diff --git a/Week 4/README.md b/Week 4/README.md
new file mode 100644
index 0000000..4d26a88
--- /dev/null
+++ b/Week 4/README.md	
@@ -0,0 +1,3 @@
+# Week 3
+
+Lab sheet: https://web.archive.org/web/20210813101313/https://teaching.csse.uwa.edu.au/units/CITS2002/labsheets/labsheet3.php
diff --git a/Week 4/easy_tasks/palindrome.c b/Week 4/easy_tasks/palindrome.c
new file mode 100644
index 0000000..228b803
--- /dev/null
+++ b/Week 4/easy_tasks/palindrome.c	
@@ -0,0 +1,39 @@
+/*
+* A palindrome is a word that reads the same forwards as it does backwards. For
+  example, the words noon and madam are palindromes. Write a function named
+  isPalindrome() which determines if a string, supplied as the single character
+  array argument to the function, is a palindrome or not, returning a Boolean.
+  Use the strlen() function to determine the length of the argument string.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+
+bool isPalindrome(char const str[]) {
+    int str_length = strlen(str);
+    for (size_t i = 0; i < str_length/2; i++)
+    {
+        if (str[i] != str[str_length-i-1]) {
+            return false;
+        }
+    }
+    return true;
+}
+
+int main(int argc, char const *argv[])
+{
+    for (size_t i = 1; i < argc; i++)
+    {
+        if (isPalindrome(argv[i]))
+        {
+            printf("yes ");
+        } else {
+            printf("no ");
+        }
+    }
+    printf("\n");
+    exit(EXIT_SUCCESS);
+    return 0;
+}
diff --git a/Week 4/easy_tasks/passwd.c b/Week 4/easy_tasks/passwd.c
new file mode 100644
index 0000000..4b320db
--- /dev/null
+++ b/Week 4/easy_tasks/passwd.c	
@@ -0,0 +1,52 @@
+/*
+* A computer password is often consider "safe" (i.e. hard to guess) if it
+  contains a mixture of uppercase and lowercase characters and digits. Write a
+  function named isSafe() to determine if a potential password (a string) has at
+  least two uppercase characters, two lowercase characters, and two digits. Your
+  function should take a single character array as its argument and return a
+  Boolean value to indicate whether the password is considered safe or not.
+* See the online documentation (man pages) for help with the isalpha(),
+  islower(), and isupper() functions. Include the appropriate header file
+  <ctype.h> into your program.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <ctype.h>
+
+bool isSafe(char const passwd[]) {
+    size_t i = 0;
+    unsigned int uppercase = 0;
+    unsigned int lowercase = 0;
+    unsigned int digit = 0;
+    while (passwd[i] != '\0')
+    {
+        if (isalpha(passwd[i])) {
+            if (isupper(passwd[i])) {
+                uppercase++;
+            } else {
+                lowercase++;
+            }
+        } else {
+            digit++;    //Assume non-alphabet characters as digits
+        }        
+        i++;
+    }
+    return uppercase > 1 && lowercase > 1 && digit > 1;
+}
+
+int main(int argc, char const *argv[])
+{
+    for (size_t i = 1; i < argc; i++)
+    {
+        if (isSafe(argv[i])) {
+            printf("safe ");
+        } else {
+            printf("unsafe ");
+        }
+    }
+    printf("\n");
+    exit(EXIT_SUCCESS);
+    return 0;
+}
diff --git a/Week 4/easy_tasks/rand.c b/Week 4/easy_tasks/rand.c
new file mode 100644
index 0000000..758f3ca
--- /dev/null
+++ b/Week 4/easy_tasks/rand.c	
@@ -0,0 +1,99 @@
+/*
+* Each call to the standard C11 function rand() returns a different random
+  integer. Running the same program multiple times results in exactly the same
+  sequence of random integers. While this is generally unexpected ("hey, they
+  are not random"!), it is very helpful for debugging programs without the
+  randomness.
+* We can provide each execution with a more random sequence of random numbers by
+  seeding the random number generator with the C statement srand( time(NULL) );
+    * Write a simple program to fill an array of 10 integers with random numbers
+      from rand(). Run the program several times, printing the contents of the
+      array.
+    * Now, use srand() to seed the generation of random numbers. Run the program
+      several times, printing the contents of the array.
+    * Extend your program by passing the initialised array to another function
+      which finds and prints the largest value in the array.
+    * Finally, extend the program's function to place the array's largest value
+      into the array's first element (index position 0), "pushing" all other
+      values down in the array (0→1, 1→2, 2→3, and so on).
+*/
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <time.h>
+#include <string.h>
+
+#define ARRAY_LENGTH 10
+
+void randomize_array(int arr_len, int arr[])
+{
+    for (size_t i = 0; i < arr_len; i++)
+    {
+        arr[i] = rand();
+    }
+}
+
+int max(int arr_len, int arr[])
+{
+    int max = -__INT32_MAX__;
+    for (size_t i = 0; i < arr_len; i++)
+    {
+        if (arr[i] > max)
+        {
+            max = arr[i];
+        }
+    }
+    return max;
+}
+
+int count(int arr_len, int arr[], int n) {
+    int count = 0;
+    for (size_t i = 0; i < arr_len; i++)
+    {
+        if (arr[i] == n)
+        {
+            count++;
+        }
+        
+    }
+    return count;
+}
+
+// Implementation will account for very rare occurence of duplicate max values.
+// (even though it is very, VERY unlikely).
+int shift_max(int arr_len, int arr[]) {
+    int arr_[arr_len];
+    memcpy(arr_, arr, sizeof(int)*arr_len);
+    int m = max(arr_len, arr);
+    int m_count = count(arr_len, arr, m);
+    int idx = m_count;
+    for (size_t i = 0; i < m_count; i++)
+        arr[i] = m;
+    for (size_t i = 0; i < arr_len; i++)
+    {
+        if (arr_[i] != m)
+        {
+            arr[idx] = arr_[i];
+            idx++;
+        }
+    }
+}
+
+int main(void)
+{
+    srand(time(NULL));
+    int rand_arr[ARRAY_LENGTH];
+
+    randomize_array(ARRAY_LENGTH, rand_arr);
+    shift_max(ARRAY_LENGTH, rand_arr);
+
+    printf("Max: %d\n", max(ARRAY_LENGTH, rand_arr));
+    for (size_t i = 0; i < ARRAY_LENGTH; i++)
+    {
+        printf("%d ", rand_arr[i]);
+    }
+    printf("\n");
+
+    exit(EXIT_SUCCESS);
+    return 0;
+}
diff --git a/Week 4/easy_tasks/strcmp.c b/Week 4/easy_tasks/strcmp.c
new file mode 100644
index 0000000..657cc9e
--- /dev/null
+++ b/Week 4/easy_tasks/strcmp.c	
@@ -0,0 +1,63 @@
+/*
+* Write a function named my_strcmp() to determine if one string is
+  (lexicographically, or alphabetically) less than, equal to, or greater than
+  another string. Your function should accept the two character arrays as its
+  arguments, and return either: -1 if the first string is less than the second
+  string, 0 if the two strings are equal, or 1 if the first string is greater
+  than the second string.
+* Call your function from the main() function with the code:
+  my_strcmp(argv[1], argv[2]).
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+#include <string.h>
+
+// Note that in many programming languages, uppercase characters have a lower
+// value than lowercase characters.
+int alphabet_index(char c) {
+    if (isalpha(c))
+    {
+        if (isupper(c)) {
+            return c - 'A';
+        } else {
+            return c - 'a' + 26;
+        }
+    }
+    return 0;
+}
+
+int my_strcmp(char s1[], char s2[]) {
+    int s1_len = strlen(s1);
+    int s2_len = strlen(s2);
+    size_t i = 0;
+    while (s1[i] != '\0' && s2[i] != '\0')
+    {
+        int s1_idx = alphabet_index(s1[i]);
+        int s2_idx = alphabet_index(s2[i]);
+        if (s1_idx > s2_idx) {
+            return 1;
+        } else if (s1_idx < s2_idx) {
+            return -1;
+        }
+        i++;
+    }
+    if (s1_len > s2_len) {
+        return 1;
+    } else if (s2_len > s1_len) {
+        return -1;
+    }
+    return 0;
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc != 3)
+    {
+        printf("Only two strings for comparison allowed\n");
+        exit(EXIT_FAILURE);
+    }
+    printf("%d\n",my_strcmp(argv[1], argv[2]));
+    return 0;
+}
diff --git a/Week 4/easy_tasks/strlen.c b/Week 4/easy_tasks/strlen.c
new file mode 100644
index 0000000..cd2d80f
--- /dev/null
+++ b/Week 4/easy_tasks/strlen.c	
@@ -0,0 +1,31 @@
+/*
+* Write a function named my_strlen() that calculates and returns the length of a
+  string. Your function should take one argument, a character array that
+  represents the string, and return an integer - the length of the string. The
+  calling function (the main() function) should print the integer returned by
+  your my_strlen() function.
+* Test your function with some string constants and by passing to it some
+  command-line arguments.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int my_strlen(char const str[]) {
+    int i = 0;
+    while (str[i] != '\0')
+    {
+        i++;
+    }
+    return i;
+}
+
+int main(int argc, char const *argv[])
+{
+    for (size_t i = 1; i < argc; i++)
+    {
+        printf("%d ", my_strlen(argv[i]));
+    }
+    printf("\n");
+    return 0;
+}
diff --git a/Week 4/multistat/stat.c b/Week 4/multistat/stat.c
new file mode 100644
index 0000000..544b54f
--- /dev/null
+++ b/Week 4/multistat/stat.c	
@@ -0,0 +1,42 @@
+/*
+* 🌶 Lecture 6 introduced C11's structures and presented an example of how
+  application programs can request and receive information from the underlying
+  operating system The example involved the gettimeofday() system-call and the
+  struct timeval structure.
+* Similarly, applications can determine information about a file's attributes
+  using the stat() system-call and the struct stat structure. Note that we need
+  to read Section 2 of the online manual:
+
+	prompt> man 2 stat
+	
+  otherwise we'll receive the documentation from Section 1.
+
+* Write a program which accepts a number of filenames on the command-line, and
+  prints (just as an integer) the modification-time of each file.
+* Now, extend the program to also print each file's size (in bytes) and the
+  (more useful string) modification-time of each file, using the ctime()
+  function.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+#include <string.h>
+#include <time.h>
+
+int main(int argc, char const *argv[])
+{
+    struct stat *stat_result;
+    stat_result = malloc(sizeof(struct stat));
+    for (size_t i = 1; i < argc; i++)
+    {
+        stat(argv[i], stat_result);
+        const time_t *mod_time = &(stat_result->st_mtim).tv_sec;
+        char *time_human_readable = ctime(mod_time);
+        printf("File \"%s\":\n", argv[i]);
+        printf(" => last_modified: %s", time_human_readable);
+        printf(" => size: %ld\n\n", stat_result->st_size);
+    }
+    
+    return 0;
+}
diff --git a/Week 4/replace/replace.c b/Week 4/replace/replace.c
new file mode 100644
index 0000000..c28bf7a
--- /dev/null
+++ b/Week 4/replace/replace.c	
@@ -0,0 +1,118 @@
+/*
+* 🌶 Write a function named replace() that replaces all instances of one string
+  for another string in a third string. For example:
+
+    prompt> ./replace red blue aredrobin
+    abluerobin
+
+    prompt> ./replace cat bison catocathartic
+    bisonobisonhartic
+	
+* A reasonable prototype for the function is:
+    void replace( char oldword[], char newword[], char whole_sentence[] );
+* Ensure you have terminated your string correctly.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+
+bool __match(int m_len, char m[], char s_len, char s[], int start)
+{
+    int offset = start + m_len;
+    if (offset <= s_len)
+    {
+        int idx = 0;
+        for (int i = start; i < offset; i++)
+        {
+            if (m[idx] != s[i])
+            {
+                return false;
+            }
+            idx++;
+        }
+        return true;
+    }
+    return false;
+}
+
+// assume match and replacement length > 0
+void replace(char match[], char replacement[], char string[])
+{
+    int match_length = strlen(match);
+    int replacement_length = strlen(replacement);
+    int string_length = strlen(string);
+    int positions[string_length]; // easy solution - probably memory wasteful
+    int valid_positions[string_length];
+    memset(positions, -1, sizeof positions);
+    memset(valid_positions, -1, sizeof valid_positions);
+
+    int idx = 0;
+    int idx_v = 0;
+    int idx_new = 0;
+    int last_position = -__INT32_MAX__;
+
+    // get possible positions.
+    for (int i = 0; i < string_length; i++)
+    {
+        if (string[i] == match[0])
+        {
+            positions[idx] = i;
+            idx++;
+        }
+    }
+
+    // get valid positions for replacement.
+    idx = 0;
+    do
+    {
+        int position = positions[idx];
+        if (__match(match_length, match, string_length, string, position))
+        {
+            // prevent overlapping matches
+            if (position > last_position + match_length) {
+                valid_positions[idx_v] = position;
+                last_position = position;
+                idx_v++;
+            }
+        }
+        idx++;
+    } while (positions[idx] != -1);
+
+    // make new string
+    char new_string[string_length + idx_v*(match_length - replacement_length)];
+    idx = 0;
+    idx_v = 0;
+    while (string[idx] != '\0')
+    {
+        if (idx == valid_positions[idx_v]) {
+            idx_v++;
+            idx += match_length-1;
+            for (size_t i = 0; i < replacement_length; i++)
+            {
+                new_string[idx_new] = replacement[i];
+                idx_new++;
+            }
+        } else {
+            new_string[idx_new] = string[idx];
+            idx_new++;
+        }
+        idx++;
+    }
+    new_string[idx_new] = '\0';
+    strcpy(string, new_string);
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc != 4)
+    {
+        printf("Error - only three arguments allowed\nreplace pattern replacement string\n");
+        exit(EXIT_FAILURE);
+    }
+    
+    replace(argv[1], argv[2], argv[3]);
+    printf("%s\n", argv[3]);
+    return 0;
+}
diff --git a/Week 4/superloop/superloop.c b/Week 4/superloop/superloop.c
new file mode 100644
index 0000000..b40f371
--- /dev/null
+++ b/Week 4/superloop/superloop.c	
@@ -0,0 +1,65 @@
+/*
+ * 🌶 🌶 🌶 If you had to write some code that iterated through all the
+  possibilities for three variables in the range 0 to 10, you would probably
+  write code similar to:
+
+    for(int a = 0; a < 10; ++a) {
+        for(int b = 0; b < 10; ++b) {
+            for(int c = 0; c < 10; ++c) {
+                ; // LOOP-BODY USING a, b, AND c
+            }
+        }
+    }
+  But what if you had to extend this to 4, 5, or even 10+ "nested" loops?
+  Instead of further nesting more loops, it is possible to write a function that
+  acts as a "superloop", performing the equivalent of n nested loops with just
+  one loop. Write a function to do this, taking an argument n that indicates the
+  number of nested loops your function should perform. To do this, you will need
+  to keep a 1-dimensional array of n values that maintain the state of each loop
+  during the execution of your function.
+*/
+
+#include <stdlib.h>
+#include "superloop.h"
+
+struct __superloop_state {
+    void (*function)(int, int (*)[]);
+    int (*value)[];
+    int level;
+    int depth;
+    int start;
+    int end;
+    int increment;
+};
+
+void __loop(struct __superloop_state *state)
+{
+    if (state->level < state->depth)
+    {
+        for (int i = state->start; i < state->end; i += state->increment)
+        {
+            (*state->value)[state->level] = i;
+            state->level++;
+            __loop(state);
+            state->level--;
+        }
+    } else {
+        (*state->function)(state->depth, state->value);
+    }
+}
+
+void superloop(int depth, int start, int end, int increment, void (*function)(int, int (*)[]))
+{
+    int (*value)[depth];
+    value = malloc(depth*sizeof(int));
+    struct __superloop_state state = {
+        function,
+        value,
+        0,
+        depth,
+        start,
+        end,
+        abs(increment)
+    };
+    __loop(&state);
+}
\ No newline at end of file
diff --git a/Week 4/superloop/superloop.h b/Week 4/superloop/superloop.h
new file mode 100644
index 0000000..6e01394
--- /dev/null
+++ b/Week 4/superloop/superloop.h	
@@ -0,0 +1,6 @@
+#ifndef _SUPERLOOP_H
+#define _SUPERLOOP_H
+
+extern void superloop(int depth, int start, int end, int increment, void (*function)(int, int (*)[]));
+
+#endif
\ No newline at end of file
diff --git a/Week 4/superloop/test_superloop.c b/Week 4/superloop/test_superloop.c
new file mode 100644
index 0000000..7d11999
--- /dev/null
+++ b/Week 4/superloop/test_superloop.c	
@@ -0,0 +1,28 @@
+// see superloop.c for information
+// remember to add ./superloop.c to gcc for this to compile!
+
+#include <stdio.h>
+#include <stdlib.h>
+#include "superloop.h"
+
+
+void func(int length, int (*values)[]) {
+    for (size_t i = 0; i < length; i++)
+    {
+        printf("%d ", (*values)[i]);
+    }
+    printf("\n");
+}
+
+int main(int argc, char const *argv[])
+{
+    if (argc != 5)
+    {
+        printf("Expected 4 arguments\ntest_superloop <loop_depth> <start> <end> <increment>\n");
+        exit(EXIT_FAILURE);
+    }
+
+    superloop(atoi(argv[1]), atoi(argv[2]), atoi(argv[3]), atoi(argv[4]), func);
+    exit(EXIT_SUCCESS);
+    return 0;
+}