如何解决For 循环与列表推导式有何不同?
我确实尝试在网上理解和寻找答案,但我找不到关于 for 循环与列表理解有何不同的清晰和简单解释。这是一个我认为会返回相同内容的示例,但它没有。
给定一组数字,返回每个元素的倒数。正数变为负数,负数变为正数。
a_list = [1,2,-3,4,5]
for 循环
def invert(a_list):
for x in a_list:
return -x
invert(a_list)
-1
而在列表理解中...
def invert(a_list):
return [-x for x in a_list]
invert(a_list)
[-1,-2,3,-4,-5]
感谢您的任何解释,我仍然对何时使用其中之一感到有些困惑。 谢谢!
解决方法
你的两个函数不做同样的事情。在第一个示例中,您在遍历循环之前返回,因此只返回一个项目。第二个示例返回完整的新列表。
你需要做更多这样的事情:
a_list = [1,2,-3,4,5]
def invert_one(a_list):
invert_list = []
for x in a_list:
invert_list.append(-x)
return invert_list
def invert_two(a_list):
return [-x for x in a_list]
print(invert_one(a_list))
[-1,-2,3,-4,-5]
print(invert_two(a_list))
[-1,-5]
,
1/ for 循环是一种通用的循环方法,你可以在循环内做任何事情,它可以包含多行代码。 结石, 印刷, 调用其他函数...
2/ 列表理解目标是生成一个列表对象, 它通常是单行的,用来使书写和阅读变得美观而简短
3/ 在您的示例中,您并没有真正循环,因为您返回第一个迭代值:-x 你不允许第二次迭代追加
如果你想要完全相同的行为:
#ifndef CRYPTODATA
#define CRYPTODATA
typedef unsigned char byte;
#include <iostream>
#include "stdafx.h"
#include "assert.h"
using std::cerr;
using std::cout;
#include "cryptopp/hex.h"
using CryptoPP::HexDecoder;
using CryptoPP::HexEncoder;
#include "cryptopp/cryptlib.h"
using CryptoPP::AuthenticatedSymmetricCipher;
using CryptoPP::BufferedTransformation;
#include <string>
using std::string;
#include "cryptopp/filters.h"
using CryptoPP::AuthenticatedDecryptionFilter;
using CryptoPP::AuthenticatedEncryptionFilter;
using CryptoPP::StringSink;
using CryptoPP::StringSource;
#include "cryptopp/aes.h"
using CryptoPP::AES;
#include "cryptopp/gcm.h"
using CryptoPP::GCM;
using CryptoPP::GCM_TablesOption;
#include <string>
namespace CryptoData{
class CryptoData{
public:
CryptoData(){
std::string _key = "mohsensalamkojayikhoobikhosshhaa";
std::string _iv = "mohsensalamk";
std::copy(_key.begin(),_key.end(),this->key);
std::copy(_iv.begin(),_iv.end(),this->iv);
string pad(16,(char)0x00);
authCode = pad;
printf("default constractor call \n");
}
CryptoData(std::string _key,std::string _iv,std::string _pad):authCode(_pad){
std::copy(_key.begin(),this->iv);
printf("constractor call with args \n");
}
~CryptoData() {
printf("default destractur call \n");
}
std::string _encrypting(std::string _data){
return this->encryptData(_data);
}
std::string _decrypting(std::string _data) {
return this->decryptData(_data);
}
protected:
std::string decryptData(std::string _data) {
std::string radata,rpdata;
try
{
GCM<AES>::Decryption d;
d.SetKeyWithIV(key,sizeof(key),iv,sizeof(iv));
string enc = cipher.substr(0,cipher.length() - TAG_SIZE);
string mac = cipher.substr(cipher.length() - TAG_SIZE);
assert(cipher.size() == enc.size() + mac.size());
// assert(enc.size() == _data.size());
assert(TAG_SIZE == mac.size());
radata = authCode;
AuthenticatedDecryptionFilter df(d,NULL,AuthenticatedDecryptionFilter::MAC_AT_BEGIN |
AuthenticatedDecryptionFilter::THROW_EXCEPTION,TAG_SIZE);
df.ChannelPut("",(const byte*)mac.data(),mac.size());
df.ChannelPut("AAD",(const byte*)authCode.data(),authCode.size());
df.ChannelPut("",(const byte*)enc.data(),enc.size());
df.ChannelMessageEnd("AAD");
df.ChannelMessageEnd("");
bool b = false;
b = df.GetLastResult();
assert(true == b);
string retrieved;
size_t n = (size_t)-1;
df.SetRetrievalChannel("");
n = (size_t)df.MaxRetrievable();
retrieved.resize(n);
if (n > 0)
{
df.Get((byte*)retrieved.data(),n);
}
rpdata = retrieved;
printf("done6 \n");
//assert(rpdata == pad);
cout << "its a decrypted data " << radata << " \n";
}
catch (CryptoPP::InvalidArgument& e)
{
cerr << "Caught InvalidArgument..." << endl;
}
catch (CryptoPP::AuthenticatedSymmetricCipher::BadState& e)
{
cerr << "Caught BadState..." << endl;
}
catch (CryptoPP::HashVerificationFilter::HashVerificationFailed& e)
{
cerr << "Caught HashVerificationFailed..." << endl;
cerr << e.what() << endl;
}
return rpdata;
}
std::string encryptData(std::string _data){
std::string encoded;
try
{
GCM<AES>::Encryption e;
e.SetKeyWithIV(key,sizeof(iv));
AuthenticatedEncryptionFilter ef(e,new StringSink(cipher),false,TAG_SIZE); // AuthenticatedEncryptionFilter
ef.ChannelPut("AAD",authCode.size());
ef.ChannelMessageEnd("AAD");
ef.ChannelPut("",(const byte*)_data.data(),_data.size());
ef.ChannelMessageEnd("");
// Pretty print
StringSource(cipher,true,new HexEncoder(new StringSink(encoded),16," "));
}
catch (CryptoPP::BufferedTransformation::NoChannelSupport& e)
{
cerr << "Caught NoChannelSupport..." << endl;
}
catch (CryptoPP::AuthenticatedSymmetricCipher::BadState& e)
{
cerr << "Caught BadState..." << endl;
}
catch (CryptoPP::InvalidArgument& e)
{
cerr << "Caught InvalidArgument..." << endl;
}
return encoded;
}
private:
byte key[32];
byte iv[12];
const int TAG_SIZE = 16;
std::string cipher;
std::string authCode;
};
}
#endif
与以下结果相同且相同:
a_list = [1,5]
inverted = [-x for x in a_list]
print(inverted)
好处:这里的列表推导式写起来更短,因为这个列表写起来很容易/短。所以这两种方法在这里都有效,但是当你可以做到的时候,把它写成一个列表推导式会更好。
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