fold example, make format check

Makefile

@@ -35,3 +35,27 @@ clean:
 	@for dir in $(SUBDIRS); do \
 		$(MAKE) -C $$dir clean; \
 	done
+
+.PHONY: check-format-all check-format
+
+check-format-all:
+	@for dir in $(SUBDIRS); do \
+	    if [ -d "$$dir" ]; then \
+	        echo "=== clang-format check in $$dir ==="; \
+	        $(MAKE) -f $(MAKEFILE_LIST) check-format DIR=$$dir; \
+	    fi; \
+	done
+
+check-format:
+	@cd $(DIR); \
+	echo "Checking clang-format in $(DIR)"; \
+	if command -v bash >/dev/null 2>&1; then \
+	    bash -c 'shopt -s nullglob; files=( *.cpp *.hpp *.c *.h ); \
+	    if [ $${#files[@]} -ne 0 ]; then \
+	        clang-format --Werror --dry-run "$${files[@]}"; \
+	    else \
+	        echo "No source files found, skipping"; \
+	    fi'; \
+	else \
+	    echo "Bash not found; skipping clang-format check"; \
+	fi

README.md

@@ -19,6 +19,8 @@ Examples include (and will expand to):
 * Views
     * [views-zip-enumerate](./views-zip-enumerate/)
 * Atomics and memory ordering
+* Folding
+    * [fold-left-fold-right](./fold-left-fold-right/)
 * RAII and ownership patterns
 * Performance‑oriented C++ idioms
 
@@ -140,6 +142,18 @@ The `clang-format` is used to ensure the code format.
 ./clang-check.sh *.cpp *.hpp
 ```
 
+At top level, you can do:
+
+```make
+make check-format-all
+```
+
+At each example directory, you can do:
+
+```make
+make check-format
+```
+
 ---
 
 ## Continuous Integration

fold-left-fold-right/Makefile

@@ -0,0 +1,29 @@
+# pull in shared compiler settings
+include ../common.mk
+
+# per-example flags
+CXXFLAGS += -pthread
+
+TARGET := fold_left_fold_right
+SRCS   := main.cpp
+OBJS   := $(SRCS:.cpp=.o)
+
+all: $(TARGET)
+
+$(TARGET): $(OBJS)
+	$(CXX) $(CXXFLAGS) -o $@ $^
+
+%.o: %.cpp
+	$(CXX) $(CXXFLAGS) -c $< -o $@
+
+run: $(TARGET)
+	./$(TARGET) $(ARGS)
+
+clean:
+	rm -f $(OBJS) $(TARGET)
+
+# Delegates to top-level Makefile
+check-format:
+	$(MAKE) -f ../Makefile check-format DIR=$(CURDIR)
+
+.PHONY: all clean run check-format

fold-left-fold-right/main.cpp

@@ -0,0 +1,85 @@
+#include <iostream>
+#include <vector>
+#include <numeric>    // accumulate, fold_*
+#include <functional> // std::plus
+#include <optional>
+
+template <typename Range, typename T, typename Op>
+T
+fold_left(const Range& r, T init, Op op)
+{
+    for (auto&& v : r)
+        init = op(init, v);
+    return init;
+}
+
+template <typename Range, typename T, typename Op>
+T
+fold_right(const Range& r, T init, Op op)
+{
+    for (auto it = r.rbegin(); it != r.rend(); ++it)
+        init = op(*it, init);
+    return init;
+}
+
+// fold_left_first: uses the first element of the range as the initial value
+template <typename Range, typename Op>
+std::optional<typename Range::value_type>
+fold_left_first(const Range& r, Op op)
+{
+    auto it = r.begin();
+    if (it == r.end())
+        return std::nullopt;
+    auto init = *it++;
+    for (; it != r.end(); ++it)
+        init = op(init, *it);
+    return init;
+}
+
+// fold_right_first: uses the first element of the range as the initial value
+template <typename Range, typename Op>
+std::optional<typename Range::value_type>
+fold_right_first(const Range& r, Op op)
+{
+    auto it = r.rbegin();
+    if (it == r.rend())
+        return std::nullopt;
+    auto init = *it++;
+    for (; it != r.rend(); ++it)
+        init = op(*it, init);
+    return init;
+}
+
+int
+main()
+{
+    std::vector<int> numbers = {1, 2, 3, 4, 5};
+
+    // Fold left (classic)
+    int sum_left = std::accumulate(numbers.begin(), numbers.end(), 0, std::plus<int>{});
+    std::cout << "Fold left (sum): " << sum_left << '\n';
+
+    // Fold right (classic using reverse iterators)
+    int sum_right = std::accumulate(numbers.rbegin(), numbers.rend(), 0, std::plus<int>{});
+    std::cout << "Fold right (sum): " << sum_right << '\n';
+
+    // Fold left with ranges (C++23)
+    int sum_ranges = fold_left(numbers, 0, std::plus{});
+    std::cout << "Fold left with ranges (sum): " << sum_ranges << '\n';
+
+    // Fold right with ranges (C++23)
+    int sum_ranges_right = fold_right(numbers, 0, std::plus{});
+    std::cout << "Fold right with ranges (sum): " << sum_ranges_right << '\n';
+
+    // Fold left using first element as initial value
+    if (auto sum_first = fold_left_first(numbers, std::plus{})) {
+        std::cout << "Fold left first element (sum): " << *sum_first << '\n';
+    }
+
+    // Fold right using first element as initial value
+    if (auto sum_right_first = fold_right_first(numbers, std::plus{})) {
+        std::cout << "Fold right first element (sum): " << *sum_right_first << '\n';
+    }
+
+    return 0;
+}

thread-safe-queue/Makefile

@@ -22,4 +22,8 @@ run: $(TARGET)
 clean:
 	rm -f $(OBJS) $(TARGET)
 
-.PHONY: all clean run
+# Delegates to top-level Makefile
+check-format:
+	$(MAKE) -f ../Makefile check-format DIR=$(CURDIR)
+
+.PHONY: all clean run check-format

unique-ptr-basics/Makefile

@@ -19,4 +19,8 @@ run: $(TARGET)
 clean:
 	rm -f $(OBJS) $(TARGET)
 
-.PHONY: all clean run
+# Delegates to top-level Makefile
+check-format:
+	$(MAKE) -f ../Makefile check-format DIR=$(CURDIR)
+
+.PHONY: all clean run check-format

views-zip-enumerate/Makefile

@@ -22,4 +22,8 @@ run: $(TARGET)
 clean:
 	rm -f $(OBJS) $(TARGET)
 
-.PHONY: all clean run
+# Delegates to top-level Makefile
+check-format:
+	$(MAKE) -f ../Makefile check-format DIR=$(CURDIR)
+
+.PHONY: all clean run check-format