Reverse a string with pointers

Question

This is an amateur question. I searched for other posts about this topic, found lots of results, but am yet to understand the concepts behind the solution.

This is a practice problem in my C++ book. It is not assigned homework. [Instructions here][1] .

WHAT I WOULD LIKE TO DO:

string input;
getline(cin, input); //Get the user's input.

int front = 0;
int rear;

rear = input.size();

WHAT THE PROBLEM WANTS ME TO DO

string input;
getline(cin, input); //Get the user's input.

int* front = 0;
int* rear;

rear = input.size();

Error: a value of type "size_t" cannot be assigned to an entity of type int*

This makes sense to me, as you cannot assign an 'address' of an int to the value of an int. So my questions are:

• What is the correct way to go about this? Should I just forget about initializing front* or rear* to ints? Just avoid that all together? If so, what would be the syntax of that solution?
• Why would this problem want me to use pointers like this? It's clear this is a horrible usage of pointers. Without pointers I could complete this problem in like 30 seconds. It's just really frustrating.
• I don't really see an advantage to EVER using pointers aside from doing something like returning an array by using pointers.

Thanks guys. I know you like to help users that help themselves so I did some research about this first. I'm just really irritated with the concept of pointers right now vs. just using the actual variable itself.

Posts about this topic that I've previously read:

• [Example 1][2]
• [Example 2][3]

• [Example 3][4]

  [1]: https://i.stack.imgur.com/VcBjE.png "Instructions" [2]: How does reversing a string with pointers works "Post 1" [3]: Reverse string with pointers? "Post 2" [4]: Reverse char string with pointers "Post 3"

Answer 1

string.size() does not return a pointer - it returns size_t.

To revert a string try this instead:

string original = "someText";  // The original string
string reversed = original;    // This to make sure that the reversed string has same size as the original string

size_t x = original.size();    // Get the size of the original string

for (size_t i = 0; i < x; i++)        // Loop to copy from end of original to start of reversed
{
    reversed[i]=original[x-1-i];
}

If you really (for some strange reason) needs pointers try this:

string input;
getline(cin, input); //Get the user's input.

char* front = &input[0];
char* rear = &input[input.size()-1];

but I would not use pointers into a string. No need for it.

Answer 2

I guest you may not quite understand the problem here. This problem want you to COPY a C string then REVERSE it by pointer operation. There is no classes in standard C. So, the C string is quite different from string class in C++. It is actually an array of char-type elements ended with character '\0'.

After understand this, you may start to understand the problem here. If you want to copy a C string, you can not just use str_a = str_b. You need constructor here. However, in pure C style, you should REQUIRE memory space for the string at first (you can use malloc here), then copy each element. For example, you want to create a function to make a copy of input string,

#include <string.h>

char *strcopy(char* str_in) {
    int len = strlen(str_in);
    char *str_out = (char*)malloc(len+1);

    char *in  = str_in;
    char *out = str_out;
    while(*in != '\0') { *out++ = *in++; }

    return str_out;
}

As you see, we actually use char* not int* here to operate string element. You should distinguish the pointer (such as in) and the element pointed by the pointer (such as *in) at first.

I'll show you a solution in pure C style for your problem, I hope this would help you to understand it. (You should be able to compile it without modification)

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* strreverse(char* in){
    // length of input string
    int len = strlen(in);
    // allocate memory for string operation
    char *out = (char*)malloc(len+1);
    // initialize <front> and <end>
    char *front = out, *end = out + len - 1;

    char buffer;

    // copy input string
    for(int i = 0; i <= len; i++){ out[i] = in[i]; }

    // reverse string
    for(; front < end; front++, end--) {
        buffer = *front;
        *front = *end;
        *end = buffer;
    }

    return out;
}

int main() {
    printf("REVERSE >> %s\n", strreverse("Hello, World!"));
    return 0;   
}

This is not you would do by C++ in actual programming, however, I guess the problem here is trying to let you understand mechanism of pointers. In this aspect, original C style would help a lot.