Question

C Question: What's wrong with my program? [2]

Answer 1

This is supposed to be a program that accepts an integer from 1 to 3,999 and converts it to Roman numeral form.

Initially the values for the digits I got were correct, but when I added the switch sections of the code I get decimal numbers for the digits. Also, the switches themselves don't seem to be working well as the program can only choose between case 1 and default. I suspect that the error has something to do with how I dealt with the strings.

#include <stdio.h>

int main (void) {

int Num, Digit[4];
char *RThou[5], *RHund[5], *RTens[5], *ROnes[5];

// Digit[3]: Thousands digit | RThou[5]: Thousands digit in Roman numerals
// Digit[2]: Hundreds digit | RHund[5]: Hundreds digit in Roman numerals
// Digit[1]: Tens digit | RTens[5]: Tens digit in Roman numerals
// Digit[0]: Ones digit | ROnes[5]: Ones digit in Roman numerals

printf("Input an integer from 1 to 3,999: ");
scanf("%d", &Num);

if (Num < 1 || Num > 3999)
printf("Invalid input.\n");
else {

Digit[3] = Num/1000 % 10;
Digit[2] = Num/100 % 10;
Digit[1] = Num/10 % 10;
Digit[0] = Num % 10;

switch (Digit[3]) {
case 1: RThou[5] = "M"; printf("M"); break;
case 2: RThou[5] = "MM"; printf("MM"); break;
case 3: RThou[5] = "MMM"; printf("MMM"); break;
default: RThou[5] = ""; break;
}

switch (Digit[2]) {
case 1: RHund[5] = "C"; break;
case 2: RHund[5] = "CC"; break;
case 3: RHund[5] = "CCC"; break;
case 4: RHund[5] = "CD"; break;
case 5: RHund[5] = "D"; break;
case 6: RHund[5] = "DC"; break;
case 7: RHund[5] = "DCC"; break;
case 8: RHund[5] = "DCCC"; break;
case 9: RHund[5] = "CM"; break;
default: RHund[5] = ""; break;
}

switch (Digit[1]) {
case 1: RTens[5] = "X"; break;
case 2: RTens[5] = "XX"; break;
case 3: RTens[5] = "XXX"; break;
case 4: RTens[5] = "XL"; break;
case 5: RTens[5] = "L"; break;
case 6: RTens[5] = "LX"; break;
case 7: RTens[5] = "LXX"; break;
case 8: RTens[5] = "LXXX"; break;
case 9: RTens[5] = "XC"; break;
default: RTens[5] = ""; break;
}

switch (Digit[0]) {
case 1: ROnes[5] = "I"; break;
case 2: ROnes[5] = "II"; break;
case 3: ROnes[5] = "III"; break;
case 4: ROnes[5] = "IV"; break;
case 5: ROnes[5] = "V"; break;
case 6: ROnes[5] = "VI"; break;
case 7: ROnes[5] = "VII"; break;
case 8: ROnes[5] = "VIII"; break;
case 9: ROnes[5] = "IX"; break;
default: ROnes[5] = ""; break;
}

printf("%d %d %d %d\n", Digit[3], Digit[2], Digit[1], Digit[0]);
printf("%c %c %c %c\n", *RThou[5], *RHund[5], *RTens[5], *ROnes[5]);

}

}

Answer 2

I don't know C but problem is NOT with switch statements

printf("%c %c %c %c\n", *RThou[5], *RHund[5], *RTens[5], *ROnes[5]);

maybe this prints ONLY 1 character?

Answer 3

You're accessing memory that's out of the boundaries of your array in ALL of your switch() statements. For example, you are writing RThou[5] even though RThou[] is a char* array of size 5; arrays in C (and other languages like C++, Java, Python, etc) are zero-indexed, which means you can only subscript an array from 0 to array_size-1: i.e. if I declare an array A[10], I can only use A[0], A[1], ..., A[9] and anything else would be undefined behavior.

Answer 4

that's what i thought, because always 1 char was written

Answer 5

but in others languages i would use String for it :P

Answer 6

in C, you could make good use of <string.h> here

Answer 7

@Tomas.A: Yep, I realized soon enough that changing %c to %s should allow me to print more than 1 character.

@agdgdgdgwngo: I'm afraid I'm not sure if I understand you. So if we imagine RThou[5] to look like this:
[] [] [] [] []
0 1 2 3 4
Does it mean in my switch statements, I'm trying to jam all those characters in a slot that does not exist? How can I make them fit into slots 0-4?

Answer 8

right. try dereferencing valid slots.

Answer 9

But if that's the case, how come it works for the first 2 digits, and fails for the last 2?

Answer 10

switch (Digit[3]) { case 1: RThou[5] = "M"; printf("M"); break; case 2: RThou[5] = "MM"; printf("MM"); break; case 3: RThou[5] = "MMM"; printf("MMM"); break; default: RThou[5] = ""; break; }

you are using printf statements there.

Answer 11

Even without the printf statements, the program is able to identify the correct values for the thousands and hundreds digits. Just not the tens and ones digits.

Answer 12

Okay, it seems that changing Digit[0] to Digit[4] makes the program output the correct value for the ones digit.

Answer 13

digit[4] doesn't give you a segfault?

Answer 14

For some weird reason, no.

Answer 15

post the code on a site like http://ideone.com so we can debug it

Answer 16

Here: http://ideone.com/nK1OV

Answer 17

http://ideone.com/vaYtT works alright O.o

Answer 18

Ah. The tens digit will always output to 1. '3333' should not work.

Answer 19

http://ideone.com/2L3MA

got rid of all the out of array bounds stuff

Answer 20

Hmm. Never knew you could just do away with declaring the number of slots in a string. Does that work for arrays of other data types?

Answer 21

I don't think so :(

Answer 22

If I *really* wanted to specify the number of slots in the strings, what should I have done?

Answer 23

you can do lots of stuff, like defining char arrays of specific sizes, malloc(), etc.

Answer 24

Another question: How come removing the array bounds made the code work?

Answer 25

It actually works even when you are storing your strings in places outside the bounds of your arrays, since you are operating on the stack.

Answer 26

Since you declared the thousands, hundreds, tens and ones consecutively, when you were going beyond the end of the thousands you could possibly have been putting the thousands value into the hundreds, then the hundreds values into the tens, and sooner or later putting the value into who knows what.