Assignment and Conditional check in C - c

Question

I came across this below program and I don't understand the output.
Can someone please shed some light on it?
#include <stdio.h>
int main()
{
int i=1,j=1;
for(;j;printf("%d %d\n",i,j))
j=i++ <=5;
return 0;
}
And its output is:
2 1
3 1
4 1
5 1
6 1
7 0

Answer 1

#include <stdio.h>
int main()
{
int i=1,j=1;
//for(initialisation; condition; operations)
// here no initialisation,
// 1. condition is j, if j is true
// 2. then it will execute block statements
// 3. then finally it execute operations, here printf
// 4. again check step 1.
for(;j;printf("%d %d\n",i,j))
j=i++ <=5; // j = (i <= 5); i++;
return 0;
}
Your question can be simplified as follows
#include <stdio.h>
int main()
{
int i=1,j=1;
while(j) {
j = (i++ <=5);
printf("%d %d\n",i,j);
}
return 0;
}

Answer 2

As Ajay have given an alternative code to the original one (I'll copy-paste it below for convenience)
#include <stdio.h>
int main()
{
int i=1,j=1;
while(j) {
j = (i++ <=5);
printf("%d %d\n",i,j);
}
return 0;
}
Now, let's look at the execution of the while loop
i=1, j=1, j is True, in loop, j = (1 <= 5) = true = 1, i=2 (post increment).
i=2, j=1, j is True, in loop, j = (2 <= 5) = true = 1, i=3 (post increment).
i=3, j=1, j is True, in loop, j = (3 <= 5) = true = 1, i=4 (post increment).
i=4, j=1, j is True, in loop, j = (4 <= 5) = true = 1, i=5 (post increment)
i=5, j=1, j is True, in loop, j = (5 <= 5) = true = 1, i=6 (post increment).
i=6, j=1, j is True, in loop, j = (6 <= 5) = false = 0, i=7 (post increment).
i=7, j=0, j is False, out of loop.
Program Stops.
Hope this execution explanation helps.

Answer 3

The following statement:
j=i++ <=5;
evaluates the value of the variable i before incrementing it. This evaluated value is used in the comparison <= 5. The result of a comparison will be either 0 or 1 depending whether the condition is false or true respectively.
In this case, it has the same effect as writing it in two separate statements:
j = i <= 5;
++i;

Answer 4

syntax of for loop:
for (initialization Statement; check condition; increment/decrements operation)
{
// codes
}
The initialization statement is executed only once. Then, the check condition is evaluated. If the condition is true (nonzero), then codes inside the body of for loop is executed and the update expression is updated. if condition is false(0), then for loop terminated.
In your code,
j = 1 then condition become true and enter into body of for loop. Then, check
j = i++ <= 5 = true, and control goes to increment/decrements operation part and print i = 2 and j = 1.
then, again code repeated until the test condition is false.

Answer 5

Same can be represent in this manner:
#include <stdio.h>
int main()
{
int i,j;
for(i=1,j=1;j!=0;){
j=i++ <=5;
printf("%d %d\n",i,j);
}
return 0;
}
here, as per convention of for loop.
i & j is initialized to 1
check condition whether j != 0
i <= 5 so j remains 1 && i becomes 2 (after increment)
prints i = 2, j =1
again, i <= 5 so j remains 1 && i becomes 3 (after increment)
prints i = 3, j =1
again, i <= 5 so j remains 1 && i becomes 4 (after increment)
prints i = 4, j =1
and this goes on, it j becomes 0, as it violent the condition as i becomes greater than 5