diff --git a/labs31/README.md b/labs31/README.md
index e360ab1..effc3c7 100644
--- a/labs31/README.md
+++ b/labs31/README.md
@@ -2,22 +2,29 @@
 
 最直接的办法就是循环计算，因为2的20次幂就达到1M的大小了，这么大的内存块其实没有缓存的必要了，所以循环最多也就20次。
 
-但是我很蛋疼了测试了sort包的Search函数，switch循环展开、if循环展开和人肉折半查找。
+但是我很蛋疼了测试了sort包的Search函数，switch循环展开、if循环展开、人肉折半查找、浮点数取指数。
 
 我电脑上测试结果如下：
 
-```
+
+go version 1.7.3
+
 $ go test -bench="."
+PASS`
+Benchmark_Normal-8    	100000000	        22.8 ns/op
+Benchmark_Search-8    	100000000	        21.5 ns/op
+Benchmark_Search2-8   	200000000	         6.62 ns/op
+Benchmark_Switch-8    	200000000	         7.54 ns/op
+Benchmark_IF1-8       	300000000	         4.33 ns/op
+Benchmark_IF2-8       	500000000	         2.71 ns/op
+Benchmark_IF3-8       	2000000000	         1.09 ns/op
+Benchmark_IF4-8       	200000000	         7.12 ns/op
+Benchmark_IF5-8       	2000000000	         0.61 ns/op
 PASS
-Benchmark_Normal-4	50000000	        25.6 ns/op
-Benchmark_Search-4	100000000	        21.7 ns/op
-Benchmark_Switch-4	100000000	        12.5 ns/op
-Benchmark_IF1-4   	200000000	         9.14 ns/op
-Benchmark_IF2-4   	300000000	         3.89 ns/op
-Benchmark_IF3-4   	1000000000	         2.37 ns/op
-ok  	github.com/idada/go-labs/labs31	11.822s
-```
+ok  	command-line-arguments	18.027s
+
 
 呵呵，蛋好疼。
 
-补充：最后一个IF3测试是由群里的v1zze同学提供的算法，果然牛逼。
\ No newline at end of file
+补充：最后一个IF3测试是由群里的v1zze同学提供的算法，果然牛逼。
+IF4 和 IF5 使用浮点数取指数计
diff --git a/labs31/labs31_test.go b/labs31/labs31_test.go
index 977e402..90194cd 100644
--- a/labs31/labs31_test.go
+++ b/labs31/labs31_test.go
@@ -1,6 +1,7 @@
 package labs31
 
 import (
+	"math"
 	"math/rand"
 	"sort"
 	"testing"
@@ -23,11 +24,14 @@ func Test_All(t *testing.T) {
 		d := IF2(n)
 		e := Search(n)
 		f := IF3(n)
+		g := IF4(n)
+		h := IF5(n)
+		i := SearchInt(n)
 		if f > 21 {
 			f = 21
 		}
-		if a != b || b != c || c != d || d != e || e != f {
-			t.Log(n, a, b, c, d, e, f)
+		if a != b || b != c || c != d || d != e || e != f || f != g || g != h || h != i {
+			t.Log(n, a, b, c, d, e, f, g, h, i)
 			t.Fail()
 		}
 	}
@@ -45,6 +49,12 @@ func Benchmark_Search(b *testing.B) {
 	}
 }
 
+func Benchmark_Search2(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		SearchInt(benchx)
+	}
+}
+
 func Benchmark_Switch(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Switch(benchx)
@@ -69,6 +79,18 @@ func Benchmark_IF3(b *testing.B) {
 	}
 }
 
+func Benchmark_IF4(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		IF4(benchx)
+	}
+}
+
+func Benchmark_IF5(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		IF5(benchx)
+	}
+}
+
 func Normal(n int) uint {
 	var i = uint(0)
 	for ; n > (1<<i) && i <= 20; i++ {
@@ -85,6 +107,25 @@ func Search(n int) uint {
 	return uint(sort.SearchInts(classes, n))
 }
 
+// copy from search.go:59
+// func Search(n int, f func(int) bool) int
+func SearchInt(n int) uint {
+	// Define f(-1) == false and f(n) == true.
+	// Invariant: f(i-1) == false, f(j) == true.
+	i, j := 0, len(classes)
+	for i < j {
+		h := i + (j-i)/2 // avoid overflow when computing h
+		// i ≤ h < j
+		if classes[h] < n {
+			i = h + 1 // preserves f(i-1) == false
+		} else {
+			j = h // preserves f(j) == true
+		}
+	}
+	// i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
+	return uint(i)
+}
+
 func Switch(n int) uint {
 	switch {
 	case n == 1:
@@ -299,3 +340,23 @@ func IF3(n int) uint {
 	}
 	return c
 }
+
+func IF4(n int) uint {
+	n--
+	ix := math.Ilogb(float64(n))
+	if ix > 20 {
+		ix = 20
+	}
+	ix++
+	return uint(ix)
+}
+
+func IF5(n int) uint {
+	n--
+	ix := (math.Float32bits(float32(n))>>23)&0xff - 127
+	if ix > 20 {
+		ix = 20
+	}
+	ix++
+	return uint(ix)
+}