diff --git a/intro/ex1_20_entab.c b/intro/ex1_20_detab.c
similarity index 94%
rename from intro/ex1_20_entab.c
rename to intro/ex1_20_detab.c
index 7b869bd..e830956 100644
--- a/intro/ex1_20_entab.c
+++ b/intro/ex1_20_detab.c
@@ -13,9 +13,10 @@ main()
   char line[MAXLINE];         /* current input line */
   char detab_line[MAXLINE];   /* detabbed line */
 
-  while ((len = my_getline(line, MAXLINE)) > 0)
+  while ((len = my_getline(line, MAXLINE)) > 0) {
     detab(detab_line, line);
     printf("%s", detab_line);
+  }
   return 0;
 }
 
diff --git a/intro/ex1_21_entab.c b/intro/ex1_21_entab.c
new file mode 100644
index 0000000..88652ea
--- /dev/null
+++ b/intro/ex1_21_entab.c
@@ -0,0 +1,58 @@
+#include <stdio.h>
+
+#define MAXLINE 1000    /* maximum characters in a line */ 
+#define TABWIDTH 4      /* tab size */ 
+
+int my_getline(char line[], int maxline);
+void entab(char to[], char from[]);
+
+
+main()
+{
+  int len;                    /* current line length */ 
+  char line[MAXLINE];         /* current input line */
+  char entab_line[MAXLINE];   /* detabbed line */
+
+  while ((len = my_getline(line, MAXLINE)) > 0) {
+    entab(entab_line, line);
+    printf("%s", entab_line);
+  }
+  return 0;
+}
+
+/* my_getline: reads in a line, return the length */
+int my_getline(char line[], int lim)
+{
+  int c, i;
+
+  for (i = 0; i < lim-1 && (c=getchar()) != EOF && c != '\n'; ++i)
+    line[i] = c;
+  if (c == '\n') {
+    line[i] = c;
+    ++i;
+  }
+  line[i] = '\0';
+  return i;
+}
+
+/* entab: replace blanks with tab in line; does not handle the limit */
+void entab(char to[], char from[])
+{
+  int i, j, k;
+
+  i = j = k = 0;
+  while ((to[j] = from[i]) != '\0') {
+    if (from[i] == ' ') {
+      ++k;
+      if (k == TABWIDTH) {
+        k = 0;
+        j = j - TABWIDTH;
+        to[j] = '\t';
+      }
+    } else {
+      k = 0;
+    }
+    ++j;
+    ++i;
+  }
+}