Add do...while documentation

doc/userdoc.org

@@ -1880,7 +1880,7 @@ We give a final illustration
 
 *** ~for~ loops
 
-~pugs~ allow to write ~for~ loops. It follows the ~C++~ syntax
+~pugs~ allows to write ~for~ loops. It follows the ~C++~ syntax
 #+BEGIN_SRC pugs :exports code
   for (declarationinstruction ; condition ; postinstruction) statement
 #+END_SRC
@@ -1937,8 +1937,43 @@ To fix the previous code, one can write
 One gets
 #+results: for-no-decl
 
+*** ~do...while~ loops
 
-*** TODO do while loops
+The second kind of loops that can be written is the ~do...while~
+construction which executes /at least/ one time the enclosed ~statement~.
+#+BEGIN_SRC pugs :exports code
+  do statement while (condition);
+#+END_SRC
+
+The ~statement~ is either a single instruction or a block of
+instructions. The ~condition~ is an expression of boolean value (type
+~B~).
+
+#+NAME: do-while-instr
+#+BEGIN_SRC pugs :exports both :results output
+  let sum:N, sum = 0;
+  let i:N, i = 0;
+  do
+    sum += i++;
+  while (i<=10);
+  cout << "sum = " << sum << "\n";
+#+END_SRC
+This code produces
+#+results: do-while-instr
+
+However, to ease the reading, it is probably better to write
+#+NAME: do-while-block
+#+BEGIN_SRC pugs :exports both :results output
+  let sum:N, sum = 0;
+  let i:N, i = 0;
+  do {
+    sum += i;
+    ++i;
+  } while (i<=10);
+  cout << "sum = " << sum << "\n";
+#+END_SRC
+It gives also
+#+results: do-while-block
 
 *** TODO while loops