[Adult Learning Log] Data Structures – Week 2 Review

○ Key Takeaways from Week 2

• Learned the concept of arrays and array-based sequential lists.
• Studied 1D/2D array declarations, initialization, sparse matrix representation, and transpose matrices.
• Learned how strings are implemented and the role and danger of omitting the \0 null character.
• Understood how arrays behave when passed as function arguments.
• Covered structure definitions, declarations, operations, and nested structures.
• Learned about pointers and dynamic memory allocation.

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○ Sequential List

• A list implemented using arrays.
• An array is a collection of elements of the same data type.
• Stored sequentially in memory, both logically and physically (addresses are also consecutive).
• Offers fast access via index but can be slow for insert/delete operations.
• Important to understand the abstract data type (ADT) concept of a list and how to implement it using arrays.
• Later, this will be compared with linked lists (pointer-based).

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○ 1D Array

• A set of pairs. Accessing by index yields the corresponding value.
• Declared as data_type array_name[size];
  • int A[6];
  • int A[6] = {1, 2, 3, 4, 5, 6};
  • int A[] = {1, 2, 3, 4, 5, 6};
• A holds the address of A[0].
• Local variables in C are not auto-initialized to 0, so uninitialized values may contain garbage data.

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○ 2D Array

• Declared as data_type array_name[rows][columns];
  • int A[2][3]; creates a 2x3 matrix.
• Initialized like:
  • int A[2][3] = {{1,2,3}, {4,5,6}};
  • int A[2][3] = {1,2,3,4,5,6};
• Sparse matrices save space but make operations more complex.
• Represented as triplets: {row, column, value}
• Transpose matrix: swaps rows and columns, useful when column-wise operations are needed.

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○ Strings = Character Array + Null Terminator \0

• Example: char S1[16] = "Hello World"; automatically adds \0 at the end.
• Without \0, memory is read until a null byte is found—may cause garbage output or segmentation fault.
• Use strcpy() or strcpy_s() to initialize character arrays safely.

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○ Passing Arrays to Functions

• C uses call-by-value, but arrays are passed by reference (by address).
• Changes inside the function affect the original array.
• Must also pass the size of the array explicitly.

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○ Structure (struct) Basics

• Combines different data types under one unit.
• Must be defined before use.

typedef struct {
    int id;
    char name[20];
    double score;
} Student;

Student a = {202403156, "Hong Gil-dong", 96.3};
a.id = 242403156;
strcpy(a.name, "Hong Gil-dong");

• Use . to access members.
• strcpy() is needed for character arrays (strings).

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○ Struct Operations and Nested Structures

• Only assignment (=) is supported. Other operations like comparison (>) are not allowed.

typedef struct {
    int month;
    int date;
} Birthday;

typedef struct {
    char name[20];
    Birthday birthday;
} Friend;

Friend list[45];

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○ Passing Structs to Functions

• Passed by value (copied), so changes inside the function don’t affect the original.
• Use pointers to modify original struct values.

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○ Polynomial Representation

• Practice in storing and manipulating structured data.
• Store coefficients in arrays, usually in ascending order by degree:
  • int coef[] = {7, 5, 2, 3}; → index 0 = constant, index 3 = highest degree term.
  • Easier coding and readability when using index = degree.

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○ Pointer Basics

• A pointer holds the address of another variable.
• Use * to access or modify the value being pointed to.
• Use & to get a variable's address.
• Arrays act like pointers in many contexts.

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○ Dynamic Memory Allocation

• Allocates memory at runtime, allows efficient use of memory.
• malloc() is used to request memory from the heap.
• Always free() the memory to avoid memory leaks.

int *pi;
pi = (int*)malloc(sizeof(int));
// use pi
free(pi);