added function cif::Column::erase()

requested in #251

Author: wojdyr

docs/cif.rst

@@ -940,7 +940,7 @@ The C++ signature of ``find_values`` is::
   // Returns underlying Item (which contains either Pair or Loop).
   Item* item();
 
-  // Returns pointer to the DOM structure containing the whole table.
+  // If it's in Loop, returns pointer to Loop; otherwise, nullptr.
   Loop* get_loop() const;
 
   // Get raw value (no bounds checking).
@@ -952,6 +952,9 @@ The C++ signature of ``find_values`` is::
   // Short-cut for cif::as_string(column.at(n)).
   std::string str(int n) const;
 
+  // Erases item for name-value pair; removes column for Loop
+  void erase();
+
 ``Column`` also provides support for C++11 range-based ``for``::
 
   // mmCIF _chem_comp_atom is usually a table, but not always
@@ -1011,6 +1014,16 @@ to this ``Loop`` in the DOM. Otherwise it returns ``None``.
   >>> column.get_loop()
   <gemmi.cif.Loop 12 x 7>
 
+``erase()`` removes the column from a loop, if this column is in a loop;
+if it's a tag-value pair it erases the containing item.
+
+.. doctest::
+
+  >>> loop = column.get_loop()
+  >>> column.erase()
+  >>> loop  # one column less
+  <gemmi.cif.Loop 12 x 6>
+
 Table
 =====
 

include/gemmi/cifdoc.hpp

@@ -199,6 +199,11 @@ struct Loop {
     int n = find_tag(column_name);
     if (n == -1)
       fail("remove_column(): tag not found: " + column_name);
+    remove_column_at(n);
+  }
+
+  /// \pre: n < tags.size()
+  void remove_column_at(size_t n) {
     tags.erase(tags.begin() + n);
     vector_remove_column(values, tags.size(), n);
   }
@@ -264,6 +269,8 @@ class Column {
   Item* item() { return item_; }
   size_t col() const { return col_; }
 
+  void erase();
+
 private:
   Item* item_;
   size_t col_;  // for loop this is a column index in item_->loop
@@ -657,6 +664,13 @@ inline std::string* Column::get_tag() {
   return &item_->pair[0];
 }
 
+inline void Column::erase() {
+  if (Loop* loop = get_loop())
+    loop->remove_column_at(col_);
+  else if (item_)
+    item_->erase();
+}
+
 inline Loop* Column::get_loop() const {
   return item_ && item_->type == ItemType::Loop ? &item_->loop : nullptr;
 }

python/cif.cpp

@@ -370,6 +370,7 @@ void add_cif(py::module& cif) {
         self.at(idx) = value;
     })
     .def("str", &Column::str, py::arg("index"))
+    .def("erase", &Column::erase)
     .def("__repr__", [](const Column &self) {
         std::string s = "<gemmi.cif.Column ";
         if (const std::string* tag = self.get_tag())

tests/test_cif.py

@@ -350,6 +350,16 @@ def test_case_sensitivity(self):
         self.assertEqual(block.get_index('_lBBB'), 5)
         self.assertEqual(block.find_pair('_Three'), ('_thrEE', '3'))
 
+        values.erase()
+        block.find_values('_two').erase()
+        block.find_values('_nonloop_b').erase()
+        expected = """\
+            data_test
+            _One 1 _thrEE 3
+            _NonLoop_a alpha
+            loop_ _lbBb _ln  B 1  D 2"""
+        self.assertEqual(block.as_string().split(), expected.split())
+
 class TestQuote(unittest.TestCase):
     def test_quote(self):
         self.assertEqual(cif.quote('a.b-c'), 'a.b-c')