CODEWITHSUNDEEP
c++string
darthsidious
darthsidious

Reputation: 3091

Reverse a string in C++ without using reverse

I wrote the following code to reverse the string without using <algorithm> reverse . I wrote:

#include <string.h>
#include <iostream>
using namespace std;


void reverse(char *str){
    int sizeStr=strlen(str);
    int firstChar,lastChar,temp;
    for(lastChar = (sizeStr - 1),firstChar = 0 ; lastChar>firstChar ; lastChar--, firstChar++){
        temp = str[firstChar];
        str[firstChar]=str[lastChar];
        str[lastChar] = temp;
    }
}
int main() {
    char *str;
    str = "myname";
    reverse(str);
    cout << str << endl; 
    return 0;
}

I am getting segfault at str[firstChar]=str[lastChar];. Please help me figure out my mistake.

NOTE: I declared char str[] = "myname" and worked perfectly. But as per your all explanations got to learn differences for *str and str[]. Thanks a lot. Goes a long way in helping a new programmer.

Upvotes: 1

Views: 3253

Answers (7)

sampathsris
sampathsris

Reputation: 22290

You cannot modify a constant string literal.

For C++

Use std::string (thanks @chris for this). Use std::swap for swapping the characters. (See C++ string swap character places)

For C

malloc a buffer (with the length of the string), and strcpy the string to it, and then do the reverse for the newly allocated buffer.

Upvotes: 5

denisvm
denisvm

Reputation: 740

#include <string.h>
#include <iostream>

void myreverse(std::string &s) {
    size_t begin = 0, end = s.length();

    while ((begin != end) && (begin != --end))
        std::swap(s[begin++], s[end]);
}

int main() {
    const char *str = "myname";
    std::cout << str << std::endl; 

    std::string newstr(str);
    myreverse(newstr);
    std::cout << newstr << std::endl; 
    return 0;
}

Upvotes: 0

wshcdr
wshcdr

Reputation: 967

#include <string.h>
#include <iostream>
using namespace std;


void reverse(char *str){
        int sizeStr=strlen(str);
        int firstChar,lastChar,temp;
        for(lastChar = (sizeStr - 1),firstChar = 0 ; lastChar>firstChar ; lastChar--, firstChar++){
        temp = str[firstChar];
        str[firstChar]=str[lastChar];
        str[lastChar] = temp;
   }
}

int _tmain(int argc, _TCHAR* argv[])
{
    char *str;
    str = "myname";
    char *str2 = new char [strlen(str)+1];
    strcpy(str2,str);
    reverse(str2);
    cout << str2 << endl; 
    delete [] str2;
    str2 = NULL;
    cout << str << endl; 
    return 0;
}

Upvotes: 0

Bob Ezuba
Bob Ezuba

Reputation: 510

using std::string, you can use the method below to reverse a string

string reverse_string(string arg){
    int length = arg.length();
    char res[length];
    for(int i = length - 1; i >= 0; i--){
        res[length - (i+1)] = arg[i];
    }
    string result(res);
    return result;
}

Let me know if you have any further questions.

Upvotes: 1

r-s
r-s

Reputation: 1045

I think you might be going about this problem the hard way. If you use a std::string the problem becomes much easier. Below is a more generic C++1y solution which will be able to reverse many different types of containers. The container must override the [ ] operator. 

#include <iostream>

template <typename T>
auto rev(T &&a){
  for (auto i = 0; i < a.size()/2; ++i){
    std::swap(a[i], a[a.size()-1-i]);
  }
  return a;
}

int main(){
  std::string x = "hello";
  std::cout<< rev(x);
}

Upvotes: 0

Santosh A
Santosh A

Reputation: 5361

The reason is you have not allocated the memory to the character pointer str.

#include <string.h>
#include <iostream>
using namespace std;


void reverse(char *str){
    int sizeStr=strlen(str);
    int firstChar,lastChar,temp;
    for(lastChar = (sizeStr - 1),firstChar = 0 ; lastChar>firstChar ; lastChar--, firstChar++){
        temp = str[firstChar];
        str[firstChar]=str[lastChar];
        str[lastChar] = temp;
    }
}
int main() {
    char *str;
    // Here use malloc to allocate memory to the character pointer str
    // Or instead of using a char pointer use a character array to initialize the string
    // char *str = malloc(strlen("myname") + 1)   or usr char str[10] = "myname"
    str = "myname";
    reverse(str);
    cout << str << endl; 
    return 0;
}

Upvotes: 0

wshcdr
wshcdr

Reputation: 967

The content of str is constant, you should use char pointer instead.

Upvotes: 0

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