Python和C++使用的一些比较

局部变量和全局变量

Python程序

def f():
    print(a)

if __name__ == "__main__":
    a = 1
    f()

和

def f():
    print(a)
a = 1

if __name__ == "__main__":
    f()

输出结果都是1。

C++程序

#include 

int a = 1;

void f()
{
	std::cout << a << std::endl;
}

int main()
{
	f();
	return 0;
}

输出结果1，而程序

#include 

void f()
{
	std::cout << a << std::endl;
}

int main()
{
	int a = 1;
	f();
	return 0;
}

报错：“a”: 未声明的标识符。

形参和实参

Python程序

def f(a, b):
    a[0] += 1
    b += 1
    print("f:", a, b)

if __name__ == "__main__":
    a = [1, 2]
    b = 1
    f(a, b)
    print("main:", a, b)

和

import numpy as np

def f(a, b):
    a[0] += 1
    b += 1
    print("f:", a, b)

if __name__ == "__main__":
    a = np.array([1, 2])
    b = 1
    f(a, b)
    print("main:", a, b)

的输出都是：

f: [2, 2] 2
main: [2, 2] 1

说明在函数中修改list或者array的值会影响函数外原list或者原array的值，而修改int的值不会影响函数外原int的值。若把 a = [1, 2]换成a = (1, 2)则报错：‘tuple’ object does not support item assignment；换成a = {1, 2}则报错：‘set’ object is not subscriptable。

C++程序

#include 

void f(int* a, int b)
{
	a[0] += 1;
	b += 1；
	std::cout << "f:{" << a[0] << ", " << a[1] << "} " << b << std::endl;
}

int main()
{
	int a[] = { 1, 2 };
	int b = 1;
	f(a, b);
	std::cout << "main:{" << a[0] << ", " << a[1] << "} " << b << std::endl;
	return 0;
}

的输出结果

f:{2, 2} 2
main:{2, 2} 1

和Python的类似。而C++程序

#include 
#include 

void f(std::vector<int> a, int b)
{
	a[0] += 1;
	b += 1;
	std::cout << "f:{" << a[0] << ", " << a[1] << "} " << b << std::endl;
}

int main()
{
	std::vector<int> a = { 1, 2 };
	int b = 1;
	f(a, b);
	std::cout << "main:{" << a[0] << ", " << a[1] << "} " << b << std::endl;
	return 0;
}

输出结果

f:{2, 2} 2
main:{1, 2} 1

说明在函数中修改vector的值不会影响函数外原vector的值。