Why doesn't my code reverse string in c++?

Question

As a novice in c++, I try to use it could reverse a string with this

class Solution {
public:
    void reverses(string s,int begin,int end){
        char c;
        for(int i=begin,j=end-1;i<j;i++,j--){  
            c=s[i];  
            std::cout<<s[i]<<std::endl;
            s[i]=s[j];  
            s[j]=c;  
            std::cout<<s[j]<<std::endl;
        }  
    }
    string reverseLeftWords(string s, int n) {
        reverses(s,0,n);
        return s;
    }
};

But it give me the original string. But when i use it in *char `

void Reverse(char *s,int n){  
    for(int i=0,j=n-1;i<j;i++,j--){  
        char c=s[i];  
        s[i]=s[j];  
        s[j]=c;  
    }  
}  

int main()  
{  
    char s[]="hello";  
    Reverse(s,5);
    cout<<s<<endl;
    return 0;
}

it out put olleh, what's different between them?

Also consider std::swap. You should also be careful with const char* string literals. I would expect your code to crash on some systems. — Paul Floyd
– Paul Floyd, Commented Apr 25, 2020 at 10:21

nanofarad · Accepted Answer · 2020-04-23 15:02:59Z

2

In the second example, you use a pointer to a character, which means that the function is changing the original string data, in place, which works. Contrast this with the first sample, where you pass in a std::string by value (meaning that the function is working on a copy of that string), reverse it in-place, and then discard the result.

If you want to use an std::string you can either take it by reference or by pointer-to-object:

void reverses(string& s,int begin,int end){
        char c;
        for(int i=begin,j=end-1;i<j;i++,j--){  
            c=s[i];  
            std::cout<<s[i]<<std::endl;
            s[i]=s[j];  
            s[j]=c;  
            std::cout<<s[j]<<std::endl;
        }  
    }

or

void reverses(string* s,int begin,int end){
        char c;
        for(int i=begin,j=end-1;i<j;i++,j--){  
            c=(*s)[i];  
            std::cout<<(*s)[i]<<std::endl;
            (*s)[i]=(*s)[j];  
            (*s)[j]=c;  
            std::cout<<(*s)[j]<<std::endl;
        }  
    }

    string reverseLeftWords(string s, int n) {
        reverses(&s,0,n);
        return s;
    }

answered Apr 23, 2020 at 15:02

nanofarad

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1 Comment

luo Bei Over a year ago

Thanks for your detailed explanation and example! It help me so much!

Paulius Liekis · Accepted Answer · 2020-04-23 15:05:26Z

2

You need to pass your string a reference:

void reverses(string& s,int begin,int end){

Or return it as a result

string reverses(string s,int begin,int end){
   ...
   return s;
}

answered Apr 23, 2020 at 15:05

Paulius Liekis

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1 Comment

luo Bei Over a year ago

Thanks for you so much! Good to know why it could not work!

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