September 2017 Update

Task/Substring/C/substring-3.c

@@ -0,0 +1,74 @@
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+char *substring(const char *s, size_t n, ptrdiff_t m)
+{
+  char *result;
+  /* check for null s */
+  if (NULL == s)
+    return NULL;
+  /* negative m to mean 'up to the mth char from right' */
+  if (m < 0)
+    m = strlen(s) + m - n + 1;
+
+  /* n < 0 or m < 0 is invalid */
+  if (n < 0 || m < 0)
+    return NULL;
+
+  /* make sure string does not end before n
+   * and advance the "s" pointer to beginning of substring */
+  for ( ; n > 0; s++, n--)
+    if (*s == '\0')
+      /* string ends before n: invalid */
+      return NULL;
+
+  result = malloc(m+1);
+  if (NULL == result)
+    /* memory allocation failed */
+    return NULL;
+  result[0]=0;
+  strncat(result, s, m); /* strncat() will automatically add null terminator
+                          * if string ends early or after reading m characters */
+  return result;
+}
+
+char *str_wholeless1(const char *s)
+{
+  return substring(s, 0, strlen(s) - 1);
+}
+
+char *str_fromch(const char *s, int ch, ptrdiff_t m)
+{
+  return substring(s, strchr(s, ch) - s, m);
+}
+
+char *str_fromstr(const char *s, char *in, ptrdiff_t m)
+{
+  return substring(s, strstr(s, in) - s , m);
+}
+
+
+#define TEST(A) do {		\
+    char *r = (A);		\
+    if (NULL == r)		\
+      puts("--error--");	\
+    else {			\
+      puts(r);			\
+      free(r);			\
+    }				\
+  } while(0)
+
+int main()
+{
+  const char *s = "hello world shortest program";
+
+  TEST( substring(s, 12, 5) );		// get "short"
+  TEST( substring(s, 6, -1) );		// get "world shortest program"
+  TEST( str_wholeless1(s) );		// "... progra"
+  TEST( str_fromch(s, 'w', 5) );	// "world"
+  TEST( str_fromstr(s, "ro", 3) );	// "rog"
+
+  return 0;
+}