Added strlen function (#606)

* added strlen function

* updating DIRECTORY.md

* Update strings/strlen.c

Co-authored-by: David Leal <halfpacho@gmail.com>

* Update strings/strlen_recursion.c

Co-authored-by: David Leal <halfpacho@gmail.com>

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: David Leal <halfpacho@gmail.com>

DIRECTORY.md

@@ -382,3 +382,7 @@
   * [Shell Sort](https://github.com/TheAlgorithms/C/blob/master/sorting/shell_sort.c)
   * [Shell Sort2](https://github.com/TheAlgorithms/C/blob/master/sorting/shell_sort2.c)
   * [Stooge Sort](https://github.com/TheAlgorithms/C/blob/master/sorting/stooge_sort.c)
+
+## Strings
+  * [Strlen](https://github.com/TheAlgorithms/C/blob/master/strings/strlen.c)
+  * [Strlen Recursion](https://github.com/TheAlgorithms/C/blob/master/strings/strlen_recursion.c)

strings/strlen.c

@@ -0,0 +1,46 @@
+/**
+ * @file
+ * @brief Program to calculate length of string.
+ *
+ * @author [Du Yuanchao](https://github.com/shellhub)
+ */
+#include <assert.h>
+#include <string.h>
+
+/**
+ * @brief Returns the length of string
+ * @param str the pointer of string.
+ * @return the length of string.
+ */
+int length(const char *str)
+{
+    int count = 0;
+    for (int i = 0; *(str + i) != '\0'; ++i)
+    {
+        count++;
+    }
+    return count;
+}
+
+/**
+ * @brief Test function
+ * @return void
+ */
+static void test()
+{
+    assert(length("") == strlen(""));
+    assert(length(("a")) == strlen("a"));
+    assert(length("abc") == strlen("abc"));
+    assert(length("abc123def") == strlen("abc123def"));
+    assert(length("abc\0def") == strlen("abc\0def"));
+}
+
+/**
+ * @brief Driver Code
+ * @returns 0 on exit
+ */
+int main()
+{
+    test();
+    return 0;
+}

strings/strlen_recursion.c

@@ -0,0 +1,38 @@
+/**
+ * @file
+ * @brief Program to calculate length of string using recursion.
+ *
+ * @author [Du Yuanchao](https://github.com/shellhub)
+ */
+#include <assert.h>
+#include <string.h>
+
+/**
+ * @brief Returns the length of string using recursion
+ * @param str the pointer of string.
+ * @return the length of string.
+ */
+int length(const char *str) { return *str == '\0' ? 0 : 1 + length(++str); }
+
+/**
+ * @brief Test function
+ * @return void
+ */
+static void test()
+{
+    assert(length("") == strlen(""));
+    assert(length(("a")) == strlen("a"));
+    assert(length("abc") == strlen("abc"));
+    assert(length("abc123def") == strlen("abc123def"));
+    assert(length("abc\0def") == strlen("abc\0def"));
+}
+
+/**
+ * @brief Driver Code
+ * @returns 0 on exit
+ */
+int main()
+{
+    test();
+    return 0;
+}