From d63f426eacb5dcbd95fc8ff9b03a37db5e1fdc18 Mon Sep 17 00:00:00 2001
From: Marcus Mohr <marcus.mohr@lmu.de>
Date: Mon, 13 Dec 2021 17:32:37 +0100
Subject: [PATCH] Adds frist draft of sub-section on lambda expression
---
notebooks/08_Functors+Lambdas.ipynb | 638 +++++++++++++++++++++++++++-
1 file changed, 617 insertions(+), 21 deletions(-)
diff --git a/notebooks/08_Functors+Lambdas.ipynb b/notebooks/08_Functors+Lambdas.ipynb
index f0c89de..9b38def 100644
--- a/notebooks/08_Functors+Lambdas.ipynb
+++ b/notebooks/08_Functors+Lambdas.ipynb
@@ -8,15 +8,6 @@
"# Functors, Lambdas and std::function"
]
},
- {
- "cell_type": "markdown",
- "id": "43efc656",
- "metadata": {},
- "source": [
- " \n",
- " "
- ]
- },
{
"cell_type": "markdown",
"id": "008aa425",
@@ -222,7 +213,7 @@
},
{
"cell_type": "code",
- "execution_count": 11,
+ "execution_count": 8,
"id": "856ee747",
"metadata": {},
"outputs": [
@@ -278,7 +269,7 @@
},
{
"cell_type": "code",
- "execution_count": 12,
+ "execution_count": 9,
"id": "4adb7f0b",
"metadata": {},
"outputs": [],
@@ -288,7 +279,7 @@
},
{
"cell_type": "code",
- "execution_count": 13,
+ "execution_count": 10,
"id": "f9bfde7f",
"metadata": {},
"outputs": [
@@ -409,12 +400,12 @@
"metadata": {},
"source": [
"##### **Example 5:**\n",
- "Now lets use a functor with a slightly more complicated interface"
+ "Now let us use a functor with a slightly more complicated interface"
]
},
{
"cell_type": "code",
- "execution_count": 15,
+ "execution_count": 20,
"id": "ff8d558b-da83-46d9-9842-8e0d9d5ba56d",
"metadata": {},
"outputs": [],
@@ -427,7 +418,7 @@
},
{
"cell_type": "code",
- "execution_count": 16,
+ "execution_count": 21,
"id": "b5790b96-bf62-4d6e-ad9c-e56839882728",
"metadata": {},
"outputs": [
@@ -437,7 +428,7 @@
"6"
]
},
- "execution_count": 16,
+ "execution_count": 21,
"metadata": {},
"output_type": "execute_result"
}
@@ -465,7 +456,7 @@
},
{
"cell_type": "code",
- "execution_count": 18,
+ "execution_count": 22,
"id": "646b302f-cdf5-40ed-98f8-01d98a9b277a",
"metadata": {},
"outputs": [],
@@ -482,7 +473,7 @@
},
{
"cell_type": "code",
- "execution_count": 19,
+ "execution_count": 26,
"id": "b56b370a-654f-4665-a86d-1b4a1497a813",
"metadata": {},
"outputs": [],
@@ -492,7 +483,7 @@
},
{
"cell_type": "code",
- "execution_count": 20,
+ "execution_count": 27,
"id": "b267e62f",
"metadata": {},
"outputs": [
@@ -519,13 +510,618 @@
"#### Lambdas\n",
"Using *function pointers* or *functors* implies some significant coding overhead.\n",
"\n",
- "C++11 introduced **lambda (functions)**, also known as **closures** as a leight-weight alternative, that can be written inline in the source code."
+ "C++11 introduced **lambda (functions)**, also known as **closures**, as a leight-weight alternative, that can be written inline in the source code."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "41bbe5a3",
+ "metadata": {},
+ "source": [
+ "Let's start with an example"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 43,
+ "id": "24ad0ef1",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ " 1 4 9 16 25"
+ ]
+ }
+ ],
+ "source": [
+ "#include <vector>\n",
+ "#include <iostream>\n",
+ "#include <algorithm>\n",
+ "\n",
+ "std::vector< int > numbers{1,2,3,4,5};\n",
+ "\n",
+ "for_each( numbers.begin(), numbers.end(), []( int& n ) { n *= n; } );\n",
+ "\n",
+ "for( int v: numbers ) std::cout << \" \" << v;"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "ff3d112c",
+ "metadata": {},
+ "source": [
+ "The expression **`[]( int& n ) { n *= n; }`** is an *anonymous*, so called, lambda, lambda expression, lambda function or closure.\n",
+ "\n",
+ "- The `(...)` part specifies the interface.\n",
+ "- If a lambda function does not return a value we can omit the return type.\n",
+ "- The `{...}` part is for the body of the function.\n",
+ "- The `[...]` part allows to *capture* variables from the current scope.\n",
+ "\n",
+ "lambda functions do not really provide extra functionality. We could accomplish the same with a free-function or a functor. In this sense they are only syntactic sugar, \"but of the sweetest kind\" (Rainer Grimm)."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "5764a9c9",
+ "metadata": {},
+ "source": [
+ "##### Captures:\n",
+ "\n",
+ "The `[ ]` part of the lambda expression can contain a comma-separated list of stuff we want to *capture*. This allows using it inside the body of the lambda without explicitely passing it through the interface. There are also two *defaults* we can speficy:\n",
+ "\n",
+ "| | effect |\n",
+ "|:-----:|:-------|\n",
+ "| & | implicitly capture the used automatic variables by reference |\n",
+ "| : | implicitly capture the used automatic variables by copy |\n",
+ "| a | capture a from surrounding scope by copy |\n",
+ "| &a | capture a from surrounding scope by reference |\n",
+ "\n",
+ "See e.g. <a href=\"https://en.cppreference.com/w/cpp/language/lambda#Lambda_capture\">cppreference.com</a> for a full list of possibilities."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "645373e5",
+ "metadata": {},
+ "source": [
+ "We are going to take a look at three examples from <a href=\"https://www.grimm-jaud.de/index.php/modernes-c-in-der-praxis-linux-magazin-a\">Modernes C++ in der Praxis</a> by Rainer Grimm."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 1,
+ "id": "3514f84c",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "#include <iostream>\n",
+ "\n",
+ "int main() {\n",
+ "\n",
+ " std::cout << std::endl;\n",
+ "\n",
+ " std::string copy = \"original\";\n",
+ " std::string ref = \"original\";\n",
+ "\n",
+ " auto lambda = [copy,&ref] { std::cout << copy << \" \" << ref << std::endl; };\n",
+ "\n",
+ " lambda();\n",
+ "\n",
+ " copy = \"changed\";\n",
+ " ref = \"changed\";\n",
+ "\n",
+ " lambda();\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 2,
+ "id": "7bca56ac",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "\n",
+ "original original\n",
+ "original changed\n"
+ ]
+ }
+ ],
+ "source": [
+ "main();"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "b237f560",
+ "metadata": {},
+ "source": [
+ "In line #10 we have our lambda expression.\n",
+ "- It does not receive arguments through its interface. Thus, we can omit the `( )` part.\n",
+ "- When we want to re-use a lambda we can store it in a variable. For this we need to use `auto`, asSmalleSmalle.\n",
+ "- Note the difference in the output between the two invocations of `lambda()`. As we captured `copy` by copy its value inside `lambda()` is not affected by its change in the surrounding scope, while that of `ref` is. The latter was captured by reference."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "ac5c13c5",
+ "metadata": {},
+ "source": [
+ "***\n",
+ "Small detour: Coming back to what the type is. Let's try to figure out using `typeid`:"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 3,
+ "id": "f0fd0543",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "5.0 is of type d\n",
+ "type of a is i\n",
+ "What is lambda? ZN11__cling_N5415__cling_Un1Qu33EPvE3$_1\n"
+ ]
+ }
+ ],
+ "source": [
+ "std::cout << \"5.0 is of type \" << typeid(5.0).name() << std::endl;\n",
+ "int a;\n",
+ "std::cout << \"type of a is \" << typeid(a).name() << std::endl;\n",
+ "\n",
+ "\n",
+ "auto lambda = [] (int a) { std::cout << a << std::endl; };\n",
+ "std::cout << \"What is lambda? \" << typeid(lambda).name() << std::endl;"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "3393bdde",
+ "metadata": {},
+ "source": [
+ "Not very helpful, hugh. So maybe let's try something else"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 61,
+ "id": "a32b1635",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Overwriting lambda.cpp\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%file lambda.cpp\n",
+ "\n",
+ "void test() {\n",
+ "\n",
+ "auto lambda = [] (int &a) { a++; };\n",
+ "\n",
+ "int b = 1;\n",
+ "lambda(b); // avoid optimizing out\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 62,
+ "id": "85373b9f",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "!g++ -c lambda.cpp"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 63,
+ "id": "fa038d35",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ " U _GLOBAL_OFFSET_TABLE_\n",
+ " U __stack_chk_fail\n",
+ "000000000000001e T test()\n",
+ "0000000000000000 t test()::{lambda(int&)#1}::operator()(int&) const\n"
+ ]
+ }
+ ],
+ "source": [
+ "!nm -C lambda.o"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "ab79093e",
+ "metadata": {},
+ "source": [
+ "Okay, so that really looks very specific. Indeed **\"the type of a closure cannot be named\"** (at least not explicitely), but it can be detected by **`auto`**."
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "d65d40b3",
+ "metadata": {},
+ "source": [
+ "***\n",
+ "Another example from *Modernes C++ in der Praxis*. Here we implement a join method for strings."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 4,
+ "id": "03a872a9",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Overwriting join.cpp\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%file join.cpp\n",
+ "\n",
+ "#include <iostream>\n",
+ "#include <algorithm>\n",
+ "#include <string>\n",
+ "#include <vector>\n",
+ "\n",
+ "std::string join( std::vector< std::string >& str, std::string sep = \" \" ) {\n",
+ "\n",
+ " std::string joinStr = \"\";\n",
+ "\n",
+ " // small mod from original\n",
+ " // if( not str.size()() ) return joinStr;\n",
+ " if( str.size() == 0 ) return joinStr;\n",
+ "\n",
+ " std::for_each( str.begin(), str.end()-1,\n",
+ " [ &joinStr, sep ] (std::string v) { joinStr += v + sep; } );\n",
+ "\n",
+ " joinStr += str.back(); // back() returns reference to last entry\n",
+ " return joinStr;\n",
+ "}\n",
+ "\n",
+ "int main() {\n",
+ "\n",
+ " std::vector< std::string > myVec;\n",
+ " std::cout << join( myVec ) << std::endl;\n",
+ "\n",
+ " myVec.push_back( \"One\" );\n",
+ " std::cout << join( myVec ) << std::endl;\n",
+ "\n",
+ " myVec.push_back( \"Two\" );\n",
+ " std::cout << join( myVec ) << std::endl;\n",
+ "\n",
+ " myVec.push_back( \"Three\" );\n",
+ " std::cout << join( myVec, \":\" ) << std::endl;\n",
+ "\n",
+ " myVec.push_back( \"Four\" );\n",
+ " std::cout << join( myVec, \"/\" ) << std::endl;\n",
+ "\n",
+ " myVec.push_back( \"Five\" );\n",
+ " std::cout << join( myVec, \"XXX\" ) << std::endl;\n",
+ "\n",
+ " std::cout << std::endl;\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 5,
+ "id": "933de670",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "!g++ -Wall -Wextra join.cpp"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 6,
+ "id": "c485b43f",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "\n",
+ "One\n",
+ "One Two\n",
+ "One:Two:Three\n",
+ "One/Two/Three/Four\n",
+ "OneXXXTwoXXXThreeXXXFourXXXFive\n",
+ "\n"
+ ]
+ }
+ ],
+ "source": [
+ "!./a.out"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "f839546e",
+ "metadata": {},
+ "source": [
+ "Final example from Modernes C++ in der Praxis. Here we implement a flexible filter to operate on a vector of random numbers."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 1,
+ "id": "dcaec377",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Overwriting filter.cpp\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%file filter.cpp\n",
+ "\n",
+ "#include <algorithm>\n",
+ "#include <functional>\n",
+ "#include <iostream>\n",
+ "#include <random>\n",
+ "\n",
+ "// This is going to be our filter\n",
+ "std::function< bool(int) > inRange( int low, int up ) {\n",
+ " return [low,up] ( int d ) { return d >= low && d <= up; };\n",
+ "}\n",
+ "\n",
+ "\n",
+ "int main() {\n",
+ "\n",
+ " std::cout << std::boolalpha << std::endl;\n",
+ "\n",
+ " std::cout << \"4 inRange(5,10): \" << inRange(5,10)(4) << std::endl;\n",
+ "\n",
+ " auto filt = inRange(5,10);\n",
+ "\n",
+ " std::cout << \"5 inRange(5,10): \" << filt(5) << std::endl;\n",
+ "\n",
+ " std::cout << std::endl;\n",
+ "\n",
+ " const int NUM = 60;\n",
+ "\n",
+ " std::random_device seed;\n",
+ " std::mt19937 engine( seed() ); // \n",
+ "\n",
+ " // distribution\n",
+ " std::uniform_int_distribution< int > six(1,6);\n",
+ "\n",
+ " std::vector< int > dice;\n",
+ " for( int i = 1; i <= NUM; ++i ) dice.push_back( six( engine ) );\n",
+ "\n",
+ " std::cout << std::count_if( dice.begin(), dice.end(), inRange(6,6) )\n",
+ " << \" of \" << NUM << \" inRange(6,6)\" << std::endl;\n",
+ "\n",
+ " std::cout << std::count_if( dice.begin(), dice.end(), inRange(4,6) )\n",
+ " << \" of \" << NUM << \" inRange(4,6)\" << std::endl;\n",
+ "\n",
+ " // remove all elements for 1 to 4\n",
+ " dice.erase( std::remove_if( dice.begin(), dice.end(), inRange(1,4) ),\n",
+ " dice.end() );\n",
+ "\n",
+ " // remove_if returns a past-the-end iterator for the new end of the range.\n",
+ "\n",
+ " std::cout << \"All numbers 5 and 6: \";\n",
+ " for( auto v: dice ) std::cout << v << \" \";\n",
+ "\n",
+ " std::cout << \"\\n\\n\";\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 2,
+ "id": "b45b0e4f",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "!g++ -Wall -Wextra filter.cpp"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 3,
+ "id": "f85b1bc8",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "\n",
+ "4 inRange(5,10): false\n",
+ "5 inRange(5,10): true\n",
+ "\n",
+ "13 of 60 inRange(6,6)\n",
+ "38 of 60 inRange(4,6)\n",
+ "All numbers 5 and 6: 5 5 6 6 5 6 6 5 5 6 6 5 6 6 5 5 5 6 5 6 6 6 6 5 5 \n",
+ "\n"
+ ]
+ }
+ ],
+ "source": [
+ "!./a.out"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "0ac98da0",
+ "metadata": {},
+ "source": [
+ "***\n",
+ "We had seen that we can store a lambda expression as an object (of some implicit type). However, we can also convert it into a\n",
+ "- function pointer\n",
+ "- std::function"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 2,
+ "id": "cf33e115",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ "Writing convert.cpp\n"
+ ]
+ }
+ ],
+ "source": [
+ "%%file convert.cpp\n",
+ "\n",
+ "#include <iostream>\n",
+ "#include <functional>\n",
+ "\n",
+ "int main() {\n",
+ "\n",
+ " using fType = int (*) (int);\n",
+ "\n",
+ " using sType = std::function< int(int) >;\n",
+ "\n",
+ " auto impl = [] (int a) { return a+1; };\n",
+ "\n",
+ " std::cout << \" 1 + 1 = \" << impl( 1 ) << std::endl;\n",
+ "\n",
+ " fType expl1 = impl;\n",
+ " std::cout << \" 2 + 1 = \" << expl1( 2 ) << std::endl;\n",
+ "\n",
+ " sType expl2 = impl;\n",
+ " std::cout << \" 3 + 1 = \" << expl2( 3 ) << std::endl;\n",
+ "\n",
+ " std::cout << \"Type of impl is function pointer? \" \n",
+ " << std::boolalpha\n",
+ " << std::is_same< fType, decltype( impl ) >::value\n",
+ " << std::endl;\n",
+ "\n",
+ " std::cout << \"Type of expl1 is function pointer? \" \n",
+ " << std::is_same< fType, decltype( expl1 ) >::value\n",
+ " << std::endl;\n",
+ "\n",
+ " std::cout << \"Type of expl2 is instance of std::function? \" \n",
+ " << std::is_same< sType, decltype( expl2 ) >::value\n",
+ " << std::endl;\n",
+ "}"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 3,
+ "id": "3fbad575",
+ "metadata": {},
+ "outputs": [],
+ "source": [
+ "!g++ convert.cpp"
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 4,
+ "id": "0584e221",
+ "metadata": {},
+ "outputs": [
+ {
+ "name": "stdout",
+ "output_type": "stream",
+ "text": [
+ " 1 + 1 = 2\n",
+ " 2 + 1 = 3\n",
+ " 3 + 1 = 4\n",
+ "Type of impl is function pointer? false\n",
+ "Type of expl1 is function pointer? true\n",
+ "Type of expl2 is instance of std::function? true\n"
+ ]
+ }
+ ],
+ "source": [
+ "!./a.out"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "b4fc6130",
+ "metadata": {},
+ "source": [
+ "A closer look at line 12 shows that in the lambda expression, we did not explicitely specify a **return type**. In this case it will automatically be determined from the `return` statement.\n",
+ "\n",
+ "When we want or need to explicitely specify the return type, then this must be done via the *trailing-return-type* syntax."
+ ]
+ },
+ {
+ "cell_type": "code",
+ "execution_count": 7,
+ "id": "ae57e2ba",
+ "metadata": {},
+ "outputs": [
+ {
+ "data": {
+ "text/plain": [
+ "10.000000"
+ ]
+ },
+ "execution_count": 7,
+ "metadata": {},
+ "output_type": "execute_result"
+ }
+ ],
+ "source": [
+ "auto lFunc = [] (double a) -> double { return 2.0*a; };\n",
+ "lFunc( 5.0 )"
+ ]
+ },
+ {
+ "cell_type": "markdown",
+ "id": "e04b7032",
+ "metadata": {},
+ "source": [
+ "***\n",
+ "In HyTeG we often make use of lambdas. Especially for initialising functions and setting boundary conditions. Like in this snippet: \n",
+ "```c++\n",
+ "real_t k = 2.0;\n",
+ "real_t m = 5.0;\n",
+ "\n",
+ "std::function< real_t(const Point3D&) > solFunc =\n",
+ " [k, m](const Point3D& x) { sin( k * pi * x[0] ) * sin( m * pi * x[1]; };\n",
+ "\n",
+ "P1Function< real_t > uExact( \"u_analytic\", storage, maxLevel, maxLevel );\n",
+ "uExact.interpolate( solFunc, maxLevel );\n",
+ "```"
]
},
{
"cell_type": "code",
"execution_count": null,
- "id": "8729d298",
+ "id": "328f2a8e",
"metadata": {},
"outputs": [],
"source": []
--
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