Fundamentals of Computer Programming

A computer's processor understands only one thing: patterns of 0s and 1s. People, on the other hand, think in words and numbers. A programming language is the bridge between the two. This lesson explains the kinds of programming language, the translator programs that turn what we write into what the machine runs, the three kinds of mistake every programmer makes and how to find them, four languages you will meet in school and beyond, and the basic building blocks of every program: constants, variables and data types. Lesson 20 then puts all of it to work in QBASIC.

19.1 Programming languages: low-level (machine and assembly) and high-level languages

Programming languages are divided into two groups according to how close they are to the machine.

Low-level languages

A low-level language is close to the hardware: each instruction does one tiny job inside the processor, and a program written for one type of processor does not work on another. There are two low-level languages.

Machine language (first generation, 1GL) is written entirely in binary — 0s and 1s. It is the only language the processor understands directly, so it needs no translation and runs fastest. But it is almost impossible for people to read, very easy to get wrong, and different for every family of processors (it is machine-dependent). The first computers were programmed this way, by setting switches or punching holes in cards.

Assembly language (second generation, 2GL) replaces each binary instruction with a short word called a mnemonic: MOV (move), ADD, SUB (subtract), JMP (jump). It is easier to read than binary, but it is still machine-dependent and needs one instruction for each tiny step. A program called an assembler translates it into machine language. Assembly language is still used where speed and size matter most, as in parts of device drivers and the tiny chips inside washing machines and remote controls.

High-level languages

A high-level language (third generation, 3GL, and later) uses English words and ordinary mathematical signs, such as PRINT, IF, INPUT and C = A + B. One high-level statement may stand for many machine instructions. High-level programs are easy to read, write and correct, and the same program works on many kinds of computer (it is portable, or machine-independent). They must be translated into machine language by a compiler or an interpreter before they run. QBASIC, Python, C, C++ and Java are high-level languages. Some books also name fourth-generation languages (4GL) such as SQL, used to ask a database for information, and fifth-generation languages (5GL) such as Prolog, used in artificial intelligence.

19.2 Language processors: compiler, interpreter and assembler

Compiler. A compiler reads the whole source program, checks all of it, and translates it at once into an object program — on Windows, usually an .exe file. If there are errors, it lists them all after it has finished reading, and no object program is made until they are corrected. Once compiled, the program runs on its own, without the compiler, as often as you like, and it runs fast. C, C++ and FORTRAN programs are compiled; GCC (the GNU Compiler Collection) and the compiler in Microsoft Visual Studio are examples.

Interpreter. An interpreter takes the source program one line (statement) at a time: it translates that line, carries it out at once, and only then moves to the next line. It stops at the first error it meets and reports it, which makes errors easy to find and fix — ideal for learning. But no separate object program is made, so the interpreter is needed every time the program runs, and the program runs more slowly. QBASIC and Python are run by interpreters.

Assembler. An assembler translates a program in assembly language into machine language, one mnemonic at a time into the matching binary instruction.

19.3 Syntax errors, logical errors and run-time errors; testing and debugging

An error in a program is called a bug. The word was used by engineers long before computers, but it became famous on 9 September 1947, when operators of the Harvard Mark II computer found a moth stuck in one of its relays, taped it into their log book and wrote "First actual case of bug being found."

Program errors are of three kinds.

print("Namaste"

SyntaxError: '(' was never closed

total = 500
students = 0
print("Share =", total / students)

ZeroDivisionError: division by zero

Testing and debugging

Testing means running a program with chosen test data — inputs whose correct outputs you have worked out beforehand — and comparing the program's output with them. Good test data includes normal values (a mark of 65), boundary values (0, 100, and exactly the pass mark), and wrong values (a negative mark, or a word where a number is expected), to see whether the program copes.

Debugging means finding where an error is and correcting it. Useful ways are:

1. Read the error message: it names the error and usually the line.
2. Dry run (trace) the program on paper with a trace table, as in Lesson 18.
3. Print the values of variables at different points to see where they first go wrong.
4. Step through the program one line at a time and watch the variables. In QBASIC, F8 runs one line at a time and F9 sets a breakpoint, a line where the program pauses.
5. Check one part at a time, and test again after every correction.

CLS
INPUT "Enter marks in English and Maths: ", E, M
AVG = E + M / 2
PRIMT "Average ="; AVG
END

Enter marks in English and Maths: 60, 80
Average = 100

CLS
INPUT "Enter marks in English and Maths: ", E, M
AVG = (E + M) / 2
PRINT "Average ="; AVG
END

Enter marks in English and Maths: 60, 80
Average = 70

19.4 Common high-level languages: QBASIC, Python, C and Java

Hundreds of high-level languages exist. The first to be widely used was FORTRAN (FORmula TRANslation), released by John Backus and his team at IBM in 1957 for science and engineering; COBOL (COmmon Business-Oriented Language), from 1959, ran the accounts of banks and governments for decades. Four languages matter most to you.

QBASIC (Quick Beginner's All-purpose Symbolic Instruction Code). The first BASIC was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College in the USA; its first program ran on 1 May 1964. Microsoft's QBASIC came with MS-DOS 5.0 in 1991. It is run by an interpreter inside its own simple editor, its keywords are plain English (PRINT, INPUT, IF, FOR), and it needs no setting up, so it is still a favourite first language in Nepali schools. Modern free programs such as QB64 and FreeBASIC run QBASIC programs on today's computers.

Python. Created by Guido van Rossum in the Netherlands and first released in February 1991. Python code is short and very readable, it uses indentation instead of END words, and it comes with a huge collection of ready-made libraries. It is used for artificial intelligence, data science, websites and automation, and it is the programming language of the CDC grade 9 and 10 course.

C. Developed by Dennis Ritchie at Bell Laboratories in the USA in 1972, to write the UNIX operating system. C programs are compiled; they are fast and work closely with the hardware, so C is used for operating systems (much of Linux and Windows), device drivers and the small computers inside machines. C++, an extension of C made by Bjarne Stroustrup, came in 1985.

Java. Developed by James Gosling and his team at Sun Microsystems and released in 1995. A Java compiler turns the program into bytecode, which the Java Virtual Machine (JVM) then runs on any computer — "write once, run anywhere". Java is widely used for Android apps, banking systems and large business software. (JavaScript, the language of web pages, is a different language with a similar name.)

A = 25
B = 17
PRINT "Sum ="; A + B
END

a = 25
b = 17
print("Sum =", a + b)

#include <stdio.h>

int main(void)
{
    int a = 25, b = 17;
    printf("Sum = %d\n", a + b);
    return 0;
}

public class Sum {
    public static void main(String[] args) {
        int a = 25, b = 17;
        System.out.println("Sum = " + (a + b));
    }
}

Sum = 42

19.5 Constants, variables and data types

Every program works with data: numbers such as marks, prices and ages, and text such as names and addresses. A program stores data in two ways, as constants and as variables.

Rules for naming variables in QBASIC

1. A name must begin with a letter (A to Z).
2. It may contain letters, digits and the full stop (period), up to 40 characters.
3. No spaces and no other symbols are allowed, except one type symbol (%, &, !, # or $) at the end.
4. A keyword (reserved word) such as PRINT, INPUT or END cannot be a variable name.
5. Capital and small letters are the same in QBASIC: age, Age and AGE are one variable. (In Python they are three different variables.)

Choose meaningful names — TOTAL, PRICE, NAME$ — rather than single letters, so that the program explains itself. Python's rules are similar: a name starts with a letter or underscore, may contain letters, digits and underscores, cannot be a keyword, and is case-sensitive.

Data types

The data type of a value tells the computer what kind of data it is, how much memory to give it and what can be done with it. The two basic kinds are numeric data, used in calculations, and string (character) data, which is text: letters, digits and symbols treated as characters, not as amounts. In QBASIC the type of a variable is shown by a symbol at the end of its name; a name with no symbol is SINGLE.

CLS
REM Variables of different data types
N$ = "Sita Thapa"
AGE% = 14
HEIGHT! = 1.52
POP& = 29164578
PRINT N$; " is"; AGE%; "years old and"; HEIGHT!; "m tall."
PRINT "Population of Nepal, census 2021:"; POP&
END

Sita Thapa is 14 years old and 1.52 m tall.
Population of Nepal, census 2021: 29164578

name = "Sita Thapa"
age = 14
height = 1.52
population = 29164578
is_student = True
print(name, "is", age, "years old and", height, "m tall.")
print(type(name), type(age), type(height), type(is_student))

Sita Thapa is 14 years old and 1.52 m tall.
<class 'str'> <class 'int'> <class 'float'> <class 'bool'>

Practice questions

Group A — Objective (1 mark each)

1. Machine language is written using [1]aEnglish wordsbmnemonicsc0s and 1sdflowchart symbols
2. The translator that converts assembly language into machine language is [1]aa compilerban interpretercan assemblerdan editor
3. Which translator converts the whole program at once and makes an object program? [1]ainterpreterbcompilercassemblerddebugger
4. QBASIC programs are run by [1]aan assemblerban interpretercan operating systemda modem
5. Which of these is a low-level language? [1]aPythonbJavacassembly languagedQBASIC
6. Dividing a number by zero while a program is running causes [1]aa syntax errorba logical errorca run-time errordno error
7. A program runs to the end but prints a wrong answer because of a wrong formula. This is [1]aa syntax errorba logical errorca run-time errorda hardware fault
8. Python was created by [1]aDennis RitchiebJames GoslingcGuido van RossumdJohn Kemeny
9. In QBASIC, the symbol that marks a string variable is [1]a%b&c!d$
10. Which is a valid QBASIC variable name? [1]a1STNAMEbFIRST NAMEcTOTAL.MARKSdPRINT
11. The range of an INTEGER variable in QBASIC is [1]a0 to 255b−32,768 to 32,767c−2,147,483,648 to 2,147,483,647dunlimited
12. A program written by a programmer in a high-level language is called [1]aobject codebsource codecmachine codedbytecode

Group B — Short answer (2 marks each)

1. Write any two differences between a low-level and a high-level language. [2]
2. What is a language processor? Name its three types. [2]
3. Write any two differences between a compiler and an interpreter. [2]
4. What is assembly language? Why does it need an assembler? [2]
5. Define syntax error and logical error with one example each. [2]
6. What is a run-time error? Give two examples. [2]
7. What is debugging? Write any two ways of debugging a program. [2]
8. Define constant and variable with one example of each. [2]
9. Write any four rules for naming a variable in QBASIC. [2]
10. Name the five data types of QBASIC with their type symbols. [2]

Group C — Long answer (4 marks each)

1. What is a programming language? Explain machine language, assembly language and high-level language, giving one advantage and one disadvantage of each. [1+3]
2. Explain compiler, interpreter and assembler. Which translator is used for QBASIC programs and which for C programs? [3+1]
3. What are testing and debugging? Explain the three types of programming error with one example each. [1+3]
4. Write short notes on any two of these languages: QBASIC, Python, C, Java. [2+2]
5. Study the program below. Find the errors, name the type of each, and write the correct program and its output when the length is 12 and the breadth is 5. [4]

CLS
INPUT "Enter length and breadth: ", L, B
A = L + B
PRINT "Area ="; A
P = 2 * L + B
PRNT "Perimeter ="; P
END

CLS
INPUT "Enter length and breadth: ", L, B
A = L * B
PRINT "Area ="; A
P = 2 * (L + B)
PRINT "Perimeter ="; P
END

Enter length and breadth: 12, 5
Area = 60
Perimeter = 34