Trying to reverse a C++ array and at the same time

问题:

I'm trying to reverse an array and simultaneously store it in a new array with for loops. I have achieved it somewhat manually but not with loops. The first code below uses loops, the latter manually or literally.

char wrd[26],rev[26];

int y,i,s;

s=strlen(wrd)-1;

for(y=0;y<s;y++) for(i=s;i>=0;i--)
{
    rev[y]=wrd[i];
}

Below is the same done manually; it works, but you get random chars used to fill the rest of the array:

{
    char drw[26], wrd2[26];
    int s,i=0;

    s=strlen(wrd2)-1;

    drw[0]=wrd2[s];
    drw[1]=wrd2[s-1];
    drw[2]=wrd2[s-2];
    drw[3]=wrd2[s-3];
    drw[4]=wrd2[s-4];
    drw[5]=wrd2[s-5];
    drw[6]=wrd2[s-6];
    drw[7]=wrd2[s-7];
    drw[8]=wrd2[s-8];
    drw[9]=wrd2[s-9];
    drw[10]=wrd2[s-10];
    drw[11]=wrd2[s-11];
    drw[12]=wrd2[s-12];
    drw[13]=wrd2[s-13];
    drw[14]=wrd2[s-14];
    drw[15]=wrd2[s-15];
    drw[16]=wrd2[s-16];
    drw[17]=wrd2[s-17];
    drw[18]=wrd2[s-18];
    drw[19]=wrd2[s-19];
    drw[20]=wrd2[s-20];
    drw[21]=wrd2[s-21];
    drw[22]=wrd2[s-22];
    drw[23]=wrd2[s-23];
    drw[24]=wrd2[s-24];
    drw[25]=wrd2[s-25];

    cout << drw;
}

回答1:

You have two nested loops, and they are not doing what you think they are doing. You only need one:

int y, s;

s = strlen(wrd) - 1;

for (y = 0; y<=s; y++)
{
    rev[y] = wrd[s - y];
}

and don't forget to terminate:

rev[s + 1] = 0;

回答2:

Since you're using C++, take advantage of the libraries it offers. Depending on whether or not you want memory-sequential access to the elements (if you're going to read it out as a string, you probably do) you could use std::vector with reverse or alternatively a list/forward_list if contiguous access isn't an issue. std::list has the benefit of directly offering a reverse method.

#include <vector>
#include <algorithm>

using namespace std;

//int main() {
    vector<char> chars;
    const char *mystr = "abcdef";
    chars.reserve(6);
    chars.assign(mystr, mystr+6); //assuming we don't want the null terminator.
    reverse(chars.begin(), chars.end());
    cout << string(&chars.front(), chars.size()) << endl;
//}

The reserve is included to maximise the performance of the copy operation.

Additionally, if mystr was declared as a char array (or at least, pointed to writable memory) you could simply do reverse(mystr, mystr+6) too.