Explain and give output pliz. I alone cannot solve

Question

1.00/5 (3 votes)

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int p=1;
do
{
printf("%d",p);
}
while(p--);

What I have tried:

int p=1;
do
{
printf("%d",p);
}
while(p--);

Posted 1-Oct-22 20:06pm

Member 10357461

Updated 1-Oct-22 23:32pm

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OriginalGriff · Answer 1 · 2022-10-01T20:16:00

Solution 1

p starts at 1.
You print the value of p, then check if it's zero.
If it isn't, you will go back around the loop.
But because you have a post-decrement on p before it actually makes the jump or not p is reduced by one.

It's the same thing as writing it like this:

C

#include <stdio.h>

int main()
    {
    int p=1;
    while (p >= 0)
        {
        printf("%d",p);
        p = p - 1;
        }
    return 0;
    }

This may help: Why Does x = ++x + x++ Give Me the Wrong Answer?[^] - it explains what the pre- and post- increment / decrement operators actually do.

Posted 1-Oct-22 20:16pm

OriginalGriff

Comments

KarstenK 2-Oct-22 4:55am

wouldnt while(--p); do the job?

OriginalGriff 2-Oct-22 5:41am

Nope: the current prints N to zero inclusive. Pre-decrement stops early!

KarstenK 3-Oct-22 3:30am

Are you serious? Not in the original code with do-loop.

OriginalGriff 3-Oct-22 4:07am

The original code was a do ... while
Run it, and see.

KarstenK 4-Oct-22 2:56am

WTF??? Didnt expect that. Can you explain. Didnt help me at: https://en.cppreference.com/w/cpp/language/do

OriginalGriff 4-Oct-22 3:17am

do
   {
   code
   } while (condition);

Always executes code at least once, then checks the condition. If it's true the loop goes round again.
Since the original condition was p-- it checks the value of p before it reduces it:

label:
   print p;
   temp = p;
   p = p - 1;
   if (temp != 0) go to label;

So in the original code, p starts at 1, so it gets printed.
Then since p is not zero, it gets reduced to zero and the loop goes around again.
p sets printed (zero this time) and then checked. This time, it is zero, so p is reduced by one and the loop exits.

Does that make sense?

OriginalGriff 4-Oct-22 3:19am

I don't like this new CRLF feature - it messes up comments badly ... all the newlines in the code blocks are gone and I can't get them back ...

merano99 · Answer 2 · 2022-10-01T23:32:00

Solution 2

One could also write it much simpler and do everything with a for() loop:

C

for (int p=1; p >= 0; p--) {
	printf("%d", p);
}

Posted 1-Oct-22 23:32pm

merano99