Question

C++,

Help needed! I need a Program that does this: http://screencast.com/t/BFLvrQUh5b
Apparently there is something wrong with my code but I don't know why! The behaviour is not correct.

Answer 1

#include <iostream>
using namespace std;
int main()
{
int InitialValue,NumberOfYears,counter=9,counter2=1,counter3=1,ValueAfterAYear;
cout << "Give me the initial value : ";
cin >> InitialValue;
cout << "Give me the number of the years of that car : ";
cin >> NumberOfYears;
ValueAfterAYear = InitialValue-InitialValue*30/100;
if (NumberOfYears <= 8)
{
while (counter3 <= NumberOfYears)
ValueAfterAYear = ValueAfterAYear-ValueAfterAYear*10/100;
counter3++;

}

if (NumberOfYears > 8)
{
while (counter2 <= 8)
{
ValueAfterAYear = ValueAfterAYear-ValueAfterAYear*10/100;
counter2++;
}


while (counter < NumberOfYears)
{
ValueAfterAYear = ValueAfterAYear-ValueAfterAYear*5/100;
NumberOfYears--;
}
}
if (ValueAfterAYear < 500)
ValueAfterAYear = 500;

cout << "The value of the car is " << ValueAfterAYear;

return 0;
}

Answer 2

I'll open up Visual Studio in a minute, just tell me exactly what you want to the program to do. I'm assuming you're getting a logic error since you say "The behaviour is not correct".

Answer 3

Yes right, I am getting a logic error. My code's syntax is 100% correct. I don't want you to do me the program. If it's possible just tell me where it lacks logic so that I can learn to make better

Answer 4

* * * The program has to do this: http://screencast.com/t/BFLvrQUh5b

Answer 5

Look here:
while (counter3 <= NumberOfYears)
ValueAfterAYear = ValueAfterAYear-ValueAfterAYear*10/100;
counter3++;
It's an infinite loop. Remember that if you don't include {} only the FIRST line is included in the loop. Wrap it in {}.

Answer 6

Oh right! Hold on lemme check!

Answer 7

okay

Answer 8

Ok now runs and it gives me results. for example this:

Give me the initial value : 5000
Give me the number of the years of that car : 5
The value of the car is 2068

But with the calculator it gives me 2551.5

Answer 9

Are you using an IDE, like Visual Studio, or a CLI compiler like gpp/g++?
Just wondering since debugging things like this is easy with breakpoints, etc

Answer 10

hm I am using Xcode (Mac application)

Answer 11

Alright, well I haven't used that, but just for future reference learning how to use breakpoints will really help things :)
Anyway: I'll check the logic of your program, sec

Answer 12

I also have Visual Studio 2010 and Visual Studio 2012 I can run the code on those programs too on Virtual Machine

Answer 13

check the answer of the equation:
Initial value: 5000
years: 5
5000 - 5000 * 30/100 = 3500
3500 - 3500 * 10/100 = 3150
3150 - 3150 * 10/100 = 2835
2835 - 2835 * 10/100 = 2551.5 (2552)
That accounts for 5 years
Your program IS, in fact, actually getting that result
consider the value of counter 3, and how many times it's looping

Answer 14

I hope that's a good hint for you

Answer 15

Really? For you it gets this result? Look here: http://screencast.com/t/3uQDOhBe

This is the result from me

Answer 16

You're changing the value from 5000 to 3500 BEFORE the loop;
so:
First iteration: 3150
Second iteration: 2835
###Third iteration: 2552 ###
Fourth iteration: 2297

Answer 17

also, you'll notice that it proves the wrong results consistently except for the third iteration in this case

Answer 18

i'm 90% sure i know why that is, let me just check

Answer 19

ok

Answer 20

Actually, it's fine

Answer 21

??

Answer 22

Think of this

Answer 23

Initial value: 5000
years: 5
5000 - 5000 * 30/100 = 3500
3500 - 3500 * 10/100 = 3150
3150 - 3150 * 10/100 = 2835
2835 - 2835 * 10/100 = 2551.5 (2552)

Answer 24

and then this

Answer 25

You're changing the value from 5000 to 3500 BEFORE the loop;
so:
First iteration: 3150
Second iteration: 2835
###Third iteration: 2552 ###
Fourth iteration: 2297

Answer 26

Do you see the problem?

Answer 27

You're looping through 5 times even though a) the first year (5000) is taken care of, and the second year (removing 30%) is taken care of

Answer 28

ooh

Answer 29

see the problem?

Answer 30

I removed that "="

Answer 31

well consider how many years have already passed when the while loop starts

Answer 32

i would advise setting your counter to start at that

Answer 33

thanks dude

Answer 34

counter3=2, and while (counter3 < NumberOfYears)
fixes your problem

Answer 35

Yea I did this and the first part is completely done

Answer 36

so it's all fixed? :)

Answer 37

Hey something last, Would it be possible to tell me how to make use of those "breakpoints" in Visual Studio?

Answer 38

sure

Answer 39

Ok Should I open 2010 or 2012 ??

Answer 40

2010

Answer 41

ok

Answer 42

http://i.imgur.com/BMAdkTm.png
see that red dot the arrow points to?

Answer 43

yes

Answer 44

That's a breakpoint. By putting that there, it tells the program to pause execution at that point.

Answer 45

You can then press F10 to step through the program, line by line, or F5 to continue until the next breakpoint

Answer 46

the value of this is it lets you see the values stored in your variables when the program isn't finished yet

Answer 47

so you can see the use of this when it comes to debugging. For example, I figured out your problem by putting a breakpoint inside the while loop, and checking the value of valueAfterAYear every iteration.

Answer 48

So put a breakpoint next to ValueAfterAYear = ValueAfterAYear-ValueAfterAYear*10/100; (You can do this by clicking on the gray bar on the far left)

Answer 49

lemme know when you're put a breakpoint there :)

Answer 50

Ok done! Now F10??

Answer 51

have you started the program in debug mode, and entered your values (5000, and 5) in this case?

Answer 52

yes done!

Answer 53

http://i.imgur.com/uoHs6hZ.png
look at where ive highlighted in a red box

Answer 54

do you see something similar to this?

Answer 55

yea there is the same exact box on me too

Answer 56

k
put your mouse over ValueAfterAYear in the code

Answer 57

when you do that it should come up with "3500", you can do that with any variable within the scope when you're debugging

Answer 58

try it with other variables too

Answer 59

ooh I see!! very useful!!

Answer 60

ok
press F10

Answer 61

you'll notice the error jump down a line

Answer 62

and ValueAfterAYear has now changed to 3150

Answer 63

you can look at it from either the box or from mouseovering it

Answer 64

I noticed the 3150 but what do you mean by an error??

Answer 65

sorry i mean arrow***

Answer 66

the arrow on the left

Answer 67

aaah yea there is one

Answer 68

ok, keep pressing F10 until you're back at where we are now

Answer 69

it's 2835 now

Answer 70

if you want to skip going line by line, you can just pres F5 and it will go to the next breakpoint (since were in a loop it goes back to the same breakpoint)

Answer 71

you can also set breakpoints while your code is running

Answer 72

and if you want, double click the value of ValueAfterAYear in the auto's box, you can actually change the value if you want

Answer 73

things like this are so useful when trying to fix a logic problem

Answer 74

but if I change the value will the code change too, so that it corresponds to the changed value?

Answer 75

the actual code itself doesn't change, but within the program itself, the value changes, so it executes as per your changed value

Answer 76

you don't change the value mid program much, it's just a useful thing to know in case you ever need to do that

Answer 77

oh I see

Answer 78

think you understand breakpoints now?

Answer 79

Yea I will use them a lot with nested loops and such! You have no idea how this helps !! :)

Answer 80

haha np

Answer 81

glad i could help