I've been having an issue with parsing through a file and the use of seekg(). Whenever a certain character is reached in a file, I want to loop until a condition is met. The loops works fine for the first iteration but, when it loops back, the file seemingly skips a character and causes the loop to not behave as expected.
Specifically, the loop works fine if it is all contained in one line in the file, but fails when there is at least one newline within the loop in the file.
I should mention I am working on this on Windows, and I feel like the issue arises from how Windows ends lines with \r\n.
Using seekg(-2, std::ios::cur) after looping back fixes the issue when the beginning loop condition is immediately followed by a newline, but does not work for a loop contained in the same line.
The code is structured by having an Interpreter class hold the file pointer and relevant variables, such as the current line and column. This class also has a functional map defined like so:
// Define function type for command map
typedef void (Interpreter::*function)(void);
// Map for all the commands
std::map<char, function> command_map = {
{'+', increment_cell},
{'-', decrement_cell},
{'>', increment_ptr},
{'<', decrement_ptr},
{'.', output},
{',', input},
{'[', begin_loop},
{']', end_loop},
{' ', next_col},
{'\n', next_line}
};
It iterates through each character, deciding if it has functionality or not in the following function:
// Iterating through the file
void Interpreter::run() {
char current_char;
if(!this->file.eof() && this->file.good()) {
while(this->file.get(current_char)) {
// Make sure character is functional command (ie not a comment)
if(this->command_map.find(current_char) != this->command_map.end()) {
// Print the current command if in debug mode
if(this->debug_mode && current_char != ' ' && current_char != '\n') {
std::cout << this->filename << ":" << this->line << ":"
<< this->column << ": " << current_char << std::endl;
}
// Execute the command
(this->*(command_map[current_char]))();
}
// If it is not a functional command, it is a comment. The rest of the line is ignored
else{
std::string temp_line = "";
std::getline(file, temp_line);
this->line++;
this->column = 0;
}
this->temp_pos = file.tellg();
this->column++;
}
}
else {
std::cout << "Unable to find file " << this->filename << "." << std::endl;
exit(1);
}
file.close();
}
The beginning of the loop (signaled by a '[' char) sets the beginning loop position to this->temp_pos:
void Interpreter::begin_loop() {
this->loop_begin_pointer = this->temp_pos;
this->loop_begin_line = this->line;
this->loop_begin_col = this->column;
this->run();
}
When the end of the loop (signaled by a ']' char) is reached, if the condition for ending the loop is not met, the file cursor position is set back to the beginning of the loop:
void Interpreter::end_loop() {
// If the cell's value is 0, we can end the loop
if(this->char_array[this->char_ptr] == 0) {
this->loop_begin_pointer = -1;
}
// Otherwise, go back to the beginning of the loop
if(this->loop_begin_pointer > -1){
this->file.seekg(this->loop_begin_pointer, std::ios::beg);
this->line = this->loop_begin_line;
this->column = this->loop_begin_col;
}
}
I was able to put in debugging information and can show stack traces for further clarity on the issue.
Stack trace with one line loop ( ++[->+<] ):
+ + [ - > + < ] [ - > + < ] done.
This works as intended.
Loop with multiple lines:
++[
-
>
+<]
Stack trace:
+ + [ - > + < ] > + < ] <- when it looped back, it "skipped" '[' and '-' characters.
This loops forever since the end condition is never met (ie the value of the first cell is never 0 since it never gets decremented).
Oddly enough, the following works:
++[
-
>+<]
It follows the same stack trace as the first example. This working and the last example not working is what has made this problem hard for me to solve.
Please let me know if more information is needed about how the program is supposed to work or its outputs. Sorry for the lengthy post, I just want to be as clear as possible.
Edit 1:
The class has the file object as std::ifstream file;.
In the constructor, it is opened with
this->file.open(filename), where filename is passed in as an argument.
