Posted by Cartesian : One example telling you why python is more readable than C++¶
By Rymoon, 2022/03/21 15:18
Wrote in Jupyter Notebook with xues-cling, C++17.
The Cartesian example is from C++17STL Cookbook.
Here is a very good example to show why Python scripts are more readable than C++ ones. Ths example is beyond the simple judgements like “runtime type”, but in aspect of that which language could express complex intentions more directly.
In [1]:
! gcc --version
gcc (Ubuntu 9.4.0-1ubuntu1~20.04) 9.4.0 Copyright (C) 2019 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
First , I give out the algorithm in a Python script:
# cartesian.py
def call_cart(f,x,*rest):
for r in rest:
if x<r:
f(x,r)
def cartesian (*xs):
def _inner(f):
for x in xs:
call_cart(f,x,*xs)
return _inner
print_cart = cartesian(1,2,3)
# Print catesian to console
# Require the calling when imported
# To simulate the compiling time of C++
print_cart(print)
Next, a C++ version implemented by C++17 STL that prints cartesian pairs in compiling time.
With lambdas instead of templates, we could have things like auto f(auto x, auto ... x).
Variables and functions decorated by constexpr are ran in compiling time.
In [2]:
#include <iostream>
void print(int x, int y){
std::cout << "(" <<x <<","<<y<<")\n";
}
In [3]:
{
constexpr auto call_cart (
[=](auto f, auto x, auto ...rest) constexpr {
(void) std::initializer_list<int>{
(((x<rest)
? (void)f(x,rest)
: (void) 0)
,0)...
};
});
constexpr auto cartesian ([=](auto ... xs) constexpr {
return [=](auto f)constexpr{
(void)std::initializer_list<int>{
((void)call_cart(f,xs,xs...),0)...
};
};
});
constexpr auto print_cart = (cartesian(1,2,3));
print_cart(print);
}
(1,2) (1,3) (2,3)
The following piece won’t work, with strange errors. So, use lambdas.
In [ ]:
#include <iostream>
template<typename F, typename X, typename... Rs>
auto call_cart(F f,X x, Rs ...rest){
std::initializer_list<int>{
((x<rest)?(void)f(x,rest):(void)0,0)...
};
}
/*
auto call_cart (
[=](auto f, auto x, auto ...rest) constexpr {
(void) std::initializer_list<int>{
(((x<rest)
? (void)f(x,rest)
: (void) 0)
,0)...
};
});
*/
template<typename... Xs>
auto cartesian(Xs... xs){
return [=](auto f){
std::initializer_list<int>{
((void)call_cart(f,xs,xs...),0)...
};
};
}
auto print_cart = (cartesian(1,2,3));
print_cart(print);
input_line_11:26:21: note: in instantiation of function template specialization 'cartesian<int, int, int>' requested here auto print_cart = (cartesian(1,2,3)); ^ input_line_11:27:1: error: unknown type name 'print_cart' print_cart(print); ^
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