Help Tracing through code

Hey guys, I'm currently in a Summer class dealing with C Programming and I'd like some help tracing through this code so I understand how it works exactly. Any help would be greatly appreciated!

Code:

#include <stdio.h>
void flipper(int,char *);
int main(void){

char path[5] ;
path[4]=0;

flipper(0,path);

system("pause");
return 0;
}

void flipper(int level, char* path){
*(path+level)='A'; /* coin flip side A */
if(level==3) /* flipped coin 4 report results */
printf("%s\n",path);
else
flipper(level+1,path); /* flip next coin */
*(path+level)='B'; /* coin flip side 'B' */
if(level==3) /* flipped four coins if you want to change the number of coin tosses shange the 3 to something else both here and above*/
printf("%s\n",path); /* report the path */
else
flipper(level+1,path); /* not done flip next coin */

return;
}

This is supposed to give the possible permutations for flipping a coin. I'm a bit confused on a few things done here.

Like... what exactly the char path[5]; is, path[4] i believe would be to say there are 4 flips. Then I'm also confused about the * (path+level) part and the if statements. Thanks again!

Step 1... indent the code properly. You'll be surprised how much easier it is to read and follow...

Step 2... BE the computer... just do what each line tells you to do, keeping track of the values of the variables as you go.

My apologies on the indention, I think the biggest issue I'm having right now is just being unsure of what char path[5] ; is doing. I just looked back through and I understand what's going on with the level variable.

Originally Posted by NICKD54

My apologies on the indention, I think the biggest issue I'm having right now is just being unsure of what char path[5] ; is doing. I just looked back through and I understand what's going on with the level variable.

That's an array declaration.... 5 characters stored in the variable named path.

To understand that code you need to have an understanding of the following

1. Recursive functions
2. Pointer
3. Pointer Arithmetic
4. Arrays

Do you know what they are? Do you have understanding of any of them? If not, it probably too early to start looking into this. I would strongly recommend getting your foundation right.

ssharish

I do understand these things for the most part ssharish, it's been a couple semesters since my last C class so I'm still getting back into the hang of syntax and everything. I believe I was mixing up the array declaration and the next line.

Why would that array be declared to 5? And then the next line, path[4] = 0; sets the fifth spot in that array equal to 0 correct?

Code:

path[4] = 0

This code could have been

Code:

path[4] = '\0';

This is to specify the NULL char to terminate the string.
And that should answer your question.

ssharish

Arrays in C start at an index of 0 and go through SIZE-1. Thus:

Code:

type array[ SIZE ];

Provides elements:

Code:

array[ 0 ]
...
array[ SIZE-1 ]

You cannot access array[ SIZE ] because that is out of bounds.


Quzah.

Originally Posted by ssharish2005

Code:

path[4] = 0

This code could have been

Code:

path[4] = '\0';

This is to specify the NULL char to terminate the string. Clearly there is no necessity to declare and array of 5 char's. 4 char array length would have done the job, with no issues clearly. It’s just poor design.

And that should answer your question.

ssharish

If a four-character array had been declared, how would you be able to access path[4]?

Agreed I totally misunderstood that :-/

ssharish

I tried playing with the char path[5] ; and I get the same output as long as the number is greater than or equal to the number in the line below it ( path[4]=0; )

So now tracing through this recursion, I see how level is incremented from 0 to 3 and I believe the *(path+level) = 'A'; assigns A to the spot in the array which would initially be 0+0, then 0+1, until level = 3, then it prints the string. Then path[3] becomes B and it prints again, but then we have the return statement and I'm not sure what happens after that.

Please let me know if I sound like I'm on the right track and what the next step here is.

*(path+level) is the same as path[ level ]


Quzah.

thank you for pointing that out to me quzah, I figured it meant something like that at least

Could anyone help me with understanding what happens after the first two calls? I'm a little lost there.

Proper indentation goes a long way.

Code:

void flipper(int level, char* path){
    *(path+level)='A'; /* coin flip side A */
    if(level==3) /* flipped coin 4 report results */
        printf("%s\n",path);
    else
        flipper(level+1,path); /* flip next coin */
    *(path+level)='B'; /* coin flip side 'B' */
    if(level==3) /* flipped four coins if you want to change the number of coin tosses shange the 3 to something else both here and above*/
        printf("%s\n",path); /* report the path */
    else
        flipper(level+1,path); /* not done flip next coin */

    return;
}

Write it out on paper:

Code:

f(0)
    here + 0 = A
    f(1)
        here + 1 = A
            f(2)
                here + 2 = A
                f( 3 )
                    here + 3 = A
                    print string
                    here + 3 = B
                    print string
                 < return
                here + 2 = B
                f( 3 )
                    here + 3 = A
                    print string
                    here + 3 = B
                    print string
                here + 2 = B
                < return

I will leave the rest as an exercise to the reader.


Quzah.