:

• int is used to store integer values. Here, numb represents the number 89.

• char* (character pointer) is used to store strings. In this case, str points to the string Bello Osagie.

• bool represents boolean values (true or false). The variable isTrue is assigned the value true because the expression 5 > 3 evaluates to true.

By utilizing different data types, we can handle various kinds of information in our programs.

Note

These are not all the data types we have in C. We will explore others later.

Array: List of Characters as a String

The code char text[] = "This is a text"; represents a character array, which is commonly used to store strings in C. In this case, the array text is initialized with the string "This is a text".

char text[] = "This is a text";

Although it is common to use character arrays to store strings in C, it's important to note that there is no separate data type specifically for strings. Instead, strings are represented as arrays of characters terminated by a null character ('\0'), indicating the end of the string.

Modification of a Character at a specific index

Let's visually represent the interpretation of the given code below and explain the zero-indexed string, modification of the character at index 8, and the memory addresses before and after the modification:

char text[] = "This is a text";
text[8] = '1';

1. Declaration and Initialization of the Character Array (String):

char text[] = "This is a text";

This code declares and initializes a character array text with the string "This is a text". The characters of the string are stored in sequential memory locations.

Visual representation:

Memory:
-----------------------------------------------------
| Address |   Value   |          Variable           |
-----------------------------------------------------
|  1000   |    'T'    |    text[0] (char, 'T')      |
|  1001   |    'h'    |    text[1] (char, 'h')      |
|  1002   |    'i'    |    text[2] (char, 'i')      |
|  1003   |    's'    |    text[3] (char, 's')      |
|  1004   |    ' '    |    text[4] (char, ' ')      |
|  1005   |    'i'    |    text[5] (char, 'i')      |
|  1006   |    's'    |    text[6] (char, 's')      |
|  1007   |    ' '    |    text[7] (char, ' ')      |
|  1008   |    'a'    |    text[8] (char, 'a')      |
|  1009   |    ' '    |    text[9] (char, ' ')      |
|  1010   |    't'    |    text[10] (char, 't')     |
|  1011   |    'e'    |    text[11] (char, 'e')     |
|  1012   |    'x'    |    text[12] (char, 'x')     |
|  1013   |    't'    |    text[13] (char, 't')     |
|  1014   |   '\0'    |    text[14] (char, '\0')    |
-----------------------------------------------------

2.) Modification of Character at Index 8:

text[8] = '1';

This code replaces the character at index 8 (zero-indexed) of text with the character '1'.

Visual representation (after modification):

Memory:
-----------------------------------------------------
| Address |   Value   |          Variable           |
-----------------------------------------------------
|  1000   |    'T'    |    text[0] (char, 'T')      |
|  1001   |    'h'    |    text[1] (char, 'h')      |
|  1002   |    'i'    |    text[2] (char, 'i')      |
|  1003   |    's'    |    text[3] (char, 's')      |
|  1004   |    ' '    |    text[4] (char, ' ')      |
|  1005   |    'i'    |    text[5] (char, 'i')      |
|  1006   |   's'     |    text[6] (char, 's')      |
|  1007   |    ' '    |    text[7] (char, ' ')      |
|  1008   |    '1'    |    text[8] (char, '1')      |
|  1009   |    ' '    |    text[9] (char, ' ')      |
|  1010   |    't'    |    text[10] (char, 't')     |
|  1011   |    'e'    |    text[11] (char, 'e')     |
|  1012   |    'x'    |    text[12] (char, 'x')     |
|  1013   |    't'    |    text[13] (char, 't')     |
|  1014   |   '\0'    |    text[14] (char, '\0')    |
-----------------------------------------------------

In a zero-indexed string or array, the first element is accessed using index 0. So, in the given code, text[8] represents the character at the ninth position in the string. By assigning '1' to text[8], the original character 'a' at index 8 is replaced with '1'.

The memory addresses before and after the modification remain the same, only the values stored at those addresses are changed. This is because the array text is mutable, allowing us to modify its elements even if it is declared as a character array (string) or a constant.

Arrays: Storing Multiple Values

Arrays allow for the storage of multiple values of the same data type. They provide a convenient way to group related data together. Let's analyze the usage of arrays in the code snippet.

const int myNumbers[] = { 4, 8, 1, 9, 7 }; // array
const char *myString[] = { "money", "people", "love" }; // array

The code snippet demonstrates the declaration and initialization of two arrays, myNumbers and myString. myNumbers is an array of integers, while myString is an array of character pointers (strings).

Visual Representation of Array

Let's visually represent the given code and explain the concept of zero-based indexing in arrays and strings:

1. Declaration and Initialization of Integer Array:

const int myNumbers[] = { 4, 8, 1, 9, 7 };

This code declares and initializes an integer array myNumbers with 5 elements. The elements are initialized with the provided values.

Visual representation:

myNumbers:
| int | index
--------------
|  4  | 0
--------------
|  8  | 1
--------------
|  1  | 2
--------------
|  9  | 3
--------------
|  7  | 4
--------------

In C and many programming languages, arrays are zero-based indexed. It means that the index of the first element in the array is 0, the second element is 1, and so on. In the myNumbers array, the values are accessed as follows:

myNumbers[0] = 4
myNumbers[1] = 8
myNumbers[2] = 1
myNumbers[3] = 9
myNumbers[4] = 7

2.) Declaration and Initialization of String Array:

const char *myString[] = { "money", "people", "love" };

This code declares and initializes a string array myString with 3 elements. Each element is a pointer to a string literal.

Visual representation:

myString:
| str      | index
--------------
| "money"  | 0
--------------
| "people" | 1
--------------
|  "love"  | 2
--------------

Similarly, in C, strings are also represented as arrays of characters. Each character in a string has its index starting from 0. In the myString array, the values are accessed as follows:

myString[0] = "money"
myString[1] = "people"
myString[2] = "love"

Remember that the index represents the position of an element in the array or a character in a string, starting from 0.

Zero-based indexing is a convention used in programming languages, and it provides a consistent and efficient way to access elements in arrays and strings. Starting the index at 0 aligns with the underlying memory representation and allows for simple arithmetic operations when working with array elements.

Constants: Immutable Values

In C programming, constants are fixed values that cannot be modified during the execution of a program. They are declared using the const keyword. Let's examine the usage of constants in our code.

/* Constants */

const int numb = 89;
const char *str = "Bello Osagie";
const bool isTrue = 5 > 3;
char text[] = "This is a text";
text[8] = '1';
const char greetings[] = { 'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', '\0' };
const int myNumbers[] = { 4, 8, 1, 9, 7 }; // array
const char *myString[] = { "money", "people", "love" }; // array
const int indexNumb = myNumbers[2];
const char *indexStr = myString[1];
const int myNumbers2D[2][3] = { {3, 5, 8 }, {3, 0, 1} };
const int indexMyNumber2D = myNumbers2D[1][0];
int replaceIndexMyNumber2D = myNumbers2D[1][0];
replaceIndexMyNumber2D = 10;

The constant numb is declared as an integer with the value 89. Since it is defined as a constant, its value cannot be changed later in the program. Similarly, str is declared as a constant character pointer pointing to the string Bello Osagie.

The boolean constant isTrue is assigned the result of the expression 5 > 3, which evaluates to true. Again, as a constant, its value remains unchanged.

The array greetings is initialized with the characters 'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd', and '\0', representing a null-terminated string constant.

myNumbers and myString are arrays of integers and character pointers, respectively. These arrays are initialized with fixed values and cannot be modified later.

Next, we have the 2D array myNumbers2D, which is initialized with two rows and three columns of integers.

We have variables such as indexNumb and indexStr, which store specific elements from the constant arrays.

Finally, we have the variable replaceIndexMyNumber2D which initially holds the value of myNumbers2D[1][0] and later gets reassigned to the value 10.

Constants in Array

Let's visually represent the interpretation of the given code and the output after the modification. Then, I will explain why the changes in the array elements are possible despite using const.

const int indexMyNumber2D = myNumbers2D[1][0];
int replaceIndexMyNumber2D = indexMyNumber2D;
replaceIndexMyNumber2D = 10;

1.) Declaration and Initialization of 2D Array:

const int myNumbers2D[2][3] = { 
    {3, 5, 8},
    {3, 0, 1}
};

This code declares and initializes a 2D array myNumbers2D with 2 rows and 3 columns. The array elements are initialized with the provided values.

Visual representation:

myNumbers2D:
--------------
| 3 | 5 | 8 |
--------------
| 3 | 0 | 1 |
--------------

2.) Accessing Element of the 2D Array:

const int indexMyNumber2D = myNumbers2D[1][0];

This code assigns the value at the index [1][0] of myNumbers2D to the constant variable indexMyNumber2D.

Visual representation (after this step):

myNumbers2D:
--------------
| 3 | 5 | 8 |
--------------
| 3 | 0 | 1 |
--------------
indexMyNumber2D: 3

3.) Assignment of Element to a Non-constant Variable:

int replaceIndexMyNumber2D = myNumbers2D[1][0];
replaceIndexMyNumber2D = 10;

This code assigns the value at the index [1][0] of myNumbers2D to the variable replaceIndexMyNumber2D. Then, it reassigns the value of replaceIndexMyNumber2D to 10.

Visual representation (after this step):

myNumbers2D:
--------------
| 3 | 5 | 8 |
--------------
| 3 | 0 | 1 |
--------------
indexMyNumber2D: 3
replaceIndexMyNumber2D: 10

Even though the array myNumbers2D is declared as const, the changes in the array elements are possible because the const qualifier applies to the elements themselves, not to the variables that hold them. The const keyword ensures that the elements cannot be modified through the array variable myNumbers2D.

In memory, the array myNumbers2D is stored as a contiguous block. Each element in the array has its address. When you assign an element to a non-constant variable, like replaceIndexMyNumber2D, the value of that element is copied to the variable, including its address. The variable replaceIndexMyNumber2D now holds the value and the address of the element.

When you modify the value of replaceIndexMyNumber2D, you are changing the value stored at the address held by the variable. However, the original array elements in myNumbers2D remains unchanged. The modification only affects the variable replaceIndexMyNumber2D and its associated memory location.

const int indexMyNumber2D = myNumbers2D[1][0];
int replaceIndexMyNumber2D = indexMyNumber2D; // myNumbers2D[1][0];
replaceIndexMyNumber2D = 10;

Visual representation (address illustration):

Memory:
---------------------------------------------------
| Address |  Value  |           Variable          |
---------------------------------------------------
|  1000   |    3    | myNumbers2D[0][0] (const)   |
|  1004   |    5    | myNumbers2D[0][1] (const)   |
|  1008   |    8    | myNumbers2D[0][2] (const)   |
|  1012   |    3    | myNumbers2D[1][0] (const)   |
|  1016   |    0    | myNumbers2D[1][1] (const)   |
|  1020   |    1    | myNumbers2D[1][2] (const)   |
|  1024   |    3    | indexMyNumber2D (const)     |
|  1028   |   10    | replaceIndexMyNumber2D      |
---------------------------------------------------

In this illustration, the addresses and values of the variables are shown in memory. The const variables (myNumbers2D, indexMyNumber2D) hold the original values without any modifications. The non-constant variable replaceIndexMyNumber2D holds the modified value.

const int indexMyNumber2D = myNumbers2D[1][0];
int replaceIndexMyNumber2D = indexMyNumber2D; // myNumbers2D[1][0];
replaceIndexMyNumber2D = 10;

Printing the Values: Printing Formatted Output

To observe the values of the variables and constants in our code, we use the printf function to print them to the console (terminal). Here's how the values are printed:

/* Printing Values */

printf("%d\n", numb); // Print the value of numb
printf("%s\n", text); // Print the value of text
printf("%s\n", str); // Print the value of str
printf("%s\n", text); // Print the value of text
printf("%d\n", isTrue); // Print the value of isTrue
printf("%d\n", indexNumb); // Print the value of indexNumb
printf("%s\n", greetings); // Print the value of greetings
printf("%s\n", indexStr); // Print the value of indexStr
printf("%d\n", indexMyNumber2D); // Print the value of indexMyNumber2D
printf("%d\n", replaceIndexMyNumber2D); // Print the value of replaceIndexMyNumber2D

Each printf statement specifies a format specifier to indicate the data type of the variable or constant being printed. For example, %d is used to print integers, %s is used to print strings, and %d is also used for booleans (where true is printed as 1 and false is printed as 0).

Note

• %d is the same as %i.

• The format specifier %c is used to print a single character in C. However, the value representing the character must be enclosed in single quotes (' '), not backticks. Below is an example:
  

printf("%c\n", 'J'); // A char

This will print the character 'J' to the console followed by a newline character ('\n').

Comments: Documenting Your Code

Comments are essential for code documentation. They provide information about the code's functionality, purpose, and any other relevant details. In the given code snippet, we can see the usage of comments to document the code. Let's take a look:

// This is a single-line comment
/* 
   This is a multi-line comment
   It can span multiple lines
*/

// Code snippet starts here

const int numb = 89; // Variable to store a number
const char *str = "Bello Osagie"; // Variable to store a string
const bool isTrue = 5 > 3; // Variable to store a boolean value

char text[] = "This is a text"; // Variable to store a character array

// Code snippet ends here

In the code snippet, we can see the use of single-line comments and multi-line comments. Single-line comments start with // and continue until the end of the line. They are used to provide brief explanations or comments on a specific line of code.

Multi-line comments start with /* and end with */. They can span multiple lines and are useful for providing detailed explanations or commenting out a block of code.

Comments are not executed as part of the program and do not affect the program's functionality. They are solely meant for human readability and understanding.

Variables: Storing Data

Variables are used to store and manipulate data in C programming. They act as containers that hold values of different types. In the given code snippet, we can see the usage of variables like numb, str, isTrue, text, and more. Let's explore their significance and how they are utilized.

/* variables */

const int numb = 89;
const char *str = "Bello Osagie";
const bool isTrue = 5 > 3;
char text[] = "This is a text";
}

In this code, numb is an integer variable assigned a constant value of 89. We use the const keyword to indicate that the value is constant and cannot be modified.

Similarly, str is a character pointer variable storing the address of the string Bello Osagie. The const keyword ensures that the pointer itself is constant and cannot be used to modify the string.

isTrue is a boolean variable assigned the result of the expression 5 > 3. It will hold the value true since the expression is true.

text is a character array initialized with the string "This is a text." It allows for the manipulation of individual characters within the array.

Modifying Variables: Adding and Replacing Values

Variables can be modified by assigning new values to them. In the code snippet, we can see examples of modifying variables:

text[8] = '1';
replaceIndexMyNumber2D = 10;

The statement text[8] = '1' replaces the character at index 8 in the text array with the character '1'. Similarly, replaceIndexMyNumber2D = 10 assigns the value 10 to the variable replaceIndexMyNumber2D.

Compiling and Running the Code

When compiling a C program using the GCC compiler, you can pass various options and flags to modify the behavior of the compilation process. Let's break down the options used in the commands you mentioned:

/*
 * Run:
 * gcc -Wall -pedantic -Werror -Wextra -std=c99 basics.c -o basics
 * gcc basics.c -o basics
 * ./basics
 */

Let's discuss each option:

• -Wall: Enables compiler warnings for potential issues in the code. It stands for all warnings. When this option is used, the compiler will display a comprehensive set of warning messages to help identify possible problems in the code.

• -pedantic: Enforces strict adherence to the C language standards. It instructs the compiler to be pedantic and only allows code that strictly follows the C standard. It disables certain language extensions and non-standard features.

• -Werror: Treats all warnings as errors. When this option is enabled, any warning generated by the compiler will be treated as an error, causing the compilation to fail. This ensures that the code is warning-free and encourages developers to write clean and error-free code.

• -Wextra: Enables additional warning messages beyond those enabled by -Wall. It provides extra warnings for potentially suspicious code constructs or common programming mistakes.

• -std=c99: Specifies the C language standard to be used. In this case, it specifies the C99 standard. C99 is an ANSI/ISO standard for the C programming language, introduced in 1999. It adds several new features and improvements to the language.

The first command (gcc -Wall -pedantic -Werror -Wextra -std=c99 basics.c -o basics) enables strict warnings, treats warnings as errors, and enforces adherence to the C99 standard. It is advisable to use this command during development or when aiming for strict code quality and adherence to standards. It helps catch potential issues and encourages cleaner code.

The second command (gcc basics.c -o basics) is a simpler command that compiles the basics.c file without any specific warning options or standard enforcement. It can be used for quick compilation without strict warning checks. However, it's generally recommended to use the first command during development or in projects where code quality and adherence to standards are crucial.

By using the first command, you can catch and address potential issues early in the development process, leading to more robust and maintainable code.

Preprocessor Directives

In the code below, #include <stdio.h> and #include <stdbool.h> are preprocessor directives that include the standard C library headers stdio.h and stdbool.h, respectively. Here's an explanation of each:

#include <stdio.h>
#include <stdbool.h>

• #include <stdio.h>:

  • This directive includes the standard input/output library header. It provides functions for input and output operations, such as reading from or writing to the console or files.
  • It allows you to use functions like printf() and scanf() for formatted output and input operations, respectively.
  • In the given code, printf() function is used to print values to the console.

• #include <stdbool.h>:

  • This directive includes the header for the C99 standard's Boolean type support.
  • It defines the bool type, which represents boolean values (true and false).
  • In the given code, bool is used to declare the variable isTrue and store a boolean value (true or false).

Including these headers is necessary to access the functions and definitions provided by the respective libraries. Without including stdio.h, you wouldn't be able to use functions like printf(), and without including stdbool.h, you wouldn't be able to use the bool data type.

Practice

Assuming you have an account or are already logged in on Replit.

• Try the Replit Editor online.

• Click the Fork button.

• Edit the current code.

Conclusion

In this blog post, we explored the basics of C programming by examining data types and constants. We learned how to declare and initialize variables and constants of various data types, including integers, characters, booleans, and arrays. We also observed the results by printing the values using printf. Understanding these fundamental concepts is crucial for building more complex C programs. Stay tuned for future blog posts where we will delve deeper into C programming.

---

References

• C Data Types

• C Constants

• C Arrays

• C Boolean Data Type

• C printf Function

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Happy Coding!!!