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Saint Retro

While loop

5 posts in this topic

Hi all,

 

I am confused by a question in the Head First C# book, the task is to identify how many times a loop will loop (is this called iterations?).  I have worked out a few but one is really confusing me.  I am currently 'paper coding' as I'm at work so I can't watch the variable but an explanation would be great as I'm really puzzled.  This one asks how many times the statement 'j = j - i;' will be executed.

int j = 2;
for (int i = 1; i < 100;
i = i * 2)
{
j = j - i;
while (j < 25)
{
j = j + 5;
}
}

I have come up with many answers and none of them are what the book tells me it should be.  I think I may be getting confused on what the values are on the very first execution and this perhaps is making me go wrong.  Any advice would be greatly appreciated and hopefully set me up for the next one which looks harder.

Edited by Saint Retro
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I have come up with many answers and none of them are what the book tells me it should be.

What tells the book ? And what is your answer, and why do you think it should be this way ?

 

Just telling you the answer would not help you much, tell us how you approach this task and we might help you and identify some errors.

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What you have quoted is an extremely unfriendly loop that has no practical purpose and that you shouldn't ever see in the wild. I guess that the purpose of the exercise it to force you to learn using a debugger, setting up breakpoints and following the logic step by step. This said, that's not what I would call a good introduction to debugging.

 

Which of those loop the book is asking for ? the "while" or the "for" loop?

The "for" loop runs 7 times (i = i * 2 -> 1 2 4 8 16 32 64 -> out (128))

 

No idea how many times the "while" loop is called without wasting way too much of my time, and neither should you to be honest. That's a pointless exercise, so i definitively think it's to force you to use a debugger or to think outside the box (add a z = z + 1 in the while loop and print the result).

 

 

Otherwise :

At the start

j = 2

for start with

i = 1

j = 2 - 1 = 1

as J it less than 25  -> J is incremented in the while loop until it reach 26 (5 times)

and we go back to the for where:

i = i * 2 = 2

j = 26 - 2 = 24

as J is less than 25  -> J is incremented until it reach 24 + 5 = 29 (one time)

i = i * 2 = 4

j = 29 - 4 = 25

no while loop as 25=25 (while doesn't run)

go back to the for and ...

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Thank you both, where I have been going wrong is I presumed after the while loop adds the 5 to j, it then goes back to the top of the for loop which is why I was never getting anywhere.  At this stage it hasn't explained about the 'rules' surrounding nested loops.

 

Ashaman, thanks for your post, The book tells me it executes 7 times.  I do not see anything wrong with knowing the answer as long as I can work out how to get to that.  

 

I will do my best to show where I was going wrong.  I wrote the two variables and the 7 steps down one side as I already know it will happen 7 times once I get it right.  The way I have been working it out means it would go on forever.

 

The first time I take is that j is 2 and i is 1.  So 2-1 = 1, then plus 5 as j is less than 25.  So now J is 6.

We now multiply i by 2 so it is now 1*2 is 2, j(6) - 2 is 4, then plus 5 and j is 9.

i = 2*2, J = 9-4 which gives me 5, add 5 and J is 10.

i = 4*2, J = 10-8 which gives me 2, add 5 and J is 7.

 

Obviously I can tell this is wrong at this stage already.  This was due to me not understanding how the nested loops worked.  I should point out in the books defence, it's not down as a code example for a program, it's under the exercises for you to try work out by hand,  just before we did this breakpoints and watching expressions were introduced and were I at home I would have worked this out a lot sooner.

Edited by Saint Retro
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Oh man, I remember doing that one - it was really frustrating. Like SerialKicked mentioned above, the solution I found was to write the code, walk through it step by step in the debugger, and mark a tic on a sheet of paper each time it went through.

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Once I learnt the way the nested loops work it was a lot easier, I was able to do the one afterwards on paper no problems.  I assume you are referring to the same book as me.

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