CMS 708 · What the Course Document Leaves Out

The course document says structured programming uses "clear control structures" but never states the underlying result. It is worth quoting because it explains why goto can simply be banned:

Each construct has one entry point and one exit point · that single-entry / single-exit property is what makes structured programs traceable, and what goto destroys.

Cohesion and coupling · the measure of good modules

Modularity is examinable, and these two words are how its quality is judged:

Advantages of functions · a likely list question

1. Reusability · write once, call many times
2. Reduces code length and eliminates repetition
3. Easier debugging · faults are isolated to one function
4. Divides labour · different programmers take different functions
5. Improves readability · main() reads like a summary of the program
6. Easier maintenance · a change is made in one place only

The flowchart symbols you must be able to draw

Pseudocode conventions

BEGIN
    READ  number
    IF number MOD 2 = 0 THEN
        PRINT "Even"
    ELSE
        PRINT "Odd"
    ENDIF
END

Use BEGIN/END, READ and PRINT, IF…THEN…ELSE…ENDIF, WHILE…ENDWHILE, FOR…ENDFOR, and indent every nested block. Pseudocode is language-independent: no semicolons, no cout.

The same problem three ways · largest of two numbers

ALGORITHM (numbered steps)
Step 1: Start
Step 2: Read A and B
Step 3: If A > B then print A
Step 4: Else print B
Step 5: Stop

PSEUDOCODE
BEGIN
    READ A, B
    IF A > B THEN
        PRINT A
    ELSE
        PRINT B
    ENDIF
END

C++
#include <iostream>
using namespace std;

int main() {
    int a, b;
    cout << "Enter two numbers: ";
    cin  >> a >> b;

    if (a > b) cout << a << " is larger" << endl;
    else       cout << b << " is larger" << endl;
    return 0;
}

Debugging techniques

• Desk checking / dry running · trace the code by hand with sample values, in a table of variables
• Print statements · insert temporary cout lines to reveal intermediate values
• Breakpoints and single-stepping in a debugger
• Divide and conquer · comment out parts until the fault region is isolated
• Rubber-duck explanation · read the code aloud line by line; the mismatch usually announces itself

Test data · three kinds, and you must name all three

A dry-run table · the format to use

for (int i = 1; i <= 4; i++) { sum += i; }

 i │ sum before │ sum after │ i <= 4 ?
───┼────────────┼───────────┼──────────
 1 │     0      │     1     │  true
 2 │     1      │     3     │  true
 3 │     3      │     6     │  true
 4 │     6      │    10     │  true
 5 │    10      │    10     │  FALSE → exit

Final: sum = 10, i = 5

Draw this table whenever a question says "trace", "dry run" or "what is the output". One column per variable, one row per iteration, and a final line stating the values on exit. It is worth marks even if the final number is wrong.

Function prototypes · declaring before defining

#include <iostream>
using namespace std;

int add(int a, int b);        // PROTOTYPE · ends with ;

int main() {
    cout << add(3, 4) << endl;    // legal: compiler knows it
    return 0;
}

int add(int a, int b) {       // DEFINITION, after main()
    return a + b;
}

C++ reads a file top to bottom, so a function must be known before it is called. Either define it above main(), or declare a prototype above and define it below. The prototype gives the return type, name and parameter types, and ends with a semicolon.

Default arguments

double power(double base, int exp = 2) {   // default
    double result = 1;
    for (int i = 0; i < exp; i++) result *= base;
    return result;
}

power(5);       // 25   · uses exp = 2
power(5, 3);    // 125  · supplied value wins

Function overloading

int    maxOf(int a, int b)       { return a > b ? a : b; }
double maxOf(double a, double b) { return a > b ? a : b; }

maxOf(3, 7);      // calls the int version
maxOf(3.5, 7.1);  // calls the double version

Overloading = several functions sharing one name but differing in the number or types of their parameters. The compiler picks by matching the arguments. Note the conditional operator a > b ? a : b · a compact if-else that returns a value.

The third way to pass a parameter · by pointer

The notes give by value and by reference. C++ has a third method that older exam questions still ask for:

void incrementByPointer(int *p) {   // takes an ADDRESS
    (*p)++;                        // * dereferences it
}

int main() {
    int num = 10;
    incrementByPointer(&num);      // & takes the address
    cout << num << endl;           // 11 · original changed
    return 0;
}

Remember the two symbols: &a means "the address of a"; *p means "the value at the address p".

Storage classes · where static comes from

Declaring, filling and reading

int scores[5];                       // 5 ints, indices 0..4
int marks[5] = {70, 65, 80, 45, 90}; // declare + initialise

cout << marks[0];    // 70  · FIRST element is index 0
cout << marks[4];    // 90  · LAST is size − 1

Reading values into an array

#include <iostream>
using namespace std;

int main() {
    int n = 5, marks[5];

    for (int i = 0; i < n; i++) {
        cout << "Enter mark " << i + 1 << ": ";
        cin  >> marks[i];
    }

    for (int i = 0; i < n; i++) {
        cout << "Mark " << i + 1 << " = " << marks[i] << endl;
    }
    return 0;
}

Two-dimensional arrays

int table[3][4];              // 3 rows, 4 columns

for (int r = 0; r < 3; r++)
    for (int c = 0; c < 4; c++)
        table[r][c] = (r + 1) * (c + 1);

cout << table[2][3];          // row 2, column 3 → 12

A 2-D array always needs nested loops · outer for rows, inner for columns. This is where the nested-loop material of Module 3 pays off.

Passing an array to a function

#include <iostream>
using namespace std;

double average(int arr[], int size) {   // note: [] and a size
    int total = 0;
    for (int i = 0; i < size; i++) total += arr[i];
    return (double)total / size;        // cast! else truncates
}

int main() {
    int marks[5] = {70, 65, 80, 45, 90};
    cout << "Average: " << average(marks, 5) << endl;  // 70
    return 0;
}

Strings · the operations worth knowing

#include <iostream>
#include <string>
using namespace std;

int main() {
    string first = "Ada", last = "Okon";

    string full = first + " " + last;   // concatenation
    cout << full            << endl;    // Ada Okon
    cout << full.length()   << endl;    // 8
    cout << full.substr(0,3)<< endl;    // Ada
    cout << full[0]         << endl;    // A

    string line;
    getline(cin, line);     // reads a WHOLE line incl. spaces
    return 0;
}

for (int i = 1; i <= 10; i++) {
    if (i == 5) continue;   // skip only 5
    if (i == 8) break;      // stop entirely at 8
    cout << i << " ";
}
// prints: 1 2 3 4 6 7

Note that both are disciplined single-exit jumps, unlike goto · they leave only the construct they are in, so the flow stays traceable.

What happens when you compile

source.cpp
    │  1. PREPROCESSING   handles #include and #define
    ▼
expanded source
    │  2. COMPILATION     C++ → assembly, syntax checked here
    ▼
assembly
    │  3. ASSEMBLY        assembly → object code (.o)
    ▼
object file
    │  4. LINKING         joins your object files + libraries
    ▼
executable

Where errors surface: a missing semicolon is caught at compilation; a function you declared but never defined is caught at linking; division by zero survives all four stages and appears only at run time. That mapping is a neat one-mark answer.

Modularisation across files

// maths.h · the interface (prototypes only)
double add(double a, double b);
double multiply(double a, double b);

// maths.cpp · the implementation
#include "maths.h"
double add(double a, double b)      { return a + b; }
double multiply(double a, double b) { return a * b; }

// main.cpp · the user
#include <iostream>
#include "maths.h"
using namespace std;
int main() { cout << add(2, 3); return 0; }

This is modularisation taken to its conclusion: the header declares what a module offers, the .cpp file holds how it does it, and users include only the header. Angle brackets < > for system libraries, double quotes " " for your own files.

1 · Largest of three numbers

int a, b, c;
cin >> a >> b >> c;

if (a >= b && a >= c)      cout << a;
else if (b >= a && b >= c) cout << b;
else                       cout << c;

2 · Sum and average of n numbers

int n, x, sum = 0;
cout << "How many numbers? ";  cin >> n;

for (int i = 0; i < n; i++) { cin >> x; sum += x; }

cout << "Sum: "     << sum << endl;
cout << "Average: " << (double)sum / n << endl;

3 · Factorial, iteratively

int n, fact = 1;
cin >> n;
for (int i = 1; i <= n; i++) fact *= i;
cout << n << "! = " << fact << endl;

4 · Prime check

int n;  bool prime = true;
cin >> n;

if (n < 2) prime = false;
for (int i = 2; i * i <= n; i++) {
    if (n % i == 0) { prime = false; break; }
}
cout << (prime ? "Prime" : "Not prime") << endl;

5 · Reverse a number

int n, reversed = 0;
cin >> n;

while (n > 0) {
    reversed = reversed * 10 + n % 10;  // take last digit
    n = n / 10;                         // drop last digit
}
cout << "Reversed: " << reversed << endl;

The two-line idiom · n % 10 gives the last digit, n / 10 removes it · also answers "sum of digits", "count digits" and "palindrome number".

6 · Multiplication table for one number

int n;
cin >> n;
for (int i = 1; i <= 12; i++)
    cout << n << " x " << i << " = " << n * i << endl;

7 · Largest element in an array

int arr[5] = {23, 78, 12, 90, 45};
int largest = arr[0];              // assume the first

for (int i = 1; i < 5; i++)
    if (arr[i] > largest) largest = arr[i];

cout << "Largest: " << largest << endl;   // 90

8 · Linear search

int arr[5] = {23, 78, 12, 90, 45}, target, pos = -1;
cin >> target;

for (int i = 0; i < 5; i++)
    if (arr[i] == target) { pos = i; break; }

if (pos != -1) cout << "Found at index " << pos << endl;
else           cout << "Not found" << endl;

9 · Bubble sort

int a[5] = {5, 2, 9, 1, 7}, n = 5;

for (int i = 0; i < n - 1; i++)
    for (int j = 0; j < n - 1 - i; j++)
        if (a[j] > a[j + 1]) {
            int t = a[j];  a[j] = a[j + 1];  a[j + 1] = t;
        }

for (int i = 0; i < n; i++) cout << a[i] << " ";  // 1 2 5 7 9

10 · Count vowels in a string

string s;  int count = 0;
getline(cin, s);

for (int i = 0; i < s.length(); i++) {
    char c = tolower(s[i]);
    if (c=='a'||c=='e'||c=='i'||c=='o'||c=='u') count++;
}
cout << "Vowels: " << count << endl;

11 · Star patterns

for (int i = 1; i <= 5; i++) {        *
    for (int j = 1; j <= i; j++)      **
        cout << "*";                  ***
    cout << endl;                     ****
}                                     *****

12 · Simple calculator with switch

double a, b;  char op;
cin >> a >> op >> b;

switch (op) {
    case '+': cout << a + b; break;
    case '-': cout << a - b; break;
    case '*': cout << a * b; break;
    case '/':
        if (b != 0) cout << a / b;
        else cout << "Division by zero";
        break;
    default: cout << "Invalid operator";
}