recursively print character using while and if

Question

I'm beginner and try to understand how recursion works. what is the different of using while and if int the code? why if i use "if" it produce correct result? but "while" doesn't? while(*k) ,when it reach "\0" then it print only once then it goes back to while loop again
thank you so much for the help!!

 #include <stdio.h>
 #include <stdlib.h>
 void back(char*k){
     if(*k) // works
     //while(*k) ->not work??
         back(++k);
     printf("%c",*k);
 }

 int main()
 {
     char k[]="hellomynameis";
     back(k);
     printf("Hello world!\n");
     return 0;
 }

is there possible way to do same thing in c++ but using iterator?

#include <iostream>
#include <string>

using namespace std;

void backk(string a){
    string::iterator itr;
    for (itr=a.begin();itr!=a.end();itr++){
        if (*itr)
            backk(++itr);
        cout<<a; --> is this possible  ?

        //simple and fast solution 
        for (itr=a.end();itr!=a.begin();itr--){
            if(*itr)
                cout<<*itr;
        }
    }
}

int main()
{
    string a("hello my name is");
    backk(a);
    cout << "Hello world!" << endl;
    return 0;
}

How about `for (itr=a.rbegin();itr!=a.rend();++itr) std::cout << *itr;`? — JVApen, Jan 05 '19 at 08:17
Use recursion **instead** of using a loop. In simple cases like this recursion is just a different kind of loop and the if statement controls whether the looping continues or not. Using recursion and a while loop makes no sense (in a simple example like this). — john, Jan 05 '19 at 08:17
Read [How to debug small programs](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/). Be aware of the [call stack](https://en.wikipedia.org/wiki/Call_stack). [Use the `gdb` debugger](https://sourceware.org/gdb/current/onlinedocs/gdb/) to run your program step by step, so you'll understand what is going on. — Basile Starynkevitch, Jan 05 '19 at 08:17
@BasileStarynkevitch thankyou! im using codeblock but is gdb debugger for linux? — fiksx, Jan 05 '19 at 11:16
I don't understand your question. First, I recommend using Linux to learn C programming, since Linux is very student- and developer- friendly (e.g. it has [valgrind](http://valgrind.org/) that is very handy). Then, I guess that [MinGW](http://www.mingw.org) will provide you a port of GCC and of `gdb` to Windows. However, I never used Windows in my life. At last [Code::Blocks](http://www.codeblocks.org/) is *not* a compiler (just an IDE) — Basile Starynkevitch, Jan 05 '19 at 11:19
@BasileStarynkevitch thankyou so much but what is the advantages of using linux over windows? if i use linux can i understand how recursion, call stack and also inspect variables or debug program thoroughly? — fiksx, Jan 05 '19 at 14:59
I am biased. I never used Windows. But, AFAIK, Linux is mostly made of free software (whose source code you can study, and that is an advantage) and Linux has [valgrind](http://valgrind.org/), and its `gdb` works well. See also [this](https://stackoverflow.com/a/54041928/841108) answer. However, recursion is a difficult concept. Read also [SICP](https://mitpress.mit.edu/sicp/) — Basile Starynkevitch, Jan 05 '19 at 16:38

score 1 · Accepted Answer · answered Jan 05 '19 at 09:01

1

About your first question on why while doesn't work, lets see how code runs in this case
Starting from a simple example lets say our char[] is only GH\0 for simplicity, Well, its a little bit hard to explain but lets try
at first back is called and k points to G, as it is not \0, back is called again with k being ++ed so it is pointing to 'H' and again it is not \0 so back is called with another ++k in which this time is \0, so while is not executed and printf does its job, now the execution return to second back call, where it was called with H input, but as we ++ed it, we moved it on the stack and now it is pointing to \0 again, so second printf runs, now only first back call is remained, guess where k is pointing to? right, it is pointing to H(dont forget ++), so again while is executed, but the input is ++k which in turn is \0, so the new called back immediately returns with a printf, again we are in first back call but this time is pointing to \0 so another printf and we are done,
to understand the sequence better i suggest run this code

#include <stdio.h>
#include <stdlib.h>
void back(char*k)
{
    printf("%c",'O');

    while(*k) //->not work??
    {
        back(++k);
    }

    printf("%c",'f');
}

int main()
{
    char k[]="he";
    back(k);
    return 0;
}

the expected out put is OOOffOff

answered Jan 05 '19 at 09:01

hessam hedieh

780
5
18

thankyou so much, really help me to understand recursive. can i ask one more thing, after reached "\0" and executed printf and back to first back call where k pointing to h, is it still inside the while loop and do the call "back(++k)" again? so "while" will not become false until *k is "\0" ? it will keep call "back (++k)"? the different from "if" is if will control recursion until *k false so stop calling back(++k) and execute next line after each recursion call but "while " will keep call "back(++k)" until it reach "\0" ? – fiksx Jan 05 '19 at 11:09
@Vixf, Welcome, yes, thats it, `if` just checks if in the current call another call should be made or not, but `while` forces the string to go again and again in the function until null is reached – hessam hedieh Jan 05 '19 at 11:15
thankyou! may i know what program do you use to trace or debug simple program like this? – fiksx Jan 05 '19 at 11:31
@Vixf, I mainly develop C framework in embedded environments, and each platform has its own tool, but for example you provided I just used an [online C compiler](https://www.tutorialspoint.com/compile_c_online.php) , no debug tool, I just tried to understand how code flows by reading code and simplifying it, then tested it with `printf` outputs to make sure I was right, in this example IMHO, this was enough, but in complex problems, a good debug platform is essential, as I said I have no idea what to use for PC apps, (if u r curious, I am using Xillinx SDK as my mainly EDK for embedded :D) – hessam hedieh Jan 05 '19 at 11:39

recursively print character using while and if

1 Answers1