Fix Issue go-gota#169 - NA is still converted to NaN even in a string…

… column

Copied from this PR:
go-gota#175

dataframe/dataframe.go

@@ -1219,17 +1219,22 @@ func LoadRecords(records [][]string, options ...LoadOption) DataFrame {
 	types := make([]series.Type, len(headers))
 	rawcols := make([][]string, len(headers))
 	for i, colname := range headers {
+		t, useCustomType := cfg.types[colname]
 		rawcol := make([]string, len(records))
 		for j := 0; j < len(records); j++ {
 			rawcol[j] = records[j][i]
+			if useCustomType && t == series.String {
+				// skip the convertion when using custom string type
+				continue
+			}
 			if findInStringSlice(rawcol[j], cfg.nanValues) != -1 {
 				rawcol[j] = "NaN"
 			}
 		}
 		rawcols[i] = rawcol
 
-		t, ok := cfg.types[colname]
-		if !ok {
+		// try to auto detect the data type
+		if !useCustomType {
 			t = cfg.defaultType
 			if cfg.detectTypes {
 				if l, err := findType(rawcol); err == nil {

dataframe/dataframe_test.go

@@ -1420,6 +1420,49 @@ Spain,2012-02-01,66,555.42,00241
 	}
 }
 
+// test case for issue #169
+func TestReadCSV_Issue169(t *testing.T) {
+	// Load the data from a CSV string and try to infer the type of the
+	// columns, but NA won't be converted to NaN when data type is specified
+	// as string.
+	const ExampleData = `
+Country,Region,Date,Age,Amount,Id
+"United States",NA,2012-02-01,50,112.1,01234
+"United States",US,2012-02-01,32,321.31,54320
+"United Kingdom",GB,2012-02-01,17,18.2,12345
+"United States",NA,2012-02-01,32,321.31,54320
+"United States","NA",2012-02-01,17,321.31,54320
+"United Kingdom",GB,2012-02-01,NA,18.2,12345
+"United States",NA,2012-02-01,32,321.31,54320
+Spain,EU,2012-02-01,66,555.42,00241
+`
+
+	df := ReadCSV(
+		strings.NewReader(ExampleData),
+		WithTypes(map[string]series.Type{
+			"Region": series.String,
+			"Age":    series.String,
+		}),
+	)
+
+	if df.Err != nil {
+		t.Errorf("Expected success, got error: %v", df.Err)
+	}
+
+	for _, v := range df.Col("Region").Records() {
+		if v == "NaN" {
+			t.Errorf("Expected not to convert NA to NaN, but it does")
+		}
+	}
+
+	for _, v := range df.Col("Age").Records() {
+		if v == "NaN" {
+			t.Errorf("Expected not to convert NA to NaN, but it does")
+		}
+	}
+
+}
+
 func TestReadJSON(t *testing.T) {
 	table := []struct {
 		jsonStr string