Modulus in language C

Question

I'm having some trouble with this line of code: (sorry it is in danish)

 case 1 :
     system("cls");
       printf("K maa kun have en veardi mellem 1 og 20\nIndtast k:");
        scanf("%d", &k1);
  while(!(k1>=1 && k1 <=20))//Starten på fejlsikret tilstand
    {
     printf("K er for hoej eller har en forkert veardi.\nProev igen:  ");
     fflush(stdin);
     scanf("%d", &k1);
    }//slutning på fejlsikret tilstand

 system("cls");
 printf("X maa kun have en veardi mellem 1 og 20\nIndtast din modstanders tal: ");
 scanf("%d", &x1);
while(!(x1<21))//Starten på fejlsikret tilstand
 {
 printf("X er for høj.");
 scanf("%d", &x1);
 }//slutning på fejlsikret tilstand

            newmove=(21-x1)%(k1+1)+x1;
            printf("Du skal sige = %d", newmove);
            printf("\n\n\nTryk 1 og afslut med Enter for at komme tilbage til start");
            scanf("%d", &m);
            continue;

But it seems that the modulus part is calculating wrong. eks: If I put in the numbers: x1 = 12 and k1 = 12, it gives me 20. But it should be 17. Can anyone help?

how do you figure that it should be 17? (21-12) % (12+1) + 12 doesn't seem to add up to 17. — Art, Nov 28 '12 at 16:02
Nope.. I want to clear any number in the previous variables.. — Lasse, Nov 28 '12 at 16:02
`(21 - 12) % (12 + 1) + 12` doesn't add up to `20` either… — Julien Royer, Nov 28 '12 at 16:03
@Lasse - It's UB. See [here](http://stackoverflow.com/questions/2187474/i-am-not-able-to-flush-stdin) — Mike, Nov 28 '12 at 16:06
I have testet it on this site: http://www.wolframalpha.com/input/?i=%2812-20%29mod%2812%2B1%29%2B12 — Lasse, Nov 28 '12 at 16:09
@Lasse Cool. But those are completely different numbers than you have in your code and your description of the problem. — Art, Nov 28 '12 at 16:12

score 1 · Accepted Answer · answered Nov 28 '12 at 16:05

1

the newmove should be 21

newmove=(21-x1) % (k1 + 1) + x1;
        (21-12) % (12 + 1) + 12
        (  9    %    13  ) + 12
                9          + 12
         21

answered Nov 28 '12 at 16:05

MOHAMED

41,599
58
163
268

score 0 · Answer 2 · answered Nov 28 '12 at 16:05

0

newmove=(21-x1)%(k1+1)+x1;

So if x1=12 and k1=12

newmove=(21-12)%(12+1)+12;
newmove=9%13+12;
newmove=9+12
newmove=21

Why should it be 17 ?

answered Nov 28 '12 at 16:05

Joze

1,285
4
19
33

OH MY GOD.. So sorry!! waste of your time!! You are completly right.. i was messing out on the code.. sry.. – Lasse Nov 28 '12 at 16:11
Don't worry, it happens. – Joze Nov 28 '12 at 16:14

score 0 · Answer 3 · answered Nov 28 '12 at 16:05

0

I would expect your result to be 21. Substituting x1=12 and k1=12 into this expression:

(21-x1)%(k1+1)+x1

gives:

(21-12)%(12+1)+12 = (9)%(13)+12 = 9 + 12 = 21

answered Nov 28 '12 at 16:05

Marc Cohen

3,742
2
19
19

Modulus in language C

3 Answers3