Can you explain please

Question

#include <stdio.h>

int r(){

static int num=7;

return num--;

}

int main(){

for (r();r();r())

printf("%d",r());

return 0;

}

Answer 1

I marked the for loop as follow:
for(r1() ; r2() ; r3())
printf("%d", r4())

The loop goes like this:
r1() 7->6
r2() 6->5
r4() 5->4 //printed 5
r3() 4->3
r2() 3->2
r4() 2->1 //printed 2
r3() 1->0
r2() 0-> -1 // 0 is false. Loop ends.