5.1 Programming Concept

5.1.1 Introduction to Programming Languages

A computer only understands electrical signals in the form of 0s and 1s. A programming language gives humans a way to write instructions in a form they can actually read and reason about, which is then converted into a form the machine can run.

Writing a program generally follows the same rough sequence, no matter which language is used: understand the problem, plan the logic (using an algorithm, flowchart, or pseudocode), write the source code, translate it into machine code, and then test and fix it. Skipping the planning step is one of the most common reasons beginner programs end up buggy or hard to follow.

5.1.2 Low Level, High Level and 4GL Programming Languages

Programming languages are grouped by how close they sit to the hardware versus how close they sit to human language.

• Low Level Language: written close to the hardware, using machine code (pure binary) or assembly language (short mnemonics like MOV, ADD). Fast to run but hard to write and read.
• High Level Language: written closer to human language, using English-like keywords and readable syntax. Easier to write, debug, and maintain, though it needs to be translated before the machine can run it. Examples: C, C++, Java, Python.
• Fourth Generation Language (4GL): designed to be even more accessible than a typical high-level language, often used for a specific purpose such as database queries or report generation, with less code required to get a result. SQL is a common example.

5.1.3 Compiler, Interpreter and Assembler

Source code written by a programmer has to be translated into a form the processor can execute. Different languages use different translators to do this.

C is a compiled language: the whole program is translated by the compiler into an executable file first, and only then is it run.

5.1.4 Syntax, Semantic and Runtime Errors

• Syntax Error: a mistake in the grammar of the language, such as a missing semicolon or an unmatched bracket. The compiler catches this before the program can even run.
• Semantic Error: the code follows the rules of the language correctly but does not do what the programmer actually intended, such as using the wrong variable in a formula.
• Runtime Error: an error that only appears while the program is running, such as dividing a number by zero or trying to access an array index that doesn't exist.

5.1.5 Control Structures: Sequence, Selection and Iteration

A control structure decides the order in which the statements of a program are executed.

5.1.6 Program Design Tools: Algorithm, Flowchart and Pseudocode

Before writing actual code, a programmer usually plans out the logic of a program using one of these tools.

• Algorithm: a step-by-step written procedure for solving a problem, described in plain language rather than any particular programming language. A good algorithm is finite (it must end), definite (every step is precise and unambiguous), and effective (each step is simple enough to actually carry out).
• Flowchart: a diagram that represents the steps of a program visually, using standard symbols.
• Pseudocode: a structured, code-like description of a program's logic, written in plain English but following a programming-style layout, without worrying about the exact syntax of any real language.

5.1.7 Absolute Binary, BCD, ASCII and Unicode

A computer stores every type of data, whether it is a number, a letter, or a symbol, in binary. These four terms describe different ways that binary is used to represent information.

• Absolute Binary: data represented directly in raw 0s and 1s, the actual form in which a computer's hardware stores and processes everything.
• BCD (Binary Coded Decimal): a way of encoding numbers where each individual decimal digit is stored as its own separate 4-bit binary value, rather than converting the whole number to binary at once. For example, the decimal number 25 is stored as 0010 0101 (2 = 0010, 5 = 0101), not as the binary equivalent of 25 as a whole.
• ASCII (American Standard Code for Information Interchange): a character encoding standard that assigns a unique numeric code (originally 7-bit, later extended to 8-bit) to English letters, digits, punctuation, and control characters. For example, the capital letter 'A' is represented as 65 in ASCII.
• Unicode: a character encoding standard built to represent text from virtually every writing system in the world, not just English, using more bits per character than ASCII to fit the much larger number of symbols. This is what allows a computer to correctly display Devanagari script alongside English text.

5.2 C Programming Language

5.2.1 Introduction and Features of C

C shows up everywhere from operating systems to compilers to everyday applications, mostly because it's efficient and gives a programmer close control over how a program runs.

5.2.2 Structure of a C Program

Every C program follows roughly the same basic layout:

• Preprocessor directives start with # and are processed before compilation. #include <stdio.h> tells the compiler to bring in the standard input/output header file.
• Global variables are declared outside every function and can be accessed from anywhere in the program.
• main() is the most important part of any C program. Execution always begins and ends there, and every program must have exactly one.
• Local variables are declared inside a function and can only be accessed within it.
• return 0; inside main() signals that the program ran successfully; a non-zero value usually signals an error.

5.2.3 C Preprocessor and Header Files

A preprocessor directive is an instruction that starts with a # symbol and is handled before the actual compilation begins. The most common ones are #include, which brings in a header file, and #define, which creates a macro.

A header file (with a .h extension) contains the declarations of functions and macros that belong to the C standard library. For example, stdio.h must be included before a program can use functions like printf() and scanf(), and string.h must be included before using functions like strlen() or strcpy().

5.2.4 Character Set Used in C

The character set is the collection of characters that C recognizes as valid when writing a program. It includes:

• Alphabets: uppercase A-Z and lowercase a-z
• Digits: 0-9
• Special characters: such as + - * / % ; , . ( ) { } [ ] # &
• White space characters: blank space, tab, and newline

5.2.5 Use of Comments

A comment is text in a program that the compiler ignores completely. It exists purely to help a human reader understand the code.

• Single-line comment: starts with // and runs to the end of the line
• Multi-line comment: starts with /* and ends with */, and can span several lines

5.2.6 Identifiers, Keywords and Tokens

• Identifiers: the names a programmer gives to variables, functions, and other elements in a program, such as total or calculateArea.
• Keywords: reserved words that already have a fixed meaning in C, such as int, if, while, and return. A keyword can never be used as an identifier.
• Tokens: the smallest individual units a C compiler recognizes while reading a program, which includes keywords, identifiers, constants, operators, and punctuation.

Naming rules for an identifier:

• Must begin with an alphabet (A-Z, a-z) or an underscore (_)
• Can contain alphabets, digits (0-9), and underscores after the first character
• Cannot contain spaces or special symbols other than an underscore
• Cannot be a reserved keyword
• Case-sensitive — age and Age are treated as two different identifiers

5.2.7 Basic Data Types in C

5.2.8 Constants and Variables

• Variable: a named location in memory that holds a value which can change during program execution, such as int score = 0;. It must be declared with a data type before use.
• Constant: a fixed value that does not change while the program is running.

Types of constants: integer (10, -5), floating-point (3.14, 2.5e-3), character ('A', enclosed in single quotes), and string literals ("Nepal", enclosed in double quotes).

A constant can be defined either with the const keyword (const float PI = 3.14159;) or with the #define preprocessor directive (#define PI 3.14159).

5.2.9 Type Specifiers

A type specifier modifies a basic data type to adjust its size or the range of values it can store. C provides four: short, long, signed, and unsigned. For example, unsigned int can only store non-negative numbers, but in exchange it can hold a larger positive range than a plain signed int.

5.2.10 Simple and Compound Statements

• Simple statement: a single instruction ending with a semicolon, such as a = a + 1;
• Compound statement (block): a group of statements enclosed within curly braces { }, treated by the compiler as a single unit, commonly used inside loops and conditions.

5.2.11 Operators and Expressions

An operator is a symbol that performs an operation on operands, and an expression is a combination of operators, operands, and function calls that evaluates to a single value.

Note: pre-increment (++a) changes the value before it is used in an expression, while post-increment (a++) changes it after. The same applies to decrement.

5.2.12 Input/Output Functions

C uses two standard library functions for basic input and output, both defined in stdio.h.

Common format specifiers: %d/%i for integers, %f for floats (%.2f for 2 decimal places), %c for characters, %s for strings, \n for a newline, \t for a tab space.

The & (address-of) operator is essential with scanf(), since it tells the function the memory address to store the input at. For reading a string with scanf("%s", name), & is not needed because an array name already acts as a pointer to its own base address. Note that scanf() with %s stops reading at the first space, so fgets() is used instead when a string may contain spaces.

5.2.13 Selection Control Statement: Decisions

Selection statements let a program choose which block of code to run, based on a condition.

if statement

if-else statement

Nested if-else / if-else-if ladder

switch statement

The break statement exits the switch block once a match is found. If it's left out, execution "falls through" to the next case. The default case runs only if nothing else matched.

5.2.14 Iteration Control Statement: Looping

Iteration statements repeat a block of code as long as a condition holds true.

for loop

while loop

do-while loop

A nested loop is a loop placed inside another loop, most often used to work through a 2D array or matrix row by row.

5.2.15 Arrays: 1D and 2D

1D (One-Dimensional) Array

A 1D array is a simple, linear list of values, declared as dataType arrayName[size];.

2D (Two-Dimensional) Array

A 2D array is an array of arrays, visualized as a table of rows and columns, declared as dataType arrayName[rows][columns];. Each element needs two indices, one for the row and one for the column.

5.2.16 Strings and String Functions

Note: strrev(), strupr(), and strlwr() are not part of the official ANSI C standard, but they are available as common extensions in compilers such as Turbo C and GCC. For strict portability, the same result can be achieved manually using toupper()/tolower() from ctype.h inside a loop.

Solved Program Examples

A definition is easier to remember once it has been used in an actual program. The programs below cover the most commonly asked coding questions from this unit, each with its expected output.

Activities and Practice

Common Mistakes to Avoid

Sample Board Exam Questions

Based on the official NEB Computer Science Specification Grid 2077, Unit 5 (Programming Concepts and Logics) carries 5 marks out of the theory paper, drawn mainly from the Remembering and Understanding levels. Given the size of this unit, MCQs are the most common format, but SAQ and LAQ patterns from past board papers are included below too, since programming questions can appear this way in practice.

Important Questions

1. What is a programming language? Differentiate between low level and high level languages.
2. Differentiate between compiler and interpreter with examples.
3. Define syntax error, semantic error, and runtime error with one example of each.
4. What are control structures? Explain sequence, selection, and iteration with examples.
5. What is an algorithm? Write an algorithm to find the largest of three numbers.
6. Draw a flowchart to check whether a given number is even or odd.
7. Differentiate between ASCII and Unicode.
8. What is BCD? How is it different from absolute binary?
9. List the features of the C programming language.
10. What is a header file? Explain the use of the C preprocessor with an example.
11. Differentiate between a keyword and an identifier with examples.
12. What are the basic data types available in C?
13. Differentiate between a constant and a variable.
14. Write a C program to find the sum of two numbers entered by the user.
15. Differentiate between if-else and switch statements
16. Differentiate between while and do-while loops with an example of each.
17. Write a C program to print numbers from 1 to 10 using a for loop.
18. Differentiate between a 1D array and a 2D array with examples.
19. Write a C program to add two 3x3 matrices.
20. Define string. List any four string handling functions used in C with their purpose.
21. Write a C program to find the sum and average of 5 numbers using an array.

Frequently Asked Questions