C languange 4 digit possible variations with exception - c

Question

So i have a homework, but i can't wrap my head around why this code doesn't work.
Problem sounds something like this:
Make a code that checks all possible variations of 4 digit number, but there are no 3 same digits in a number. ( i hope that makes sense)
My code:
#include<stdio.h>
int main ()
{
int i;
char a,b,c,d;
for (i=0; i<9999; i++)
{
a = i/1000;
b = i/100%10;
c = i/10%10;
d = i%10;
if (a==b==c) {i++;}
else if (b==c==d) {i++;}
else if (c==d==a) {i++;}
else if (d==a==b) {i++;}
else if (a==b==c==d) {i++;}
else
{
printf("%d,%d,%d,%d\n", a,b,c,d);
}
}
}
I want it to print out something like this:
0,0,1,1
0,0,1,2
0,0,1,3
...
0,1,0,9
0,1,1,0
0,1,1,2
...
9,9,8,8
Instead it prints:
0,1,2,2
0,1,3,3
0,1,4,4
0,1,5,5
0,1,6,6
...
0,8,6,6
0,8,7,7
0,8,8,8
0,8,9,9
0,9,2,2
...
9,9,9,8

Answer 1

There's a problem with these comparisons:
if (a==b==c)
This doesn't find out if a, b and c are all the same. The result of == is 0 or 1, and then that is compared to the third number. So per example if all three numbers are 9, then 9==9==9 turns into 1==9, which is 0, so the if is not taken. What's more, you have false positives for things like 9==9==1.
What you instead need here is
if ((a==b) && (a==c))
Now you check if a is equal to b, and when it is, you check it with c, and the if is taken if they're all the same.
Likewise of course for the other comparisons, so instead of else if (a==b==c==d) you want else if ((a==b) && (a==c) && (a==d))

Answer 2

As other answers have noted, you can't chain comparison operators in C and get the expected results. Use logical operators to combine comparisons. It would probably be more clear to use || to avoid all of the else statements.
There is another significant problem with the posted code: i is incremented whenever a failing number is found. But then i is incremented again by the for loop. This causes the number after a failing number to be skipped, so some desired numbers will not be found. For example, 3222 should fail, and the subsequent number 3223 should pass, but this number is skipped by the posted code (after the comparison corrections), with the next number to be checked 3224. Instead, just use continue:
if ((a == b && a == c) ||
(b == c && b == d) ||
(c == d && c == a) ||
(d == a && d == b) ||
(a == b && a == c && a == d)) {
continue;
} else {
printf("%d,%d,%d,%d\n", a,b,c,d);
}

Answer 3

You can't use multiple equalities like this. You have to do them separately with and conditions (e.g. a==b && b==c).