diff --git a/docs/source/perfeval.rst b/docs/source/perfeval.rst
index 0b8e6ef..f03c503 100644
--- a/docs/source/perfeval.rst
+++ b/docs/source/perfeval.rst
@@ -14,12 +14,12 @@ Classification Performance
 
     Compute error rate of predictions given by ``pred`` w.r.t. the ground truths given in ``gt``.
 
-.. function:: confusmat(k, gt, pred)
+.. function:: confusmat(gt, pred)
 
     Compute the confusion matrix of the predictions given by ``pred`` w.r.t. the ground truths given in ``gt``.
-    Here, ``k`` is the number of classes.
 
-    It returns an integer matrix ``R`` of size ``(k, k)``, such that ``R(i, j) == countnz((gt .== i) & (pred .== j))``.
+    It returns an integer matrix ``R`` of size ``(k, k)`` where k is the number of classes in ``gt``, 
+    such that ``R(i, j) == countnz((gt .== i) & (pred .== j))``.
 
     **Examples:**
 
@@ -29,7 +29,7 @@ Classification Performance
 
         julia> pred = [1, 1, 2, 2, 2, 3, 3, 3];
 
-        julia> C = confusmat(3, gt, pred)   # compute confusion matrix
+        julia> C = confusmat(gt, pred)   # compute confusion matrix
         3x3 Array{Int64,2}:
          2  1  0
          0  2  1
diff --git a/src/perfeval.jl b/src/perfeval.jl
index 4f521e2..b67491d 100644
--- a/src/perfeval.jl
+++ b/src/perfeval.jl
@@ -6,14 +6,18 @@ correctrate(gt::IntegerVector, r::IntegerVector) = counteq(gt, r) / length(gt)
 errorrate(gt::IntegerVector, r::IntegerVector) = countne(gt, r) / length(gt)
 
 ## confusion matrix
-
-function confusmat(k::Integer, gts::IntegerVector, preds::IntegerVector)
+function confusmat(gts::IntegerVector, preds::IntegerVector)
     n = length(gts)
     length(preds) == n || throw(DimensionMismatch("Inconsistent lengths."))
+
+    gtslbl = sort(unique(gts))
+    k = length(gtslbl)
+
+    lookup = Dict(reverse.(enumerate(gtslbl)|> collect))
     R = zeros(Int, k, k)
     for i = 1:n
-        @inbounds g = gts[i]
-        @inbounds p = preds[i]
+        @inbounds g = lookup[gts[i]]
+        @inbounds p = lookup[preds[i]]
         R[g, p] += 1
     end
     return R