Matching word c++ program using getline() running infinitely? - c++

Question

I am learning c++ so bear with me and apologize for any idiocy beforehand.
I am trying to write some code that matches the first word on each line in a file called "command.txt" to either "num_lines", "num_words", or "num_chars".
If the first word of the first line does not match the previously mentioned words, it reads the next line.
Once it hits a matching word (first words only!) it prints out the matching word.
Here is all of my code:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
ifstream comm_in("commands.txt"); // opens file
string command_name = "hi"; // stores command from file
bool is_command() {
if (command_name == "num_words" || command_name == "num_chars" || command_name == "num_lines") {
return true;
} else {
return false;
}
}
// FIND a first word of a line in file THAT MATCHES "num_words", "num_chars" or "num_lines"
void get_command() {
string line;
char c;
while (!is_command()) { // if command_name does not match a command
// GET NEXT LINE OF FILE TO STRING
getline(comm_in, line);
// SUPPOSED TO GET THE FIRST WORD OF A STRING (CANT USE SSTREAM)
for (int i = 0; i < line.size(); i++) { // increment through line
c = line[i]; // assign c as index value of line
if (c == ' ' || c == '\t') { // if c is a space/tab
break; // end for loop
} else {
command_name += c; // concatenate c to command_name
} // if
} // for
} // while
return;
}
int main() {
get_command();
cout << command_name; // supposed to print "num_lines"
}
The contents of the command.txt file:
my bear is happy
and that it
great ha
num_lines sigh
It compiles properly, but when I run it in my terminal, nothing shows up; it doesn't seem to ever stop loading.
How can I fix this?

Answer 1

Unless you really want to hate yourself in the morning (so to speak) you want to get out of the habit of using global variables. You'll also almost certainly find life easier if you break get_command into (at least) two functions, one specifically to get the first word from the string containing the line.
I'd write the code more like this:
bool is_cmd(std::string const &s) {
return s == "num_words" || s == "num_chars" || s == "num_lines";
}
std::string first_word(std::istream &is) {
std::string line, ret;
if (std::getline(is, line)) {
auto start = line.find_first_not_of(" \t");
auto end = line.find_first_of(" \t", start);
ret = line.substr(start, end - start);
}
return ret;
}
void get_command(std::istream &is) {
std::string cmd;
while (!(cmd = first_word(is)).empty())
if (is_cmd(cmd)) {
std::cout << cmd;
break;
}
}
This still isn't perfect (e.g., badly formed input could still cause it to fail) but at least it's a move in what I'd say is a better direction.

Answer 2

If something goes wrong and you reach the end of file the loop will never stop. You should change getline(comm_in, line) to if(!getline(comm_in, line)) break;, or better yet, use that as the condition for the loop.
You also have to reset command_name for each pass:
while(getline(comm_in, line))
{
command_name = "";
for(int i = 0; i < line.size(); i++)
{
c = line[i];
if(c == ' ' || c == '\t')
break;
else
command_name += c;
}
if(is_command())
break;
}

Answer 3

// FIND a first word of a line in file THAT MATCHES "num_words", "num_chars" or "num_lines"
void get_command()
{
string line;
char c;
while (!is_command()) { // if command_name does not match a command
// GET NEXT LINE OF FILE TO STRING
if(getline(comm_in, line),comm_in.fail()){
// end reading
break;
}
//clear
command_name = "";
// SUPPOSED TO GET THE FIRST WORD OF A STRING (CANT USE SSTREAM)
for (int i = 0; i < line.size(); i++) { // increment through line
c = line[i]; // assign c as index value of line
if (c == ' ' || c == '\t') { // if c is a space/tab
break; // end for loop
} else {
command_name += c; // concatenate c to command_name
} // if
} // for
} // while
return;
}
The key of this problem is that you didn't clear the command_name.
What's more, you have to add a judge about whether reaching the end of the file.
ps: if(getline(comm_in, line),comm_in.fail()) is equal to if(getline(comm_in, line)),