+3-for-a-while

3-for-a-while/CMakeLists.txt

@@ -0,0 +1,15 @@
+# for
+add_executable(stars stars.c)
+add_executable(primes primes.c)
+
+# while (do-while)
+add_executable(binary-search binary-search.c)
+add_executable(binary-search-for binary-search-for.c)
+
+add_executable(digits-while digits-while.c)
+add_executable(digits-do-while digits-do-while.c)
+add_executable(digits-for digits-for.c)
+
+# for-a-while
+add_executable(selection-sort selection-sort.c)
+add_executable(palindrome palindrome.c)

3-for-a-while/README.md

@@ -0,0 +1,72 @@
+# `3-for-a-while`
+
+## `stars.c`
+
+- double loops
+- `for (int i = 0) + for (int j = 0)`
+
+## `primes.c`
+
+- double loops
+- `int is_prime = 1;`: why 1? why not 0?
+- `if (is_prime)` vs. `if (is_prime != 0)` vs. `if (is_prime == 1)`
+- testing
+  - https://www.wolframalpha.com/input?i=+primes+less+than+100000
+  - mma: `PrimePi[100000]`
+  - `number = 2`
+- `break`
+- `i * i <= number` vs. `i * i < number`
+- `stdbool.h`
+  - C89, C99, C23
+  - `bool b = 5`
+  - `(bool) 3.5`
+- [x] timing
+  - `clock_t start = clock(); clock_t end = clock(); (end - start) / CLOCKS_PER_SEC`
+
+# `binary-search.c`
+
+- already sorted array
+  - Fib
+- `int index = -1;`
+  - `printf`
+- `break`
+- testing
+  - `1`: the leftmost/rightmost one
+  - search for the leftmost/rightmost one
+- [ ] learn from the standard library???
+  - `(low + high) / 2`
+    - `low + (high - low) / 2`
+    - [ ] try it???
+
+## `digits.c`
+
+- testing
+- `do-while`
+
+## `selection-sort.c`
+
+- preparation: scanf
+- with comments
+- `swap`
+- `while (scanf ...)`
+  - https://en.cppreference.com/w/c/io/fscanf
+    - Number of receiving arguments successfully assigned (which may be zero in case a matching failure occurred before
+      the first receiving argument was assigned)
+    - or `EOF` if input failure occurs before the first receiving argument was assigned
+  - How to run this?
+    - Linux: `Ctrl + D` at the beginning of a line
+    - Mac: `Cmd + D` at the beginning of a line
+    - Windows: `Ctrl + Z` at the beginning of a line
+- more `printf` (after each iteration)
+- `sizeof`
+- Input&Output indirection
+  - Linux/Windows Cmd
+
+## `palindrome.c`
+
+- `#define`: pre-processing
+- `scanf("%20s", string);`
+- `strlen`
+- comma expression
+- `for` version
+- `while` version

3-for-a-while/binary-search-for.c

@@ -0,0 +1,34 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+
+#define LEN 10
+int dictionary[LEN] = { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 };
+
+int main(void) {
+  int key = 0;
+  scanf("%d", &key);
+
+  int index = -1;
+
+  for (int low = 0, high = LEN - 1; low <= high; ) {
+    int mid = (low + high) / 2;
+
+    if (key > dictionary[mid]) {
+      low = mid + 1;
+    } else if (key < dictionary[mid]) {
+      high = mid - 1;
+    } else {  // key == dictionary[mid]
+      index = mid;
+      high = mid - 1; // find the leftmost index of the key
+    }
+  }
+
+  if (index == -1) {
+    printf("Not found!\n");
+  } else {
+    printf("The index of %d is %d.\n", key, index);
+  }
+
+  return 0;
+}

3-for-a-while/binary-search.c

@@ -0,0 +1,39 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+
+#define LEN 10
+int dictionary[LEN] = { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 };
+
+int main(void) {
+  int key = 0;
+  scanf("%d", &key);
+
+  // TODO: binary search: search for key in dictionary[]
+  int low = 0;
+  int high = LEN - 1;
+
+  int index = -1;
+
+  while (low <= high) {
+    int mid = (low + high) / 2;
+
+    if (key > dictionary[mid]) {
+      low = mid + 1;
+    } else if (key < dictionary[mid]) {
+      high = mid - 1;
+    } else {  // key == dictionary[mid]
+      index = mid;
+      // break;  // what if `break` is removed
+      high = mid - 1; // find the leftmost index of the key
+    }
+  }
+
+  if (index == -1) {
+    printf("Not found!\n");
+  } else {
+    printf("The index of %d is %d.\n", key, index);
+  }
+
+  return 0;
+}

3-for-a-while/birth

@@ -0,0 +1 @@
+1958 1961 1969 1954 1969

3-for-a-while/digits-do-while.c

@@ -0,0 +1,21 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+
+int main(void) {
+  int number = 0;
+  scanf("%d", &number);
+
+  // TODO: number of digits
+  int num_of_digits = 0;
+
+  do {
+    number /= 10;
+    num_of_digits++;
+  } while (number > 0);
+
+  printf("Number of digits is %d\n",
+         num_of_digits);
+
+  return 0;
+}

3-for-a-while/digits-for.c

@@ -0,0 +1,20 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+
+int main(void) {
+  int number = 0;
+  scanf("%d", &number);
+
+  // Initialize the number of digits to 1
+  int num_of_digits = 1;
+
+  // For numbers other than 0, adjust number of digits
+  for (; number / 10 != 0; num_of_digits++) {
+    number /= 10;
+  }
+
+  printf("Number of digits is %d\n", num_of_digits);
+
+  return 0;
+}

3-for-a-while/digits-while.c

@@ -0,0 +1,26 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+
+int main(void) {
+  int number = 0;
+  scanf("%d", &number);
+
+  // TODO: number of digits
+  int num_of_digits = 0;
+
+  // TODO: >= (infinite loop)
+  if (number == 0) {
+    num_of_digits = 1;
+  } else {
+    while (number > 0) {
+      number /= 10;
+      num_of_digits++;
+    }
+  }
+
+  printf("Number of digits is %d\n",
+         num_of_digits);
+
+  return 0;
+}

3-for-a-while/palindrome.c

@@ -0,0 +1,51 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+
+#define LEN 21
+char string[LEN] = "";
+
+int main() {
+  // example: nolemon,nomelon
+  printf("Input a string containing at most 20 characters.\n");
+  scanf("%20s", string);
+
+//  int len = 0;
+//  while (string[len] != '\0') {
+//    len++;
+//  }
+  int len = strlen(string);
+  printf("The length of \"%s\" is %d.\n", string, len);
+
+  // TODO: test palindrome
+
+  // TODO: the for version
+  // bool is_palindrome = true;
+  // for (int i = 0, j = len - 1; i < j; i++, j--) {
+  //   if (string[i] != string[j]) {
+  //     is_palindrome = false;
+  //     break;
+  //   }
+  // }
+
+  // TODO: the while version
+  bool is_palindrome = true;
+
+  int i = 0;
+  int j = len - 1;
+  while (i < j) {
+    if (string[i] != string[j]) {
+      is_palindrome = false;
+      break;
+    }
+    i++;
+    j--;
+  }
+
+  printf("\"%s\" is %s a palindrome.\n", string,
+         is_palindrome ? "" : "not");
+
+  return 0;
+}

3-for-a-while/primes.c

@@ -0,0 +1,35 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+#include <stdbool.h>
+
+int main(void) {
+  int max = 0;
+  scanf("%d", &max);
+
+  // TODO: print primes between 1 and max
+
+  int count = 0;
+
+  for (int number = 2; number <= max; number++) {
+    // decide whether number is a prime
+    // Since C99: bool (macro extended to _Bool; with macros true and false)
+    // Since C23: will become keywords (bool, true, false); do not need stdbool.h
+    bool is_prime = true;
+    for (int factor = 2; factor * factor <= number; factor++) {
+      if (number % factor == 0) {
+        is_prime = false;
+        break;  // test: number = 18
+      }
+    }
+
+    if (is_prime) { // TODO: is_prime == 1; is_prime != 0
+      count++;
+      printf("%d ", number);  // TODO: comment this for performance
+    }
+  }
+
+  printf("\ncount = %d\n", count);
+
+  return 0;
+}

3-for-a-while/selection-sort.c

@@ -0,0 +1,57 @@
+// Created by hfwei on 2024/10/10.
+
+#include <stdio.h>
+
+#define LEN 20
+int numbers[LEN] = { 0 };
+
+int main(void) {
+  /*
+   * Input the array
+   *
+   * Note: fails to run this program in "Run" (Ctrl + D)
+   * See: https://youtrack.jetbrains.com/issue/CPP-5704
+   * Use "Terminal" instead.
+   *
+   * TODO: CLion; Terminal
+   * Linux: Ctrl + D (works now; in the new line, or Ctrl + D twice)
+   *   See https://stackoverflow.com/a/21365313/1833118 (send and clear the buffer)
+   * Windows: Ctrl + Z (does not work on my platform)
+   * TODO: Input&Output redirection
+   *   See https://stackoverflow.com/a/11788475/1833118
+   */
+  int len = -1;
+  while (scanf("%d", &numbers[++len]) != EOF);
+
+  // sizeof numbers / sizeof(numbers[0])
+  for (int i = 0; i < len; i++) {
+    printf("%d ", numbers[i]);
+  }
+  printf("\n");
+
+  // TODO: selection sort
+  for (int i = 0; i < len; i++) {
+    // find the minimum value of numbers[i .. n-1]
+    int min = numbers[i];
+    int min_index = i;
+
+    for (int j = i + 1; j <= len - 1; ++j) {
+      if (numbers[j] < min) {
+        min = numbers[j];
+        min_index = j;
+      }
+    }
+
+    // swap numbers[i] and numbers[min_index]
+    int temp = numbers[i];
+    numbers[i] = numbers[min_index];
+    numbers[min_index] = temp;
+  }
+
+  for (int i = 0; i < len; i++) {
+    printf("%d ", numbers[i]);
+  }
+  printf("\n");
+
+  return 0;
+}