How to access an array element by using a scalar variable in C?

Question

I want to use an element of an array (in this case element 3) as a loop variable giving it a simple name like "c". (This code is for the Arduino, but should look similar in standard C.) The question is: is there any way to make the compiler accept the c = 5 statement in the for-loop?

byte array[5];
#define c  (int) (byte *) array[3]  

void setup() {
  Serial.begin(9600);
  Serial.println("this only works with a work-around:");
  Serial.print("address of array: ");
  Serial.println( (int) &array );
  Serial.println("this is the loop:");
  for ( /* what you want is: c = 5 */ 
        /* what the compiler requires: */ array[3] = 5; 
        c < 10; c++) Serial.println(c); 
  // the compiler does not accept c = 5
  Serial.println("this works but it wasts time:");
  while (c != 5) c++;
  for ( ; c < 10; c++) Serial.println(c);
}

void loop() {
}

#define c ((int) array[3])

anotherdev
– anotherdev

2014-01-12 13:13:36 +00:00
Commented Jan 12, 2014 at 13:13 — anotherdev
– anotherdev, Commented Jan 12, 2014 at 13:13

user3419174 · Accepted Answer · 2014-03-14 09:21:56Z

1

// quod licet int non licet byte
   int array[10]; // byte does not work
// ^^^ that makes the difference

#define c  (int) (byte *) array[3]  
#define d  (int) (byte *) array[4]  

void setup() {
  Serial.begin(9600);
  Serial.println("this is the loop:");
  for (c = 2; c < 5; c++) {     // this loop starts at 2
    for (d = 3; d < 6; d++) {   // this loop starts at 3
      Serial.print(c);
      Serial.print(" ");
      Serial.println(d);
    }
  }
  for (int i = 0; i < 10; i++) Serial.println(array[i]);
}

void loop() {}

answered Mar 14, 2014 at 9:21

user3419174

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Comments

archetipo · Accepted Answer · 2014-01-12 15:01:32Z

0

Hi you nedd to use atoi() function:

ascii to integer converter

 c=atoi(array[3]);

hope this help you.

answered Jan 12, 2014 at 15:01

archetipo

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Comments

user3187285 · Accepted Answer · 2014-01-20 12:48:07Z

unfortunately, I had to find it out myself. This seems to be the best way to give names to certain array elements and use them in for-loops (target is the Arduino environment).

#define SET(x,y) while (x != y) x++
/* thanks to the cleverness of the compiler this macro does not waste time at run-time */

byte array[10];
#define c  (int) (byte *) array[3]  
#define d  (int) (byte *) array[4]  

void setup() {
  Serial.begin(9600);
  Serial.println("this is the loop:");
  SET(c,2);
  for ( ; c < 5; c++) {     // this loop starts at 2
    SET(d,3);
    for ( ; d < 6; d++) {   // this loop starts at 3
      Serial.print(c);
      Serial.print(" ");
      Serial.println(d);
    }
  }
  for (int i = 0; i < 10; i++) Serial.println(array[i]);
}

void loop() {}

When you print the array contents at the end you see the last values of the loop variables c and d (5 and 6).

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