Need explanation please! ! !

Question

0.00/5 (No votes)

See more:

I'm having problems explaining this code and I would really like a detailed explanation of how it works.

C++

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int x=-3, y=0, k,j;

    for( k=j=-3 ; x= x+(k<j),++j ; y+=2);
    printf("x=%d  y=%d k=%d  j=%d",x,y,k,j);

    getch();
}

By the way, the answers are x=-1 y=4 k=-3 j=0.

What I have tried:

I tried to solve it systematically but I keep getting stuck at the increment phase of the problem (++j) and my answers end up different from the run program

Posted 11-Apr-18 19:04pm

Hyeladi Bassi

Updated 11-Apr-18 20:46pm

v3

Add a Solution

Comments

GKP1992 12-Apr-18 1:45am

Are you sure this is the correct body of the for loop? There isn't a proper condition.

Hyeladi Bassi 12-Apr-18 1:48am

yes, that's the correct body it was a question given to students about two years ago in my school but the explanation was never given just the answer

CPallini 12-Apr-18 3:21am

It is truly awkward code, I don't see the point of coding like that (I don't even see the point of trying to understand it).

3 solutions

Solution 1

To understand that code you need to learn about the for loop and the prefix increment operator.

This for loop is a bit messy. Normally is in the head only the check and counter logic and the in the body.

C++

int main()
{

 for( int y = 0 ; y < 5; y+=2) //normally you check the counter
 {
   j++;
   x= x+(k<j);// strange should be x + 1 (true) ???
   if( j > y ) //alternative exit
   {
     break;
   }
 }
 
  printf("x=%d  y=%d k=%d  j=%d",x,y,k,j);

    getch();
}

This demonstration code may led to other results than your code.

Posted 11-Apr-18 20:22pm

KarstenK

Solution 2

Start here: Why does x = ++x + x++ give me the wrong answer?[^]

Posted 11-Apr-18 20:23pm

Maciej Los

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Accepted Answer · 2018-04-11T20:47:00

That is nasty code!
It's always a bad idea to have a construct like this:

C++

for (...);
printf(...);

because is't very difficult to see the ";" at the end of the for loop - so it looks to a casual eye like you wrote this:

C++

for(...)
   {
   printf(...);
   }

Which would have very different results. In fact, modern compilers will complain or at least warn you about it because it does cause a lot of confusion.
So start by restructuring your code:

C++

int main()
{
    int x = -3, y = 0, k, j;

    for( k = j = -3 ; ++j ; y += 2)
       {
       x = x + (k < j);
       }
    printf("x=%d  y=%d k=%d  j=%d",x,y,k,j);
    return 0;
}

Now it's easier to read and a lot more obvious what is going on.
1) Preset x to -3, y to 0.
2) Preset k and j to -3.
3) Enter the loop.
3.1) Compare k and j.
3.1.1) If k is less than j, add one to x (Remember, in C the result of any logical operation is zero for false, or one for true, so k < j returns 0 or 1)
3.1.2) Otherwise, add zero to x.
3.2 Add 2 to y
3.3 Add one to j, and if that value is nonzero, loop back round to (3).
4) Print the results.

See how much easier it is to read and understand when you write it out properly?
It's still nasty code though: I'd prefer to see either this:

C++

int main()
{
    int x = -3, y = 0, k, j;

    for( k = j = -3 ; ++j ; y += 2)
       {
       if (k < j)
          {
          x++;
          }
       }
    printf("x=%d  y=%d k=%d  j=%d",x,y,k,j);
    return 0;
}

Or this:

C++

int main()
{
    int x = -3, y = 0, k, j;

    for( k = j = -3 ; ++j ; y += 2)
       {
       x += (k < j) ? 1 : 0;
       }
    printf("x=%d  y=%d k=%d  j=%d",x,y,k,j);
    return 0;
}

Where the values you are playing with are more explicit.