From 95d9bb50ca6834fe65013bed0e04e6df94bedd27 Mon Sep 17 00:00:00 2001
From: Jidong Xiao <xiaoj8@rpi.edu>
Date: Tue, 11 Mar 2025 00:56:33 -0400
Subject: [PATCH] adding benchmarks and readme

---
 .../optimization/erase/map_erase_fast.cpp     | 20 ++++++
 .../optimization/erase/map_erase_slow.cpp     | 20 ++++++
 lectures/optimization/map_erase/README.md     | 69 +++++++++++++++++++
 .../optimization/map_erase/map_erase_fast.cpp | 20 ++++++
 .../optimization/map_erase/map_erase_slow.cpp | 20 ++++++
 5 files changed, 149 insertions(+)
 create mode 100644 lectures/optimization/erase/map_erase_fast.cpp
 create mode 100644 lectures/optimization/erase/map_erase_slow.cpp
 create mode 100644 lectures/optimization/map_erase/README.md
 create mode 100644 lectures/optimization/map_erase/map_erase_fast.cpp
 create mode 100644 lectures/optimization/map_erase/map_erase_slow.cpp

diff --git a/lectures/optimization/erase/map_erase_fast.cpp b/lectures/optimization/erase/map_erase_fast.cpp
new file mode 100644
index 0000000..ccbd380
--- /dev/null
+++ b/lectures/optimization/erase/map_erase_fast.cpp
@@ -0,0 +1,20 @@
+#include <iostream>
+#include <map>
+
+int main() {
+    const int N = 1000000;
+    std::map<int, int> m;
+
+    // fill the map with some values
+    for (int i = 0; i < N; ++i) {
+        m[i] = i;
+    }
+
+    // erase all elements using iterators
+    for (std::map<int, int>::iterator itr = m.begin(); itr != m.end(); ) {
+        itr = m.erase(itr); // erase using iterator and move to the next element
+    }
+
+    return 0;
+}
+
diff --git a/lectures/optimization/erase/map_erase_slow.cpp b/lectures/optimization/erase/map_erase_slow.cpp
new file mode 100644
index 0000000..5b46dd2
--- /dev/null
+++ b/lectures/optimization/erase/map_erase_slow.cpp
@@ -0,0 +1,20 @@
+#include <iostream>
+#include <map>
+
+int main() {
+    const int N = 1000000;
+    std::map<int, int> m;
+
+    // fill the map with some values
+    for (int i = 0; i < N; ++i) {
+        m[i] = i;
+    }
+
+    // erase by key
+    for (int i = 0; i < N; ++i) {
+        m.erase(i);
+    }
+
+    return 0;
+}
+
diff --git a/lectures/optimization/map_erase/README.md b/lectures/optimization/map_erase/README.md
new file mode 100644
index 0000000..0f78388
--- /dev/null
+++ b/lectures/optimization/map_erase/README.md
@@ -0,0 +1,69 @@
+# `erase by iterator` vs. `erase by key` in C++
+
+## Overview
+
+erasing by iterator in std::maps is typically faster than erasing by key.
+
+## Erasing by Iterator:
+
+- When you erase an element using an iterator, the map already has a direct reference to the node that contains the element to be erased.
+
+- The iterator is a pointer to the element (node) in the underlying tree structure (which is usually a red-black tree for std::map).
+
+## Erasing by Key:
+
+- When you erase an element by key, std::map first has to search for the key.
+
+- It does this by traversing the red-black tree to find the node that matches the key.
+
+- After finding the node, it performs the same operation as erase(iterator), but since it had to perform a search to locate the key, this is slower than directly accessing the iterator.
+
+## Benchmarks
+
+Compile and run these 2 programs to see the performance difference.
+
+[map_erase_slow.cpp](map_erase_slow.cpp) [map_erase_fast.cpp](map_erase_fast.cpp)
+
+Both programs just creates a map containing 1 millions integers, and then erase these 1 millions integers. As can be seen from the following results, erasing by iterators is much faster than erasing by key.
+
+```console
+$g++ map_erase_slow.cpp -o map_erase_slow
+$g++ map_erase_fast.cpp -o map_erase_fast
+
+$time ./map_erase_slow
+
+real    0m0.640s
+user    0m0.624s
+sys     0m0.016s
+
+$time ./map_erase_fast
+
+real    0m0.406s
+user    0m0.386s
+sys     0m0.020s
+
+$time ./map_erase_fast
+
+real    0m0.382s
+user    0m0.374s
+sys     0m0.008s
+
+$time ./map_erase_slow
+
+real    0m0.629s
+user    0m0.617s
+sys     0m0.012s
+
+$time ./map_erase_slow
+
+real    0m0.632s
+user    0m0.623s
+sys     0m0.009s
+
+$time ./map_erase_fast
+
+real    0m0.383s
+user    0m0.366s
+sys     0m0.017s
+
+```
diff --git a/lectures/optimization/map_erase/map_erase_fast.cpp b/lectures/optimization/map_erase/map_erase_fast.cpp
new file mode 100644
index 0000000..ccbd380
--- /dev/null
+++ b/lectures/optimization/map_erase/map_erase_fast.cpp
@@ -0,0 +1,20 @@
+#include <iostream>
+#include <map>
+
+int main() {
+    const int N = 1000000;
+    std::map<int, int> m;
+
+    // fill the map with some values
+    for (int i = 0; i < N; ++i) {
+        m[i] = i;
+    }
+
+    // erase all elements using iterators
+    for (std::map<int, int>::iterator itr = m.begin(); itr != m.end(); ) {
+        itr = m.erase(itr); // erase using iterator and move to the next element
+    }
+
+    return 0;
+}
+
diff --git a/lectures/optimization/map_erase/map_erase_slow.cpp b/lectures/optimization/map_erase/map_erase_slow.cpp
new file mode 100644
index 0000000..5b46dd2
--- /dev/null
+++ b/lectures/optimization/map_erase/map_erase_slow.cpp
@@ -0,0 +1,20 @@
+#include <iostream>
+#include <map>
+
+int main() {
+    const int N = 1000000;
+    std::map<int, int> m;
+
+    // fill the map with some values
+    for (int i = 0; i < N; ++i) {
+        m[i] = i;
+    }
+
+    // erase by key
+    for (int i = 0; i < N; ++i) {
+        m.erase(i);
+    }
+
+    return 0;
+}
+