Control Instructions in C

Question

I am not understanding for loop statement and expression following it. Please do help me understand.

#include<stdio.h>

int main()
{
   int x = 1;
   int y = 1;
   for( ; y ; printf("%d %d\n",x,y))
       y = x++ <= 5;
   return 0;
}

And the output I got

+1 im not clear why the downvotes, the code appears to be deliberately convoluted and the op is asking for help understanding what its doing, a valid question that might help many beginners. It is not the OP's code. — John Faulkner, Jul 22 '13 at 06:25

score 7 · Accepted Answer · answered Jul 22 '13 at 05:05

7

y = x++ <= 5; ==> y = (x++ <= 5); ==> first compare x with 5 to check whether x is small then or equals to 5 or not. Result of (x++ <= 5) is either 1, 0 assigned to y,

As x becomes > 5, (x++ <= 5) becomes 0 so y = 0 and condition false and loop break,

answered Jul 22 '13 at 05:05

Grijesh Chauhan

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@Sandeep you may like to check: [What is the name of this operator: “`-->`”?](http://stackoverflow.com/questions/1642028/what-is-the-name-of-this-operator) – Grijesh Chauhan Jul 22 '13 at 09:18
@sjas may be not as I could understand Sandeep's (OP) main confusion was expression `y = x++ <= 5`. – Grijesh Chauhan Jul 22 '13 at 11:33

score 1 · Answer 2 · edited Jul 22 '13 at 20:49

Initialize your variables:

int x = 1; int y = 1;

There are 3 statements for the for loop: -1. Initialize, 2. Condition, 3. Iteration:increment/decrement
In your case, you did not provide the initialize condition, however, you have the part of condition and incrementation. I do not think your for loop is used in the correct way.

You should swap the part of incrementation with your body like this:

for(; y; y = x++ <= 5;)
   printf("%d %d\n", x, y)

First, you check whether the condition is true or not, y is true or not. Then, you print x and y out. Then, the part of incrementation is executed, x++ <= 5 or not. The result is assigned to y. It does so until your condition is false, y == false.

NOTE: For the good programming, you should enclose your body with a curly braces.

Good practice is also keep space after `;` and `,` and expression `a<=b;` should be written like `a <= b`. I given some formatting to your answer to make it readable. btw good answer +1 — Grijesh Chauhan, Jul 22 '13 at 20:51

Steve Barnes · Answer 3 · 2013-07-22T11:05:30.560

1

Basically the for syntax is: for(StartCondition; Test; PostLoopOperation) DoWhileTestPasses;

In this case:

StartCondition == None
Test == (y != 0)
PostLoopOperation == do some printing 
DoWhileTestPasses == set y to zero if x > 5 otherwise to non-zero THEN increment x.

Which is all rather bad practice because it is confusing.

Would be better written as:

int x=0;
int y=0;
for(y=0; y = (x <= 6); x++)
{
    printff("%d %d\n",x,y);
}
return(0);

edited Jul 22 '13 at 11:05

answered Jul 22 '13 at 05:12

Steve Barnes

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In your last point, it appears to me, that since its a postfix increment, hence increment x must come after the word otherwise :-) – nIcE cOw Jul 22 '13 at 07:07
Correct @nIcEcOw - solution adjusted. – Steve Barnes Jul 22 '13 at 11:06

score 1 · Answer 4 · edited Jul 22 '13 at 18:39

1

In y = x++ <= 5;, y stores the value that is output by the condition x++ <= 5 (here x++ is post increment). If the condition is true then y = 1 else y = 0.

for( ; y ; printf("%d %d\n",x,y))

In the for loop you are printing the values of x and y after executing the for loop body.

edited Jul 22 '13 at 18:39

Grijesh Chauhan

57,103
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answered Jul 22 '13 at 05:13

Sandeep Reddy Goli

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score 0 · Answer 5 · answered Jul 22 '13 at 05:13

0

similar to this

   int x = 1;
   for( int y = 1; y!=0 ; )
   {
       if (x++ <= 5)
       {
          y = 1;
       } 
       else
       {
          y = 0;
       }
       printf("%d %d\n",x,y);
   }

answered Jul 22 '13 at 05:13

aah134

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score 0 · Answer 6 · answered Jul 22 '13 at 05:14

0

Perhaps this slightly transformed (but functionally equal) code will help:

int x = 1;
int y = 1;
while (y) {
   y = (x <= 5);
   x = x + 1;
   printf("%d %d\n", x, y)
}

answered Jul 22 '13 at 05:14

Bill Lynch

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Control Instructions in C

6 Answers6