C++ 使用指针反转字符串
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reverse a string using pointers
提问by user2420395
I have to admit, i have no idea how to use pointers, but I tried non the less. the problem with my program is that it shows the string in reverse, except for what was the first letter being missing and the entire string is moved one space forward with the first element being blank. for example it show " olle" when typing "hello".
我不得不承认，我不知道如何使用指针，但我尝试过不少。我的程序的问题在于它反向显示字符串，除了缺少第一个字母并且整个字符串向前移动一个空格而第一个元素为空白。例如，它在键入“hello”时显示“olle”。
#include <iostream>
#include <string>
using namespace std;
string reverse(string word);
int main()
{
char Cstring[50];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(string word)
{
char *front;
char *rear;
for (int i=0;i< (word.length()/2);i++)
{
front[0]=word[i];
rear[0]=word[word.length()-i];
word[i]=*rear;
word[word.length()-i]=*front;
}
return word;
}
The new code works perfectly. changed the strings to cstrings. the question technicaly asked for cstrings but i find strings easier so i work with strings then make the necesary changes to make it c string. figured out ho to initialize the rear and front as well.
新代码完美运行。将字符串更改为 cstrings。技术上要求 cstrings 的问题，但我发现字符串更容易，所以我使用字符串，然后进行必要的更改以使其成为 c 字符串。想出 ho 来初始化后部和前部。
#include <iostream>
#include <cstring>
using namespace std;
string reverse(char word[20]);
int main()
{
char Cstring[20];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(char word[20])
{
char a='a';
char b='b';
char *front=&a;
char *rear=&b;
for (int i=0;i< (strlen(word)/2);i++)
{
front[0]=word[i];
rear[0]=word[strlen(word)-1-i];
word[i]=*rear;
word[strlen(word)-1-i]=*front;
}
return word;
}
回答by
char *front;
char *rear;
then later
然后后来
front[0]=word[i];
rear[0]=word[strlen(word)-1-i];
Not good. Dereferencing uninitialized pointers invokes undefined behavior.
不好。取消引用未初始化的指针会调用未定义的行为。
Apart from that, your code is overly complicated, it calls strlen()during each iteration (and even multiple times), which is superfluous, and the swap logic is also unnecessarily complex. Try using two pointers instead and your code will be much cleaner:
除此之外，您的代码过于复杂，strlen()每次迭代（甚至多次）都会调用，这是多余的，并且交换逻辑也不必要地复杂。尝试改用两个指针，你的代码会更简洁：
void rev_string(char *str)
{
char *p = str, *s = str + strlen(str) - 1;
while (p < s) {
char tmp = *p;
*p++ = *s;
*s-- = tmp;
}
}
The thing is, however, that in C++ there's rarely a good reason for using raw pointers. How about using std::reverse()instead?
然而，问题是在 C++ 中很少有使用原始指针的充分理由。如何使用std::reverse()呢？
string s = "foobar";
std::reverse(s.begin(), s.end());
回答by stewart99
inline void swap(char* a, char* b)
{
char tmp = *a;
*a = *b;
*b = tmp;
}
inline void reverse_string(char* pstart, char* pend)
{
while(pstart < pend)
{
swap(pstart++, pend--);
}
}
int main()
{
char pstring[] = "asfasd Lucy Beverman";
auto pstart = std::begin(pstring);
auto pend = std::end(pstring);
pend -= 2; // end points 1 past the null character, so have to go back 2
std::cout << pstring << std::endl;
reverse_string(pstart, pend);
std::cout << pstring << std::endl;
return 0;
}
回答by 0decimal0
you can also do it like this:
你也可以这样做：
#include <iostream>
#include <cstring>
using namespace std;
string reverse(char word[20]);
int main()
{
char Cstring[20];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(char word[20])
{
char a='a';
char b='b';
char *front=&a;
char *rear=&b;
for (int i=0;i< (strlen(word)/2);i++)
{
*front=word[i];
*rear=word[strlen(word)-1-i];
word[i]=*rear;
word[strlen(word)-1-i]=*front;
}
return word;
}
it successfully works on my system ,i.e. on emacs+gccon windows 7
它在我的系统上成功运行，即在Windows 7上的emacs+gcc
