Yet more C problems

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#1

Hi again, I’m writing a program that will given that you have $10,000 in the bank with 4% interest and you take out $1000 every month, how many months will you be able to do that?
my program:
#include <stdio.h>
unsigned long int total=10000;
unsigned long int total_two;
unsigned long int total_three;
//unsigned int total_fn(void);
int main()
{
int months=0;
while(total>0)
{
total_two=(total-1000)*(104)/(100);
total_three=total_two;
total=total_three;
printf("%d ",total);
months++;
}

printf("If you have 250,000 dollars in your account with 4%% interest,
");
printf("And you withdraw 1,000 dollars a month, you will be able to withdraw for %d months.
",months);
return(0);
}
/unsigned int total_fn(void)
{
total=total104/100-1000;
return(total);
}*/

the problem is that when it runs, it prints wild random numbers and never gets to the printing the months because it is always greater than zero. the reason I have unsigned long int total_two&_three is because I thought that maybe the problem was total=total-1000 etc. but it still dosen’t work. What am I doing wrong? thanks

#2

You should probably use floats instead of ints. And your numbers should have .0 on the end of them.

#3

It’s funny how you answered yourself in your post. Of course unsigned long int is always greater than zero. That’s the whole point of it being unsigned :D.

When you reach the number total = 378, the next iteration goes like this
(378 - 1000) * 1.04 = 171798044

You get these results because of the internal binary representation of unsigned numbers. And yes it’s correct.

There are other problems with your algorithm - if total at the begining so big that 0.04 * total > 1000, the cycle will never end.

The number of months will sometimes be one larger that the number of moths you are allowed to withdraw.

etc. :slight_smile:

Actually you don’t have to use cycling at all, it’s strictly mathematical problem, which can be solved with one line (I won’t tell though, google for it or http://mathforum.org/dr.math/faq/faq.interest.html )

#include &lt;stdio.h&gt;
long int total = 10000;
int main()
{
        int months=0;
        while(total &gt; 0)
        {
                total = (total-1000) * 1.04;
                printf("%d
",total);

                if (total &gt;= 0)
                        months++;
        }
        printf("%d
", months);
}
#4

Thank you! it works now. Now onto a completely different project. I’m doing a trivia thing and I want something like:
if(entry==“rizzo”||entry==“Rizzo”)
then do whatever…;

but I don’t know how to compare strings. Any help would be appreciated. Thanks.

#5

I must say I like your enthusiasm :).

In C can compare strings with strcmp. However strcmp returns 0 if the strings are equal and not zero when they differ (it’s not very important which numbers it returns when they aren’t equal).

So you must do

char *buf1 = “buf1”;
char *buf2 = “buf2”;

if (strcmp(buf1, buf2) == 0)
printf("equal
");
else
printf("not equal
");

What I would suggest, is to take one day and listen to this seminar.

http://mindview.net/CDs/ThinkingInC/beta3

It’s awesome introduction to C. They explain why some things work the way they work in C and give you a generally very good insight in coding practices.