help with switch statement

can this chuck of if-else if statements can be converted into switch statement format? thanks

Code:

---

int search_opcode (char opcode[LENGTH])
{
    if (( strcmp ("add", opcode) == 0) || (strcmp ("sub", opcode) == 0)
        || (strcmp ("mul", opcode) == 0) || (strcmp ("and", opcode) == 0)
        || (strcmp ("or", opcode) == 0) || (strcmp ("xor", opcode) == 0)
        || (strcmp ("nor", opcode) == 0) || (strcmp ("sllv", opcode) == 0)
        || (strcmp ("srlv", opcode) == 0)){
            return 1;
        }else if ((strcmp ("lw", opcode) == 0) || (strcmp ("sw", opcode) == 0)){
            return 2;
        }else if ((strcmp ("addi", opcode) == 0)){
            return 3;
        }else if ((strcmp ("mfhi",opcode) == 0) || (strcmp ("mflo", opcode) == 0)
        || (strcmp ("mthi", opcode) == 0) || (strcmp ("mtlo", opcode) == 0)
        || (strcmp ("jr", opcode) == 0)){
            return 4;
        }else if ((strcmp ("beq", opcode) == 0) || (strcmp ("bne", opcode) == 0)){
            return 5;
        }else if ((strcmp ("bltz", opcode) == 0) || (strcmp ("blez", opcode) == 0)
        || (strcmp ("bgtz", opcode) == 0) || (strcmp ("bgez", opcode) == 0)){
            return 6;
        }else if ((strcmp ("b", opcode) == 0) || (strcmp ("j", opcode) == 0)
        || (strcmp ("jal", opcode) == 0)){
            return 7;
        }else if (strcmp ("nop", opcode) == 0){
            return 8;
        }else if ((strcmp ("break", opcode) == 0) || (strcmp ("syscall", opcode) == 0)){
            return 9;
        }else{
            return -1;
    }
}

---

No, it's not possible, because a switch statements needs a constant expression, something that can be evaluated at compile time, and a function call cannot.
Or consequently, it may be possible with things such as hash tables, but that may be somewhat advanced.

Well, it is possible, but personally I do not like it:

Code:

---

switch (1)
{
case a == 0 || x == 0:
    return 1;
case b == 0 || y == 0:
    return 2;
case z == 0:
    return 3;
default:
    return -1;
}

---

EDIT:
hmm... decided to check, and I am wrong. It is indeed true that the label must evaluate to a constant, so this trick does not work in C. That is good to hear, in my opinion :)

Here's one way to do it. Without a switch, but still, much more readable than what you have.

Code:

---

int search_opcode (char opcode[LENGTH])
{
    const char *one[] = {"add", "sub", /* ... */};
    const char *two[] = {"lw", "sw"};
    const char *three[] = {"addi"};
    struct {
        const char **data;
        size_t n;
    } data[] = {
        {one, sizeof one},
        {two, sizeof two},
        {three, sizeof three}
    };
    size_t x, y;
   
    for(x = 0; x < sizeof(data) / sizeof(*data); x ++) {
        for(y = 0; y < data[x].n; y ++) {
            if(!strcmp(opcode, data[x].data[y])) return x + 1;
        }
    }
   
    return -1;
}

---

I'm sure you could do it without naming the "one", "two", etc. arrays, but then you'd likely have to hard-code how many strings there were for each one. This way, all of the counting is done for you. If you want to add a string, just do it; you don't have to change any numbers or anything.

Hi. I know this is an old post but I have been unsuccessful at trying to implement the above code by dwks. At first I tried it out and it seemed only to work when I passed it a string that was present in the const char *one[] array. For everything else it gave me a Bus Error. However, upon further inspection I discovered that substituting

Code:

---

return x + 1;

---

with

Code:

---

return x;

---

revealed that the function actually returned 0.
I tried fixing the code but I cannot do much since I do not fully understand what goes on in the following loop:

Code:

---

for (x = 0; x < sizeof (data) / sizeof (*data); x++){
        for (y = 0; y < data[x].n; y++){
            if (!strcmp (opcode, data[x].data[y]))
                return x + 1;
        }
    }

---

Can someone please explain what exactly goes on in that loop and where the code can be fixed in general so it would achieve its goal of replacing the clunky code I had in the beginning of this post?
Thanks a lot.

The x+1 represents the value you return - which is 1..9 in your original code.

Since arrays start at zero, the index (x) that you find the instruction at will be zero for the first lot of values - but you return one.

--
Mats

Try this:

Code:

---

int search_opcode (const char *opcode)
{
  const char *one[] = {"add", "sub"};
  const char *two[] = {"lw", "sw"};
  const char *three[] = {"addi"};
  struct {
      const char **data;
      size_t n;
  } data[] = {
      {one, sizeof one / sizeof *one },
      {two, sizeof two / sizeof *two },
      {three, sizeof three / sizeof *three}
  };
  size_t x, y;

  for(x = 0; x < sizeof(data) / sizeof(*data); x ++) {
      for(y = 0; y < data[x].n; y ++) {
        if(!strcmp(opcode, data[x].data[y])) return x + 1;
      }
  }

  return -1;
}

int main()
{
  printf("Opcode for &#37;s: %d\n", "add", search_opcode("add"));
  printf("Opcode for %s: %d\n", "addi", search_opcode("addi"));

  return 0;
}

---

Ok now I understand that bit, but how do I solve the bus error problem?? Whenever, I pass it something like "lw" or "addi", which are both not in the first array I keep getting Bus Error and I don't know where this code goes wrong in order to fix it.
Thanks

Thanks rag_to_riches. It all seems to work now!! I will add more string arrays and test it.
Thanks a lot!!!

I'll usually just format my code to make it easier to read / maintain:

Code:

---

int search_opcode (char opcode[LENGTH])
{
        if  (
                (strcmp ("add" , opcode) == 0) ||
                (strcmp ("sub" , opcode) == 0) ||
                (strcmp ("mul" , opcode) == 0) ||
                (strcmp ("and" , opcode) == 0) ||
                (strcmp ("or"  , opcode) == 0) ||
                (strcmp ("xor" , opcode) == 0) ||
                (strcmp ("nor" , opcode) == 0) ||
                (strcmp ("sllv", opcode) == 0) ||
                (strcmp ("srlv", opcode) == 0)
                ) {
                    return 1;
        }
        else if (
                (strcmp ("lw", opcode) == 0) ||
                (strcmp ("sw", opcode) == 0)
                ) {
                        return 2;
                }
        else if (
                (strcmp ("addi", opcode) == 0)
                ) {
                        return 3;
                }
        else if (
                (strcmp ("mfhi", opcode) == 0) ||
                (strcmp ("mflo", opcode) == 0) ||
                (strcmp ("mthi", opcode) == 0) ||
                (strcmp ("mtlo", opcode) == 0) ||
                (strcmp ("jr"  , opcode) == 0)
                ) {
                        return 4;
                }
        else if (
                (strcmp ("beq", opcode) == 0) ||
                (strcmp ("bne", opcode) == 0)
                ) {
                        return 5;
                }
        else if (
                (strcmp ("bltz", opcode) == 0) ||
                (strcmp ("blez", opcode) == 0) ||
                (strcmp ("bgtz", opcode) == 0) ||
                (strcmp ("bgez", opcode) == 0)
                ){
                        return 6;
                }
        else if (
                (strcmp ("b"  , opcode) == 0) ||
                (strcmp ("j"  , opcode) == 0) ||
                (strcmp ("jal", opcode) == 0)
                ){
                        return 7;
                }
        else if (strcmp ("nop", opcode) == 0) {
                return 8;
        }
        else if (
                (strcmp ("break"  , opcode) == 0) ||
                (strcmp ("syscall", opcode) == 0)
                ){
                        return 9;
                }
        else{
                return -1;
        }
}

---

I can even use the following also (Though not switch statement)

Code:

---

#include <stdio.h>
struct {
        char opcode[5];
        int ret_val;
      } my_struct[] = {{"add", 1},
                        {"sub", 1},
                        {"lw",  2},
                        {"sw",  2},
                        {"addi",3}};

main()
{
    printf("Returned value %d\n", search_opcode("addi"));
}
int search_opcode(char *opcode)
{
    int i = 0;
    int val = 0;
    val = sizeof(my_struct)/sizeof(my_struct[0]);
    for (i=0; i <= val; i++)
    {
        if (!strcmp(opcode, my_struct[i].opcode))
        {
            return my_struct[i].ret_val;
        }
    }
    return 99;
}

---

Code:

---

for (i=0; i <= val; i++)

---

I think by using <= instead of <, you've created a buffer overrun there.

Oh, and returning 99 probably isn't a good idea. int is signed, so go with a negative number or INT_MAX or something.

That's a good way of doing it, though. I hadn't thought of that. :)