diff --git a/intro/ex1_20_entab.c b/intro/ex1_20_entab.c
new file mode 100644
index 0000000..7b869bd
--- /dev/null
+++ b/intro/ex1_20_entab.c
@@ -0,0 +1,51 @@
+#include <stdio.h>
+
+#define MAXLINE 1000    /* maximum characters in a line */ 
+#define TABWIDTH 4      /* tab size */ 
+
+int my_getline(char line[], int maxline);
+void detab(char to[], char from[]);
+
+
+main()
+{
+  int len;                    /* current line length */ 
+  char line[MAXLINE];         /* current input line */
+  char detab_line[MAXLINE];   /* detabbed line */
+
+  while ((len = my_getline(line, MAXLINE)) > 0)
+    detab(detab_line, line);
+    printf("%s", detab_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;
+}
+
+/* detab: replace tab with blanks in line; does not handle the limit */
+void detab(char to[], char from[])
+{
+  int i, j, k;
+
+  i = j = k = 0;
+  while ((to[j] = from[i]) != '\0') {
+    if (to[j] == '\t')
+      for (k = 0; k < TABWIDTH; ++k, ++j) 
+        to[j] = ' ';
+    else
+      ++j;
+    ++i;
+  }
+}