Parsing Inputs

Hi there! I am using the Jumping into C++ book as a guide to start writing C++. In the first examples, we are asked to read in some numbers and print them out. If I set the variable type to read to an int, like this: cin >> my_int;

But then when prompted to enter the input I type 2.2 (not an integer), then the rest of the line (the ".2") is stored in the input buffer and screws up later requests for data.

Could someone please recommend a good way to safely get input from the command line, check it type correctness, and throw away extra data?

thanks so much!
Aaron

Like this:

Code:

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if(!cin)
    cin.clear();
cin.ignore(numeric_limits<streamsize>::max(),'\n'); //#include <limits> for this

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Quote:

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Originally Posted by manasij7479

Like this:

Code:

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if(!cin)
    cin.clear();
cin.ignore(numeric_limits<streamsize>::max(),'\n'); //#include <limits> for this

---

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Thank you! What does the if (!cin) check for?
Aaron

Quote:

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Originally Posted by aaleclaire

What does the if (!cin) check for?

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Whether the input stream is in an error state.

Ah, okay. I will study the input error states some more later. Now, the cin.ignore will ignore and throw away the fraction part of a number if the cin is directed to a type int? I will try it later today (once I get done with my "real" job :)).

Yes, it will "flush" the input buffer.

Quote:

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Originally Posted by aaleclaire

Could someone please recommend a good way to safely get input from the command line, check it type correctness, and throw away extra data?

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The way I recommend is to read the line into a string, then parse the string to extract the specific input. This way, you don't need to "throw away extra data" because the next read will read the next line, rather than whatever is left on the line. This also avoids the problem where you read into an int, then try to read the next line into a string, only to find that you read nothing into the string because you ended up reading the rest of the previous line instead of the next line.

Unfortunately, doing this, while simple, requires more stuff that you probably don't know, unlike manasij7479's suggestion, e.g.,

Code:

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// Headers to include
#include <iostream>
#include <string>
#include <sstream>

// Read a line from standard input.
std::string line;
std::getline(std::cin, line);
// Create an input string stream from the line to extract the int
std::istringstream ss(line);
int my_int;
if ((ss >> my_int) && ss.eof())
{
    // Use my_int
}
else
{
    // Could not extract the int, or there was invalid input after extraction.
}

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Thank you laserlight! I am using getline() to read in the command line as you suggest. The part that I have not seen before is istringstream, but I get the idea. Is there a generic way to read in the command line (user data) and parse the various strings separated by some delimiter into a vector? For example, if the input were 2 2.2 abc, then the vector vec[0] = 2, vec[1] = 2.2, and vec[2] = "abc". That way the length of the vector would tell me if I got the right number of parameters. Then I could use your recommendation to check the types of each of the parameters. I am also open to other suggestions.
Thanks! aaleclaire.

You could use the delimiter of getline, or in this case as your tokens are separated by whitespace, use operator>> to read into a std::string.

Thanks again. I will give it a try. This is my second day learning C++, so I appreciate the help! I am a hardware guy who needs a change!