CSCA48_

What is Testing?

Testing is the process of verifying that your code produces correct output for all valid inputs — including edge cases that you didn't think about while writing. A program that runs without crashing is NOT the same as a program that is correct.

The 5 Testing Categories for CSCA48

Writing Test Cases in C

// Pattern: arrange → act → assert
void test_insert_head() {
    // Arrange
    Node *head = NULL;

    // Act
    insert_head(&head, 42);

    // Assert — check EVERY postcondition
    assert(head != NULL);            // list is non-empty
    assert(head->data == 42);        // correct value
    assert(head->next == NULL);      // only one node
    printf("test_insert_head: PASS\n");
}

// Test multiple cases in sequence
void test_insert_head_multiple() {
    Node *head = NULL;
    insert_head(&head, 1);
    insert_head(&head, 2);   // 2 → 1

    assert(head->data == 2);
    assert(head->next->data == 1);
    assert(head->next->next == NULL);
    printf("test_insert_head_multiple: PASS\n");
}

Test Case Design — Linked Lists

Test Case Design — BSTs

Using Valgrind

gcc -g -Wall program.c -o program  # -g adds debug symbols
valgrind --leak-check=full ./program

# Output to understand:
# "definitely lost: 0 bytes" → no memory leaks ✓
# "definitely lost: 40 bytes in 5 blocks" → leaked 5 nodes
# "Invalid read of size 4" → reading freed/invalid memory
# "Invalid write of size 4" → writing to freed/invalid memory
# "Use of uninitialised value" → uninitialized variable used

# Most common causes of valgrind errors in CSCA48:
# 1. Forgot free() on malloc'd node
# 2. free(head) without saving head->next first
# 3. Using a pointer after free()
# 4. Not initializing struct fields (use calloc not malloc)

The 5-Step Debugging Process

Common Bug Patterns in CSCA48

// ─── Off-by-one ─────────────────────────────────────────────
for (int i = 0; i <= n; i++)    // BUG: should be i < n
    arr[i] = 0;                  // arr[n] is out of bounds!

// ─── Assignment instead of comparison ────────────────────────
if (head = NULL)   // BUG: assigns NULL, always false!
    return;         // Fix: if (head == NULL)

// ─── Wrong pointer dereference ───────────────────────────────
Node *p = head;
while (p != NULL) {
    printf("%d\n", p->data);
    p->next;        // BUG: doesn't advance p! Fix: p = p->next;
}

// ─── Free before saving next ─────────────────────────────────
void free_list(Node *head) {
    while (head) {
        free(head);          // BUG: head->next now invalid!
        head = head->next;   // Fix: save next BEFORE free
    }
}

// ─── Missing NULL check after malloc ─────────────────────────
Node *n = malloc(sizeof(Node));
n->data = val;   // BUG: n could be NULL!
                 // Fix: if (!n) { handle error; return; }

// ─── Dot vs arrow ────────────────────────────────────────────
Node n;
Node *p = &n;
n->data = 5;   // BUG: n is not a pointer, use n.data
p.data  = 5;   // BUG: p IS a pointer, use p->data