for, dowhile, and while problem.

[cocopuff]

so essentially i made a program that allows that use to convert any number from base n to base 10. im soley using base 5 in this instance. the issue is, is that im trying to rewrite my function, in two ways. one way, is that it replaces the for loop with a while loop, and the for loop with a dowhile loop. i did it, but i dont receive the same results. i've been gnawing my fingers for the past 3 hours trying to figure this out but no dice. here's the code:

#include "stdafx.h"

#include <conio.h>


#include <stdlib.h>




int convert(int a,int base);


int main()
{
 int a,base,ans;
 
 printf("\nEnter your preferred number: ");
 scanf("%d", &a);

 printf("\nEnter the base of that number: ");
 scanf("%d", &base);

 ans=convert(a,base);
     
 printf("The converted base 10 is: %d", ans);
 getch();
 return 0;
}   


int convert(int a,int base)
{
int sum=a%10;
for(int i=base;(a/=10)!=0;i*=base)
sum+=a*i;
return sum;
}

/// Input 12, 5, Output, 7. Correct.

^^everything in that fragment of code functions correctly and should remain unmodified, but the dilemma is trying to switch the for cycle with a dowhile and a while. here's what i tried:

int convert(int a,int base)
{
int sum=a%10;
int i = base;
while((a/=10)!=0){
i*=base;          //added that outside of this while method, but nothing new.
sum+=a*i;         //with or without the brackets, different wrong results.
}
return sum;
}

///////////////////////////

int convert(int a,int base)
{
int sum=a%10;
int i = base;
do{
i*=base;
sum+=a*i;	
}while((a/=10)!=0);

return sum;
}

//////////////////////Input 12,5 Output, 427. Incorrect. 

any help would be appreciated. :)

[ResidentBiscuit]

Well the only difference between between a while and a do while is that that the former pretest and the latter is a post test. You have to account for this when programming. Now, have you tried moving the i*=base to after the sum+=a*i?

[ReedTompkins]

Consider how a for loop is formatted.
for( initialization ; condition ; variable update)

Here is your problem. In a for loop, the variable update is performed AFTER the statements in the for loop.

Therefore, when you run your for loop, you first run the line
sum+=a*i;
and THEN you run the variable update:
i*=base;

But in your while and do-while loops, you have things backwards! You are updating i before you are updating sum, which is not what you want.

edit: oops, ResidentBiscuit posted first. Sorry, didn't see it.

[cocopuff]

haha, no i didnt! but now it works for the while loop! :). thanks!

[cocopuff]

and thank you reed for the elaboration of the problem. but what about the dowhile cycle, is there a similar complication with that cycle, or is it more intrinsic?

[cocopuff]

any help? trying everything i can. should i alter the condition?

[ResidentBiscuit]

Just think about it a bit, a do while is a POST TEST, meaning it checks the condition AFTER it runs through it's loop. The only real difference is that a do while will always run through at least once. This is handy in some situations, sometimes it's irrelevant, and sometimes it causes problems.

[clanmjc]

In every control statement except the do while you are performing (a/=10) first. As you now have noticed the do while executes this instruction After all other computations. you will need to perform (a/=10) before entering the do while and also check that it is not zero... to keep your same logic.

[cocopuff]

but the only way i can do that is through another while statement. or if statement. im trying but according to the instructions of the assignment, i cant use anything besides the do while cycle.

[cocopuff]

ok im stuck. can anyone help me? nevermind, ill just go with the if statement. if my teacher screams at me, it wont matter...