How do you access data in a const char **& in C++? - c++

Question

example code:
const char* list[] = {"Elem_E", "Elem_T", "Elem_R", "Turtle", "Rabbit"};
const char ** patterns=0;
.
.
.
bool sec_run = false;
patterns = list;
process_data(patterns, sec_run);
process_data function:
process_data(const char **& pattern, bool sec_run){
.
.
some_variable=0;
for(int i; i < num_patterns;++i){
if(!sec_run){
some_variable = *pattern[i];
}
else{
if(/* list element contains "_" */)continue;
some_variable= /*letter after "_" */
}
if(some_variable == 'E') multiplier = 0;
else if(some_variable == 'T') multiplier = 1;
else if(some_variable == 'R') multiplier = 2;
}
}
So there is the base of what I'm trying to do. I cannot change signature for process_data. To start i do not get how some_variable = *pattern[i]; returns E,T, or R, and I cannot figure out how to iteratively access the full elements in the list. ie "Elem_E" to check for underscore and parse off the E.
I have little background in C++, but have used C numerous times. I am having a difficult time finding visual representation for char **& to help with based understanding of what is going on here, if you can point in the direction of a good tutorial with visual that will also suffice.
Sorry for confusion, forgot quotes in the list.

Answer 1

In C++, reading a parameter passed by reference (with the &) works the same as reading a parameter passed by value (without the &). The difference happens when you assign to the parameter. If the parameter was passed by value then the assignment is only visible inside the function but if it was passed by reference the assignment will be visible outside.
int mynumber = 0;
void foo(int &x)
{
printf("%d\n", x); //prints 0;
x = 10;
}
int main()
{
foo(mynumber);
printf("%d\n", mynumber); // prints 10
}
The equivalent to this in plain C would be to make the x parameter into a pointer and add the required *s and &s:
int mynumber = 0;
void foo(int *x)
{
printf("%d\n", *x);
*x = 10;
}
int main()
{
foo(&mynumber);
printf("%d\n", mynumber); // prints 10
}
Coming back to your code, I don't really know how to solve all your problems (what does the constant Elem_E mean? Is your list NULL terminated or is there a length stored somewhere?) but what I can say is that as long as you don't want to change the patterns global variable from inside process_data, using a char **& will be the same as using a char **.

Answer 2

I don't know how some_variable and multiplier will be used, but I made these changes to calculate them for each string in the list. The variable sec_run is not required in this approach. If no match is found, some_variable and multiplier are set to default values of '\0' and -1.
Output:
item=Elem_E some_variable=E multiplier=0
item=Elem_T some_variable=T multiplier=1
item=Elem_R some_variable=R multiplier=2
item=Turtle some_variable= multiplier=-1
item=Rabbit some_variable= multiplier=-1
Code:
void process_data(const char **& pattern, int num_patterns)
{
const char * item;
for (int i = 0; i < num_patterns; ++i)
{
item = pattern[i];
if ( item == NULL ) continue;
char some_variable = '\0'; // set to default for no match
int multiplier = -1; // set to default for no match
int len = strlen(item);
for (int j = 0; j < len; ++j)
{
if (item[j] == '_' && j + 1 < len)
some_variable = item[j + 1]; /*letter after "_" */
}
if (some_variable == 'E') multiplier = 0;
else if (some_variable == 'T') multiplier = 1;
else if (some_variable == 'R') multiplier = 2;
cout << "item=" << item << " some_variable=" << some_variable << " multiplier=" << multiplier << endl;
}
}
void pattern_test()
{
const char* list[] = { "Elem_E", "Elem_T", "Elem_R", "Turtle", "Rabbit" };
const char ** patterns = list;
// trick to calculate array length
// length of entire array divided by length of one element
int num_length = sizeof(list) / sizeof(list[0]);
process_data(patterns, num_length);
}