diff --git a/decoder.py b/decoder.py
index 05400d1b4113e78bd79de5765e4640727086fa85..0eab3651968d16e06225c94ecf67592e16240e5b 100755
--- a/decoder.py
+++ b/decoder.py
@@ -121,25 +121,10 @@ def ctc_beam_search_decoder(probs_seq,
     if not blank_id < probs_dim:
         raise ValueError("blank_id shouldn't be greater than probs dimension")
 
-    # assign space_id
-    if ' ' not in vocabulary:
-        raise ValueError("space doesn't exist in vocabulary")
-    space_id = vocabulary.index(' ')
-
-    # function to convert ids in string to list
-    def ids_str2list(ids_str):
-        ids_str = ids_str.split(' ')
-        ids_list = [int(elem) for elem in ids_str]
-        return ids_list
-
-    # function to convert ids list to sentence
-    def ids2sentence(ids_list, vocab):
-        return ''.join([vocab[ids] for ids in ids_list])
-
     ## initialize
     # the set containing selected prefixes
-    prefix_set_prev = {'-1': 1.0}
-    probs_b_prev, probs_nb_prev = {'-1': 1.0}, {'-1': 0.0}
+    prefix_set_prev = {'\t': 1.0}
+    probs_b_prev, probs_nb_prev = {'\t': 1.0}, {'\t': 0.0}
 
     ## extend prefix in loop
     for time_step in range(num_time_steps):
@@ -148,10 +133,6 @@ def ctc_beam_search_decoder(probs_seq,
         probs_b_cur, probs_nb_cur = {}, {}
         for l in prefix_set_prev:
             prob = probs_seq[time_step]
-
-            # convert ids in string to list
-            ids_list = ids_str2list(l)
-            end_id = ids_list[-1]
             if not prefix_set_next.has_key(l):
                 probs_b_cur[l], probs_nb_cur[l] = 0.0, 0.0
 
@@ -161,18 +142,20 @@ def ctc_beam_search_decoder(probs_seq,
                     probs_b_cur[l] += prob[c] * (
                         probs_b_prev[l] + probs_nb_prev[l])
                 else:
-                    l_plus = l + ' ' + str(c)
+                    last_char = l[-1]
+                    new_char = vocabulary[c]
+                    l_plus = l + new_char
                     if not prefix_set_next.has_key(l_plus):
                         probs_b_cur[l_plus], probs_nb_cur[l_plus] = 0.0, 0.0
 
-                    if c == end_id:
+                    if new_char == last_char:
                         probs_nb_cur[l_plus] += prob[c] * probs_b_prev[l]
                         probs_nb_cur[l] += prob[c] * probs_nb_prev[l]
-                    elif c == space_id:
-                        if ext_scoring_func is None:
+                    elif new_char == ' ':
+                        if (ext_scoring_func is None) or (len(l) == 1):
                             score = 1.0
                         else:
-                            prefix = ids2sentence(ids_list, vocabulary)
+                            prefix = l[1:]
                             score = ext_scoring_func(prefix)
                         probs_nb_cur[l_plus] += score * prob[c] * (
                             probs_b_prev[l] + probs_nb_prev[l])
@@ -185,8 +168,7 @@ def ctc_beam_search_decoder(probs_seq,
             # add l into prefix_set_next
             prefix_set_next[l] = probs_b_cur[l] + probs_nb_cur[l]
         # update probs
-        probs_b_prev, probs_nb_prev = copy.deepcopy(probs_b_cur), copy.deepcopy(
-            probs_nb_cur)
+        probs_b_prev, probs_nb_prev = probs_b_cur, probs_nb_cur
 
         ## store top beam_size prefixes
         prefix_set_prev = sorted(
@@ -198,8 +180,7 @@ def ctc_beam_search_decoder(probs_seq,
     beam_result = []
     for (seq, prob) in prefix_set_prev.items():
         if prob > 0.0:
-            ids_list = ids_str2list(seq)[1:]
-            result = ids2sentence(ids_list, vocabulary)
+            result = seq[1:]
             log_prob = np.log(prob)
             beam_result.append([log_prob, result])