Dictionary type lookup table (array)

BIOT · February 22, 2018, 5:49pm

Hi,

I have a set of about 30 paired commands like this:

const char* ON = "+";
const char* OFF = "-";

const char* RESTART = "RESTART";
const char* RESTARTED = "RESTARTED";

and require a way to lookup the corresponding varied value.

For example:
if the command received is "+", need to respond with '-'
if the command received is "RESTART", need to respond with "RESTARTED"

There isn't a dictionary type object for me to use key values, and I can't figure out map

Im thinking of two options:
Option 1: create a one dimensional array of

const char* const STATE[2] = {"+", "-"};
const char* const RESTART[2] = {"RESTART", "RESTARTED"};

Lookup function would iterate the array and check if each items[0] element == whats received and return item[1]

Option 2: use a 2 dimensional array - but I don't know how to do this.

Would appreciate any help on how to define the array and the lookup function.

Thank you

TonyWilk · February 22, 2018, 5:57pm

Hi,

BIOT:
Option 1: create a one dimensional array

You could use your 1-d array this way round:

 const char * questions[]={
    "+",
    "RESTART",
    "Foo"
 };
 const char * answers[]={
    "-",
    "RESTARTED",
    "Bar"
 };

...lookup to match the question, use that index for the answer.

Yours,
TonyWilk

BIOT · February 22, 2018, 6:06pm

TonyWilk:
Hi,

You could use your 1-d array this way round:
 const char * questions[]={

"+",
"RESTART",
"Foo"
};
const char * answers[]={
"-",
"RESTARTED",
"Bar"
};




...lookup to match the question, use that index for the answer.

Yours,
TonyWilk

Thank you Tony.

Yes, I see how that would work. I worry that it is error prone as the order of the items in both arrays needs to stay correct over time during development (will be adding many commands in the coming months).

Ideally, a 1 dim array of 2 dim items in it would be safer, but I dont understand how to define the parent 1 dim array

gfvalvo · February 22, 2018, 6:14pm

Time for an array of structs.

TonyWilk · February 22, 2018, 6:39pm

BIOT:
Yes, I see how that would work. I worry that it is error prone as the order of the items in both arrays needs to stay correct over time during development (will be adding many commands in the coming months).

Yep, you are right there. Do-able with what you know, but you want more

Follow gfvalvo's suggestion.

Yours,
TonyWilk

econjack · February 22, 2018, 6:42pm

The following code uses parallel arrays or 2-dimentioned arrays. You can toggle them by using the #define SINGLE preprocessor directive. For beginners, I would think the parallel arrays is easier to understand, but I appreiciate how they can get out of sync.

#define ELEMENTCOUNT(x) (sizeof(x) / sizeof(x[0]))

#define SINGLE  // comment out for single array

#ifdef SINGLE
char operators[] = {'+', '-', '*', '/'};
char *words[] = {"add", "subtract", "multiply", "divide"};
#else
char *tokens[][2] = { {"+","add"}, {"-", "subtract"},
                      {"*","multiply"}, {"/", "divide"}
                    };
#endif
void setup() {
  Serial.begin(9600);
}

void loop() {
  char c[2];
  int index;
  
  if (Serial.available() > 0) {
    c[0] = Serial.read();

    index = FindWord(c);

    Serial.print("For operator ");
    Serial.print(c);
    Serial.print(" the word is ");
    if (index != -1) {
#ifdef SINGLE      
      Serial.println(words[index]);
#else
      Serial.println(tokens[index][1]);
#endif
    } else {
      Serial.println("not not found in list.");
    }
  }
}

/*****
  Purpose: to search a list of operators for a match on the target operator

  Parameter list:
    char target     the operator to find

  Return value:
    int             the index for a match, -1 no match
*****/
int FindWord(char *target) 
{
  int i;

#ifdef SINGLE    
  for (i = 0; i < ELEMENTCOUNT(operators); i++) {
    if (operators[i] == target[0]) {
#else
  for (i = 0; i < ELEMENTCOUNT(tokens); i++) {
    if (strcmp(tokens[i][0], target) == 0) { 
#endif
      return i;
    }
  }
  return -1;
}

BIOT · February 22, 2018, 7:13pm

econjack:
The following code uses parallel arrays or 2-dimentioned arrays. You can toggle them by using the #define SINGLE preprocessor directive. For beginners, I would think the parallel arrays is easier to understand, but I appreiciate how they can get out of sync.
#define ELEMENTCOUNT(x) (sizeof(x) / sizeof(x[0]))

#define SINGLE // comment out for single array

#ifdef SINGLE
char operators[] = {'+', '-', '*', '/'};
char *words[] = {"add", "subtract", "multiply", "divide"};
#else
char tokens[][2] = { {"+","add"}, {"-", "subtract"},
{"","multiply"}, {"/", "divide"}
};
#endif
void setup() {
Serial.begin(9600);
}

void loop() {
char c[2];
int index;

if (Serial.available() > 0) {
c[0] = Serial.read();

index = FindWord(c);

Serial.print("For operator ");
Serial.print(c);
Serial.print(" the word is ");
if (index != -1) {
#ifdef SINGLE
Serial.println(words[index]);
#else
Serial.println(tokens[index][1]);
#endif
} else {
Serial.println("not not found in list.");
}
}
}

/*****
Purpose: to search a list of operators for a match on the target operator

Parameter list:
char target the operator to find

Return value:
int the index for a match, -1 no match
*****/
int FindWord(char *target)
{
int i;

#ifdef SINGLE
for (i = 0; i < ELEMENTCOUNT(operators); i++) {
if (operators[i] == target[0]) {
#else
for (i = 0; i < ELEMENTCOUNT(tokens); i++) {
if (strcmp(tokens[i][0], target) == 0) {
#endif
return i;
}
}
return -1;
}

Just pasted the code and ran it. OMG it worked! THANK YOU SO MUCH!

Now I just need to focus for a bit to understand whats its doing ...

BIOT · February 22, 2018, 8:56pm

ELEMENTCOUNT(x) (sizeof(x) / sizeof(x[0]))

Why does this not break give the size of each element can be different (depending on whats stored in each of the two arrays?

gfvalvo · February 22, 2018, 8:59pm

The arrays are holding pointers whose size is constant regardless of the length of the string they're pointing to.

BIOT · February 22, 2018, 10:14pm

gfvalvo:
The arrays are holding pointers whose size is constant regardless of the length of the string they're pointing to.

Many Thanks. I understand.

kikiorg · October 12, 2019, 10:43am

WARNING: This line is dangerous:

#define SINGLE // comment out for single array

A #define causes a direct copy-and-paste of the first item with the second item. It happens in the compiler before the code is compiled.

Consider this

#define three 3 // This is the number 3

x = 2 + three + 4

The compiler sees:

x = 2 + 3 // This is the number 3 + 4

Note how the + 4 is commented out. The value of x is 5, not 9 because the got put 4 in the #define comment.

Put #define comments on separate lines!

anon73444976 · October 12, 2019, 11:35am

kikiorg:
WARNING: This line is dangerous:

A #define causes a direct copy-and-paste of the first item with the second item. It happens in the compiler before the code is compiled.

Consider this
#define three 3 // This is the number 3

x = 2 + three + 4



The compiler sees:

x = 2 + 3 // This is the number 3 + 4



Note how the + 4 is commented out. The value of x is 5, not 9 because the got put 4 in the #define comment.

Put #define comments on separate lines!

...written by someone who couldn't be bothered to do a 30 second experiment to check what they were typing was bollocks.
(Hint: the compiler would have told you about your missing semicolon)

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Dictionary type lookup table (array)

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