Computer Programming using QBASIC

Problem. (a) Find the value of 10 + 8 / 2 ^ 2 * 3. (b) Write QBASIC expressions for \frac{a+b}{c}, a^2 + 2ab and \sqrt{a^2 + b^2}, and find their values when a = 3, b = 4 and c = 5.

Solution. (a) Exponent first: 2 ^ 2 = 4. Then division and multiplication from left to right: 8 / 4 = 2 and 2 * 3 = 6. Last, addition: 10 + 6 = 16.

(b)

QBASIC printed exactly these values, 1.4 33 5, when the three expressions were run. In algebra ab means a × b, but in QBASIC the * must always be written: 2AB is an error, 2 * A * B is correct.

Problem. Write a program that displays the name and address of a school, a blank line, and the class.

Solution.

CLS
REM Program 20.1: my first program
PRINT "Shree Janata Secondary School"
PRINT "Biratnagar-7, Morang"
PRINT
PRINT "Class 8 Computer Science"
END

Shree Janata Secondary School
Biratnagar-7, Morang

Class 8 Computer Science

The REM line does not appear in the output: it is a note for people reading the program.

Problem. Show the effect of ;, , and TAB in PRINT statements.

Solution.

CLS
REM Program 20.2: semicolon, comma and TAB
PRINT "Name"; "Marks"
PRINT "Name", "Marks"
PRINT "Asha", 87
PRINT "Bikash", 92
PRINT "Roll"; TAB(10); "Name"; TAB(25); "Marks"
PRINT 1; TAB(10); "Asha"; TAB(25); 87
END

NameMarks
Name          Marks
Asha           87
Bikash         92
Roll     Name           Marks
 1       Asha            87

The semicolon joined "Name" and "Marks" with no space. The comma started "Marks" at column 15; the numbers 87 and 92 begin at column 15 too, but their first character is the space for the sign. TAB lines up the columns of a table.

Problem. Read two numbers and display their sum, difference, product and quotient.

Solution.

CLS
REM Program 20.3: four operations on two numbers
INPUT "Enter the first number"; A
INPUT "Enter the second number"; B
LET S = A + B
LET D = A - B
LET P = A * B
LET Q = A / B
PRINT "Sum ="; S
PRINT "Difference ="; D
PRINT "Product ="; P
PRINT "Quotient ="; Q
END

Enter the first number? 20
Enter the second number? 8
Sum = 28
Difference = 12
Product = 160
Quotient = 2.5

Problem. Read the principal, time and rate, and display the simple interest and the amount. (This is the algorithm of Worked example 18.3, now coded.)

Solution.

CLS
REM Program 20.4: simple interest
INPUT "Principal (Rs)"; P
INPUT "Time (years)"; T
INPUT "Rate (% per year)"; R
SI = P * T * R / 100
A = P + SI
PRINT "Simple interest = Rs"; SI
PRINT "Amount = Rs"; A
END

Principal (Rs)? 25000
Time (years)? 3
Rate (% per year)? 9.5
Simple interest = Rs 7125
Amount = Rs 32125

Check: 25,000 × 3 × 9.5 ÷ 100 = 7,125 and 25,000 + 7,125 = 32,125 ✓.

Problem. Convert a temperature from degrees Celsius to degrees Fahrenheit using F = C × 9 ÷ 5 + 32.

Solution.

CLS
REM Program 20.5: Celsius to Fahrenheit
INPUT "Temperature in Celsius: ", C
F = C * 9 / 5 + 32
PRINT C; "degrees Celsius ="; F; "degrees Fahrenheit"
END

Temperature in Celsius: 37
 37 degrees Celsius = 98.6 degrees Fahrenheit

The prompt ends with a comma, so no question mark appears. The output line starts with a space because it begins with a number.

Problem. Read the radius of a circle and display its area (πr²) and circumference (2πr), taking π = 3.14.

Solution.

CLS
REM Program 20.6: area and circumference of a circle
CONST PI = 3.14
INPUT "Radius of the circle (cm)"; R
A = PI * R ^ 2
C = 2 * PI * R
PRINT "Area ="; A; "sq. cm"
PRINT "Circumference ="; C; "cm"
END

Radius of the circle (cm)? 7
Area = 153.86 sq. cm
Circumference = 43.96 cm

Problem. Display the values of several mathematical functions.

Solution.

CLS
REM Program 20.7: mathematical functions
PRINT "SQR(144) ="; SQR(144)
PRINT "ABS(-25) ="; ABS(-25)
PRINT "INT(7.8) ="; INT(7.8); "  INT(-7.8) ="; INT(-7.8)
PRINT "FIX(7.8) ="; FIX(7.8); "  FIX(-7.8) ="; FIX(-7.8)
PRINT "CINT(7.8) ="; CINT(7.8); "  CINT(-7.3) ="; CINT(-7.3)
PRINT "SGN(-9) ="; SGN(-9); "  SGN(0) ="; SGN(0)
PRINT "17 MOD 5 ="; 17 MOD 5; "  2 ^ 10 ="; 2 ^ 10
END

SQR(144) = 12
ABS(-25) = 25
INT(7.8) = 7   INT(-7.8) =-8
FIX(7.8) = 7   FIX(-7.8) =-7
CINT(7.8) = 8   CINT(-7.3) =-7
SGN(-9) =-1   SGN(0) = 0
17 MOD 5 = 2   2 ^ 10 = 1024

A negative number is printed with its minus sign in the place where a positive number has its leading space, which is why "=-8" has no gap.

Problem. Display the results of the main string functions on the word "Kathmandu".

Solution.

CLS
REM Program 20.8: string functions
W$ = "Kathmandu"
PRINT LEN(W$)
PRINT LEFT$(W$, 4)
PRINT RIGHT$(W$, 5)
PRINT MID$(W$, 3, 4)
PRINT UCASE$(W$); " "; LCASE$(W$)
PRINT ASC("A"); CHR$(66)
PRINT "["; STR$(45); "]"; VAL("12.5") + 1
PRINT "["; LTRIM$("   Nepal"); "]"; INSTR(W$, "man")
END

 9
Kath
mandu
thma
KATHMANDU kathmandu
 65 B
[ 45] 13.5
[Nepal] 5

The square brackets show where each string starts and ends: STR$(45) keeps the leading space of a positive number, and LTRIM$ removed the three spaces before "Nepal".

Problem. Read a whole number and say whether it is odd or even.

Solution. A number is even when the remainder after dividing by 2 is 0.

CLS
REM Program 20.9: odd or even
INPUT "Enter a whole number"; N
IF N MOD 2 = 0 THEN
    PRINT N; "is an even number."
ELSE
    PRINT N; "is an odd number."
END IF
END

Two runs:

Enter a whole number? 46
 46 is an even number.

Enter a whole number? -7
-7 is an odd number.

Problem. Read three numbers and display the greatest.

Solution. AND joins two conditions; >= handles equal numbers.

CLS
REM Program 20.10: the greatest of three numbers
INPUT "Enter three numbers: ", A, B, C
IF A >= B AND A >= C THEN
    G = A
ELSEIF B >= A AND B >= C THEN
    G = B
ELSE
    G = C
END IF
PRINT "The greatest number is"; G
END

Enter three numbers: 45, 72, 68
The greatest number is 72

Problem. Read a percentage and display the letter grade of the SEE grading system: A+ (90 and above), A (80 to below 90), B+ (70–80), B (60–70), C+ (50–60), C (40–50), D (35–40) and NG (below 35).

Solution. The cases are checked from the top, so CASE IS >= 80 is reached only by marks below 90.

CLS
REM Program 20.11: SEE grade from percentage
INPUT "Enter the percentage"; P
SELECT CASE P
    CASE IS > 100, IS < 0
        G$ = "not a valid percentage"
    CASE IS >= 90
        G$ = "A+"
    CASE IS >= 80
        G$ = "A"
    CASE IS >= 70
        G$ = "B+"
    CASE IS >= 60
        G$ = "B"
    CASE IS >= 50
        G$ = "C+"
    CASE IS >= 40
        G$ = "C"
    CASE IS >= 35
        G$ = "D"
    CASE ELSE
        G$ = "NG"
END SELECT
PRINT "Grade: "; G$
END

Three runs:

Enter the percentage? 86.5
Grade: A

Enter the percentage? 33
Grade: NG

Enter the percentage? 105
Grade: not a valid percentage

Problem. Read a number from 1 to 7 and display the day of the week, Sunday being day 1.

Solution.

CLS
REM Program 20.12: day of the week
INPUT "Enter a day number (1 to 7)"; D
SELECT CASE D
    CASE 1
        PRINT "Sunday"
    CASE 2
        PRINT "Monday"
    CASE 3
        PRINT "Tuesday"
    CASE 4
        PRINT "Wednesday"
    CASE 5
        PRINT "Thursday"
    CASE 6
        PRINT "Friday"
    CASE 7
        PRINT "Saturday - the weekly holiday in Nepal"
    CASE ELSE
        PRINT "There is no day number"; D
END SELECT
END

Two runs:

Enter a day number (1 to 7)? 7
Saturday - the weekly holiday in Nepal

Enter a day number (1 to 7)? 9
There is no day number 9

Problem. Read a letter and say whether it is a vowel or a consonant.

Solution. UCASE$ turns a small letter into a capital, so only capitals need to be listed. A string range such as "B" TO "Z" follows alphabetical order.

CLS
REM Program 20.13: vowel or consonant
INPUT "Enter a letter"; L$
L$ = UCASE$(L$)
SELECT CASE L$
    CASE "A", "E", "I", "O", "U"
        PRINT L$; " is a vowel."
    CASE "B" TO "Z"
        PRINT L$; " is a consonant."
    CASE ELSE
        PRINT "That is not a letter."
END SELECT
END

Three runs, with e, k and 7:

Enter a letter? e
E is a vowel.

Enter a letter? k
K is a consonant.

Enter a letter? 7
That is not a letter.

Problem. Display the multiplication table of a number.

Solution.

CLS
REM Program 20.14: multiplication table
INPUT "Enter a number"; N
FOR I = 1 TO 10
    PRINT N; "x"; I; "="; N * I
NEXT I
END

Enter a number? 7
 7 x 1 = 7
 7 x 2 = 14
 7 x 3 = 21
 7 x 4 = 28
 7 x 5 = 35
 7 x 6 = 42
 7 x 7 = 49
 7 x 8 = 56
 7 x 9 = 63
 7 x 10 = 70

Problem. Display the even numbers from 2 to 20 and their sum, then count down from 10 to 1.

Solution.

CLS
REM Program 20.15: counting with STEP
PRINT "Even numbers from 2 to 20:"
S = 0
FOR I = 2 TO 20 STEP 2
    PRINT I;
    S = S + I
NEXT I
PRINT
PRINT "Their sum ="; S
PRINT "Countdown:"
FOR I = 10 TO 1 STEP -1
    PRINT I;
NEXT I
PRINT
PRINT "Blast off!"
END

Even numbers from 2 to 20:
 2  4  6  8  10  12  14  16  18  20
Their sum = 110
Countdown:
 10  9  8  7  6  5  4  3  2  1
Blast off!

The semicolon after PRINT I keeps the numbers on one line; the empty PRINT after the loop ends that line.

Problem. Find the factorial of a number, N! = 1 × 2 × … × N.

Solution. F starts at 1 (not 0), because it is a product.

CLS
REM Program 20.16: factorial of a number
INPUT "Enter a number"; N
F = 1
FOR I = 1 TO N
    F = F * I
NEXT I
PRINT "Factorial of"; N; "is"; F
END

Enter a number? 6
Factorial of 6 is 720

Problem. Read a positive whole number. Find the sum of its digits and the number reversed, and say whether it is a palindrome (the same when reversed).

Solution. N MOD 10 gives the last digit and N \ 10 removes it; the loop goes on while digits are left. The original number is kept in M, because N is used up by the loop.

CLS
REM Program 20.17: sum of digits, reverse and palindrome
INPUT "Enter a positive whole number"; N
M = N
S = 0
R = 0
WHILE N > 0
    D = N MOD 10
    S = S + D
    R = R * 10 + D
    N = N \ 10
WEND
PRINT "Sum of the digits ="; S
PRINT "Reversed number ="; R
IF R = M THEN PRINT M; "is a palindrome." ELSE PRINT M; "is not a palindrome."
END

Two runs:

Enter a positive whole number? 4521
Sum of the digits = 12
Reversed number = 1254
 4521 is not a palindrome.

Enter a positive whole number? 12321
Sum of the digits = 9
Reversed number = 12321
 12321 is a palindrome.

Trace for 4521: digits 1, 2, 5, 4 come off one by one; S goes 1, 3, 8, 12; R goes 1, 12, 125, 1254; N goes 452, 45, 4, 0 and the loop stops.

Problem. Display the Fibonacci numbers (0, 1, 1, 2, 3, 5, …, each the sum of the two before it) that are less than a given limit.

Solution.

CLS
REM Program 20.18: Fibonacci series below a limit
INPUT "Show the Fibonacci numbers below"; L
A = 0
B = 1
DO WHILE A < L
    PRINT A;
    C = A + B
    A = B
    B = C
LOOP
PRINT
END

Show the Fibonacci numbers below? 100
 0  1  1  2  3  5  8  13  21  34  55  89

The number of terms is not known in advance, so a FOR loop does not fit; the loop runs while A is still below the limit.

Problem. Keep reading numbers until the user types 0, then display how many numbers were entered and their total.

Solution. The body must run at least once — a number has to be read before it can be tested — so the test goes at the bottom.

CLS
REM Program 20.19: add numbers until 0 is entered
T = 0
C = 0
DO
    INPUT "Enter a number (0 to stop)"; N
    T = T + N
    IF N <> 0 THEN C = C + 1
LOOP UNTIL N = 0
PRINT "You entered"; C; "numbers."
PRINT "Their total is"; T
END

Enter a number (0 to stop)? 25
Enter a number (0 to stop)? 40
Enter a number (0 to stop)? 15
Enter a number (0 to stop)? 0
You entered 3 numbers.
Their total is 80

Problem. Display the pattern 1 / 1 2 / 1 2 3 / 1 2 3 4 / 1 2 3 4 5, one row per line.

Solution. The outer loop counts the rows (I = 1 to 5); the inner loop prints the numbers 1 to I on that row.

CLS
REM Program 20.20: a number triangle (nested loops)
FOR I = 1 TO 5
    FOR J = 1 TO I
        PRINT J;
    NEXT J
    PRINT
NEXT I
END

 1
 1  2
 1  2  3
 1  2  3  4
 1  2  3  4  5

The inner loop must be completely inside the outer one: NEXT J comes before NEXT I.

Problem. Read a word, display it reversed, count its vowels, and say if it is a palindrome.

Solution. The loop runs backwards over the positions of the word, from LEN(W$) down to 1, taking one character at a time with MID$. INSTR checks whether that character appears in the list of vowels.

CLS
REM Program 20.21: reverse a word and count its vowels
INPUT "Enter a word"; W$
R$ = ""
V = 0
FOR I = LEN(W$) TO 1 STEP -1
    CH$ = MID$(W$, I, 1)
    R$ = R$ + CH$
    IF INSTR("AEIOUaeiou", CH$) > 0 THEN V = V + 1
NEXT I
PRINT "Reversed: "; R$
PRINT "Number of vowels:"; V
IF UCASE$(R$) = UCASE$(W$) THEN PRINT W$; " is a palindrome."
END

Two runs:

Enter a word? Nepal
Reversed: lapeN
Number of vowels: 2

Enter a word? Malayalam
Reversed: malayalaM
Number of vowels: 4
Malayalam is a palindrome.

Problem. This program should find the average of N numbers typed by the user, but it has three faults. Find each, name its type, and write the corrected program.

CLS
REM Average of N numbers typed by the user
INPUT "How many numbers"; N
S = 1
FOR I = 1 TO N
    INPUT "Enter a number"; X
    S = S + X
NEXT J
PRINT "Average ="; S / N
END

Solution.

1. NEXT J does not match FOR I. QBASIC cannot pair them and refuses to run the program; FreeBASIC reported "FOR/NEXT variable name mismatch" and QB64 "Incorrect variable after NEXT". It must be NEXT I.
2. S = 1 is a logical error: a sum must start at 0. With only the NEXT corrected, the numbers 12, 15, 9 and 20 gave "Average = 14.25" instead of 14, with no warning at all.
3. If the user enters 0 for N, S / N divides by zero — a run-time error. The corrected program checks N first.

CLS
REM Program 20.22: average of N numbers typed by the user
INPUT "How many numbers"; N
IF N <= 0 THEN
    PRINT "Nothing to average."
ELSE
    S = 0
    FOR I = 1 TO N
        INPUT "Enter a number"; X
        S = S + X
    NEXT I
    PRINT "Average ="; S / N
END IF
END

Two runs:

How many numbers? 4
Enter a number? 12
Enter a number? 15
Enter a number? 9
Enter a number? 20
Average = 14

How many numbers? 0
Nothing to average.

Check: (12 + 15 + 9 + 20) ÷ 4 = 56 ÷ 4 = 14 ✓.