Reversing characters from input - c++

Question

I am trying to reverse a char which has been provided in input from an user. I am having issues with the reverse function, particularly the loop. I can't get it to work- can I get advice?
#include <iostream>
using namespace std;
#include <cstring>
char* reverse(char* input) {
int len = strlen(input);
char temp[len];
for(int i=len; i>len; --i) {
temp[i]+=input[i];
}
return temp;
}
int main()
{
char input[100];
while(cin>>input) {
cout << reverse(input);
}
return 0;
}

Answer 1

Your Program has few issues
You're trying to return local variable address i.e. temp array address. The Function will return the address to main function. Since memory might get cleaned so it will print garbage value present at the address.
As Rohan Bari mentioned variable length array might cause undefined behavior. There for you can create a constant length array i.e.
char temp[100];
or you can dynamically allocate array on heap. Memory allocated on heap do not get cleared after termination of block but we have to manually delete it.
char* temp = new char[len];
As array start from 0 it goes till len-1 so loop condition should start from len-1 and has to go till 0 to reverse.
+ operator do not work's with array or char even if you are trying to add just char it preforms normal integer addition of their ASCII value.
Here is improved version of your code
#include<iostream>
using namespace std;
#include <cstring>
char* reverse(char* input) {
int len = strlen(input);
char* temp = new char [len]; // or you can use char temp[100];
int j = 0; //temp variable to enter values from 0th index if we use same as loop it just enter in the same order as original char array.
for(int i=len-1; i>=0; --i) {
temp[j++] = input[i];
}
temp[j] = '\0';
return temp;
}

Answer 2

You have got several errors in the program.
The variable-length arrays are used here:
char temp[len];
This should not be applied in C++ since this invokes undefined-behavior. Note that this is a valid statement in the C99 standard.
There is a better alternative to this. That is to take the std::string built-in type in use.
In the following line:
temp[i] += input[i];
You are not sequentially adding one character after another, but the values of them in a single integer. This could be not a problem if temp was of the type std::string.
The reverse function should look like this:
const char *reverse(char *input) {
int len = strlen(input);
std::string temp;
while (len--)
temp += input[len];
return temp.c_str();
}

Answer 3

len should actually be (len-1) and i should be >= 0 not len, so from (len-1) to 0 your loop should run.
for(int i = len-1; i >= 0; i--){}

Answer 4

You have to allocate the new array with the new keyword if you don't want to use a string. The following code does what you need:
char* reverse(char* input)
{
int len = strlen(input);
char* temp = new char[len + 1];
for (int i = len; i >= 0; --i)
{
temp[len-i-1] = input[i];
}
temp[len] = '\0';
return temp;
}

Answer 5

You could use a std::stack to reverse your input:
std::stack<char> s;
char c;
while (std::cin >> c)
{
s.push(c);
}
while (!s.empty())
{
std::cout << s.top();
s.pop();
}

Answer 6

It's 2021. Use the STL. If your instructor isn't aware of it or doesn't allow you to use it, your instructor is not keeping up-to-date and you should fire your instructor.
#include <algorithm>
#include <iostream>
#include <string>
int main() {
std::string input{};
while(std::getline(std::cin, input)) {
std::reverse(std::begin(input), std::end(input));
std::cout << input << '\n';
}
return 0;
}

Answer 7

There's quite many things wrong with the code as many people have already mentioned! Since you want to implement this without using STL it can be done this way,
#include <iostream>
using namespace std;
#include <cstring>
void reverse(char* input,int len) { //added len as argument
char temp[len];
for(int i=len-1; i>=0; --i) {
temp[len-i-1]=input[i];
cout<<temp[len-i-1]; //printing while reversing
}
cout<<endl;
}
int main()
{
char input[100];
int len=0;
//using do while since it has to run atleast once
do{
cin.getline(input,100);
len=strlen(input);
input[len]='\0';
if(len!=0)
reverse(input,len);
}while(len!=0) ;
return 0;
}