How to reverse the order of words in a string through recursion

Question

I want this to reverse my string order by words. Like if a string is "Cat is running" then it should be "running is Cat". Here is the code:

#include<iostream>
#include<string>

using namespace std;
void reverseString(string str);
int length, lastLength;

int main() {

    string s;
    cout << "Enter a string to reverse its words: ";
    getline(cin, s);
    lastLength = s.length() - 1;
    length = lastLength;
    cout << "\nThe string in reverse order is ";
    cout << endl;
}

void reverseString(string str) {

    if (length < 0)
        return;
    else {
        if (str.at[length] == " " || length == 0)
        {
            if (length == 0)
                length = -1;
            for (int i = length + 1; i < lastLength; i++)
                cout << str.at[length];
            lastLength = length - 1;
        }
        length--;
        reverseString(str);
    }
}

It shows some error of pointer and array. I don't know how to resolve this. Any help will be truly appreciated! :)

Answer 1

You have two different errors. .at is a method so it should be called as .at() not .at[] . Second, you compare char to string (" "). So, you should replace " " with ' '.

#include<iostream>
#include<string>

using namespace std;
void reverseString(string str);
int length, lastLength;

int main() {

    string s;
    cout << "Enter a string to reverse its words: ";
    getline(cin, s);
    lastLength = s.length() - 1;
    length = lastLength;
    cout << "\nThe string in reverse order is ";
    cout << endl;
}

void reverseString(string str) {

    if (length < 0)
        return;
    else {
        if (str.at(length) == ' ' || length == 0) // <- note the changes here
        {
            if (length == 0)
                length = -1;
            for (int i = length + 1; i < lastLength; i++)
                cout << str.at(length); // <- note the changes here
            lastLength = length - 1;
        }
        length--;
        reverseString(str);
    }
}

I didn't check the logic. You may continue working on the logic :)

Answer 2

std::string has many helper functions, such as string::find , string::rfind , and std::substr which you can use to manipulate the string, instead of accessing the characters individually. For example:

void reverseString(std::string str, size_t end)
{
    size_t pos = str.rfind(' ', end);
    if (pos == std::string::npos)
    {
        cout << str.substr(0, end + 1) << endl;
    }
    else
    {
        cout << str.substr(pos + 1, end - pos) << endl;
        reverseString(str, pos - 1);
    }
}

int main()
{
    std::string s = "Enter a string to reverse its words";
    cout << s << endl;
    reverseString(s, s.length());
}

Answer 3

Here's a version that attempts to preserve the logic in your solution, with only a little C++ <string> convenience:

void output_reverse_string(string str, int last, int current) {
    /* Terminating condition: we've exhausted the input: */
    if (current == 0) {
        std::cout << str.substr(current, 1 + last - current);
        return;
    }
    /* Recurse until we've found a space: */
    if (str.at(current) != ' ') {
        output_reverse_string(str, last, current - 1);
        return;
    }
    /* Since we've found a space, output the following word: */
    std::cout << str.substr(current + 1, last - current);
    /* Just for readability, can be skipped: */
    std::cout << " ";

    /* Recurse on the *remaining* string contents: */
    output_reverse_string(str, current - 1, current - 1);
}