Why i am not getting the decimal number as output? The code is explained with the help of comments

Question

// Defined three array arr0,arr1,arr2
int arr0[] = {0, 0, 0, 0, 0, 0, 0, 0};
int arr1[] = {0, 0, 0, 0, 0, 0, 0, 0};
int arr2[] = {0, 0, 0, 0, 0, 0, 0, 0};
//cord function takes binary number array as input and returns decimal 
//number to a single variable as output
int cord(int a[]) {
 int v = 0;
 for (int i = 0; i < 8; i++)
 {
 int p = 1;
 for (int j = 0; j < i; j++)
 {
 p = p * 2;
 }
 v = v + a[i] * p;
 }
 return v;
}
//function bcord takes input row & col and updates the value in arr0,arr1,arr2 to 1 wherever
// there is some value in the row and column coordinate Eg.2,0 will make 2nd index of arr0
// to 1 or 2,1 will make 2nd index of arr1 to 1
int bcord(int row[], int col[])
{
 int n = sizeof(col) / sizeof(col[0]);
 for (int i = 0; i < n; i++)
 {
 int j;
 if (col[i] == 2)
 {
 j = row[i];
 arr2[j] = 1;
 }
 if (col[i] == 1)
 {
 j = row[i];
 arr1[j] = 1;
 }
 if (col[i] == 0)
 {
 j = row[i];
 arr0[j] = 1;
 }
 }
 int led0 = cord(arr0);
 Serial.print(led0);
 int led1 = cord(arr1);
 Serial.println(led1);
 int led2 = cord(arr2);
 Serial.println(led2);
 return 0;
}
void setup()
{
 Serial.begin(9600);
 int row[] = {0, 0, 2, 2, 1};
 int col[] = {0, 1, 0, 1, 1};
 bcord(row, col);
}
void loop() {}
// Output expected is decimal numbers which are stored in led0, led1 and led2 on the serial
// monitor

• jsotola: what output are you getting?
• jsotola: your code is improperly formatted ... code that should be indented, isn't indented ... code that should not be indented, is indented ... it's a mess
• chrisl: I just used the autoindent function of the Arduino IDE. You should make more use of it, because correct indenting comes in very handy to find certain syntax errors.
• Dhruv Rastogi: Thank you jsotola, Chrisl & Edgar! But i get 57 and then 0 in next line as output. How can i get the decimal values stored in led0, led1 and led2?
• Edgar Bonet: "570" is correct: led0 is 5, led1 is 7 and led2 is 0.

Answer 1

In C++, you cannot pass an array as a parameter to a function. When you write the function prototype

int bcord(int row[], int col[], int n)

the compiler considers it a synonym of:

int bcord(int *row, int *col, int n)

In other words, both row and col are pointers, not arrays.

When you call the function like this:

bcord(row, col);

the compiler implicitly converts the call into this:

bcord(&row[0], &col[0]);

In other words, you are really passing pointers to the first element of each array. This is a standard language behavior called "decay to pointer".

When I compile your code with warnings enabled, I get the following:

In function 'int bcord(int*, int*)':
warning: 'sizeof' on array function parameter 'col'
will return size of 'int*' [-Wsizeof-array-argument]
 int n = sizeof(col) / sizeof(col[0]);
 ^
arr-ind.ino:31:30: note: declared here
 int bcord(int row[], int col[])
 ^

Thus the compiler is telling you what is wrong. The variable n is not what you expect.

The standard solution is to pass the size of the array from the caller: move the definition of n to setup(), define bcord() as:

int bcord(int row[], int col[], int n)

and call it as

bcord(row, col, n);