Update README

Author: markisus

README.md

@@ -4,6 +4,7 @@ These are the most useful operations.
 - `.sort_by_field<col_idx>()` sort all columns in tandem based on particular column 
 - `operator[row_idx]` read data out as tuple of references
 - `.get_column<col_idx>()` direct access to underlying std::vector column
+- `.view<col_idx1, col_idx2, ...>()` read subset of the columns out as a tuple of references
 
 Code Example (scratch.cpp)
 ------------------------
@@ -16,6 +17,10 @@ int main(int argc, char *argv[])
 {
     // presidents will be a soa representing
     // order, first name, last name
+
+    constexpr int ORDER = 0;
+    constexpr int FIRST_NAME = 1;
+    constexpr int LAST_NAME = 2;
     vapid::soa<int, std::string, std::string> presidents;
 
     presidents.insert(0, "Abraham", "Lincoln");
@@ -29,35 +34,48 @@ int main(int argc, char *argv[])
     std::cout << presidents << "\n";
 
     // sort by time (first column)
-    presidents.sort_by_field<0>();
+    presidents.sort_by_field<ORDER>();
     std::cout << "Presidents sorted by temporal order" << "\n";
     std::cout << presidents << "\n";
 
     // sort by first name (second column)
-    presidents.sort_by_field<1>();
+    presidents.sort_by_field<FIRST_NAME>();
     std::cout << "Presidents sorted by first name" << "\n";
     std::cout << presidents << "\n";
 
     // sort by last name (third column)
-    presidents.sort_by_field<2>();
+    presidents.sort_by_field<LAST_NAME>();
     std::cout << "Presidents sorted by last name" << "\n";
     std::cout << presidents << "\n";
 
     // operator[] returns a tuple of references
     // Let's update Joe Biden to Joseph Biden
-    auto [order, fname, lname] = presidents[0];
-    fname = "Joseph"; // Joe => Joseph
-    std::cout << "Editing the first row to update Joe=>Joseph" << "\n";
-    std::cout << presidents << "\n";
+    {
+        std::cout << "Editing the first row to update Joe => Joseph" << "\n";
+        auto [order, fname, lname] = presidents[0];
+        fname = "Joseph"; // Joe => Joseph
+        std::cout << presidents << "\n";
+    }
+
+    // view is templated to return a subset of fields 
+    // Let's update Abraham Lincoln to George Washington
+    {
+        std::cout << "Editing the third row to update Abraham Lincoln => George Washington" << "\n";
+        auto [fname, lname] = presidents.view<FIRST_NAME,LAST_NAME>(3);
+        fname = "George";
+        lname = "Washington";
+        std::cout << presidents << "\n";
+    }
 
     // get_column<idx> returns direct access
     // to the underlying std::vector.
-    // Let's update the first name column, changing
-    // the entry George to George W.
-    std::cout << "Editing the second row to update George=>George W." << "\n";
-    auto& first_names = presidents.get_column<1>();
-    first_names[1] = "George W.";
-    std::cout << presidents << "\n";
+    // Let's sum the characters of the first names.
+    std::cout << "Summing first name lengths\n";
+    int length_sum = 0;
+    for (const auto& fname : presidents.get_column<FIRST_NAME>()) {
+        length_sum += fname.length();
+    }
+    std::cout << "Total characters used in first names = " << length_sum << "\n\n";
 
     // We can pass a custom comparator when sorting
     // Let's sort based on length of last name
@@ -69,7 +87,6 @@ int main(int argc, char *argv[])
 
     return 0;
 }
-
 ```
 
 Manual Installation