Why is my while loop not terminating when i < 0? - c++

Question

sorry for the noob question.
I have the following code for an assignment in my class (includes the assignment description):
Write a program which asks the user to enter a string and then passes it to a void recursive function which displays the string in reverse. The prototype of the function would be:
void reverse(string s, int i);
The function call would be:
reverse(strIn, strIn.length());
A typical run will be:
Enter a string:
Four score and seven years ago ..
.. oga sraey neves dna erocs ruoF
// Actual code
#include <iostream>
#include <string>
void reverse(std::string, int);
int main() {
std::string input;
std::cout << "Enter a string." << std::endl;
getline(std::cin, input);
int i = input.length() - 1;
reverse(input, i);
return 0;
}
void reverse(std::string input, int i) {
while (i >= 0) {
std::cout << input.at(i);
i--;
reverse(input, i);
}
}
I assumed that by subtracting 1 each time would display the last character in the string, and setting the loop condition to i >= 0 would make the loop terminate when there's no more characters left in the string. But I'm getting the following output:
sleepy#mooncell ~/CMPSC121/Activities $ ./a.out
Enter a string.
help
plehhehhlehhehh
I can't figure out what I'm doing wrong here. Help is much appreciated!

Answer 1

This is because you are trying to perform recursion and a while loop at the same time, just get rid of the while loop and it will work, the code should look something like this:
// Actual code
#include <iostream>
#include <string>
void reverse(std::string, int);
int main() {
std::string input;
std::cout << "Enter a string." << std::endl;
getline(std::cin, input);
int i = input.length() - 1;
reverse(input, i);
return 0;
}
void reverse(std::string input, int i) {
if(i <0)
return;
std::cout << input.at(i);
i--;
reverse(input, i);
}

Answer 2

This function
void reverse(std::string input, int i) {
while (i >= 0) {
std::cout << input.at(i);
i--;
reverse(input, i);
}
}
is not pure recursive because it uses a while loop.
Also it is a bad idea to pass the string by value. And the position in the string should have the type std::string::size_type.
The second parameter can have a default argument. in this case a function call will look much simpler
reverse( input );
Pay attention to that these statements can invoke undefined behavior when an empty string is passed to the function
int i = input.length() - 1;
reverse(input, i);
Because in this case the variable i will have maximum value. That is this declaration
int i = input.length() - 1;
in fact will be equivalent to
int i = std::string::size_type( -1 );
What you need is the following
void reverse( const std::string &s, std::string::size_type i = 0 )
{
if ( i < s.size() )
{
char c = s[i];
reverse( s, i + 1 );
std::cout << c;
}
}
Here is a demonstrative program.
#include <iostream>
#include <string>
void reverse( const std::string &s, std::string::size_type i = 0 )
{
if ( i < s.size() )
{
char c = s[i];
reverse( s, i + 1 );
std::cout << c;
}
}
int main()
{
const char *s = "Hello bbchan";
std::cout << s << '\n';
reverse( s );
std::cout << '\n';
return 0;
}
Its output is
Hello bbchan
nahcbb olleH
And even better to define the function the following way as it is shown in the demonstrative program below. In this case you can for example write a string in the reverse order in a file.
#include <iostream>
#include <string>
std::ostream & reverse( const std::string &s,
std::string::size_type i = 0,
std::ostream &os = std::cout )
{
if ( i < s.size() )
{
char c = s[i];
reverse( s, i + 1, os );
os << c;
}
return os;
}
int main()
{
const char *s = "Hello bbchan";
std::cout << s << '\n';
reverse( s ) << '\n';
return 0;
}