I am creating a simple command parser using c++, and I am trying to use istream >> to check whether I am inputting a number or a character.
input:
a = 10
b = a
parser.cpp:
string inputLine, varLeft, equal, varRight;
double varValue
// please see update below
while(getline(cin, inputLine)){
istringstream stringSplitter(inputLine);
stringSplitter >> varLeft >> equal;
if(!(stringSplitter >> varValue)){
stringSplitter >> varRight;
}
}
The goal is, later in the code, if varRight is empty, I assume that the line of input was a double literal, otherwise, it's a string that denotes a variable. I understand there might be logical errors associated with mixed input starting with digits, but I am assuming that all input is correctly formatted for now. Why is the a in the second line of input discarded? What fix do you propose?
Update
The problem is not with the while loop statement, it is with the if statement at the end of the while code block.
In the actual code, the loop is not actually a while loop; I am using a vector object holding string commands, and iterating through them using a for loop which goes through the vector using iterators. But, in order to please the commenters, I have fixed it above.
If an input function fails, the stream will not allow any more extractions until you clear the failure state. You'll need to do that, after you've checked that the input to varValue failed, with std::basic_ios::clear:
if(!(stringSplitter >> varValue)){
stringSplitter.clear();
stringSplitter >> varRight;
}
I don't know how you're doing /* not end of input */ at the moment (hopefully you're not checking eof()!), but it's recommended that you do:
while (getline(cin, inputLine)) {
// ...
}
This checks that the line input was successful before diving into the loop.
Related
I have a problem with my code in C++ and need some help. There are some related questions but I couldn't really understand the answers.
I have a text file ('parameters.dat' in the example below) that I want to read in with my code written in C++. The file includes different types of variables: Boolean, doubles and integers as well as some comments which I want to skip when reading.
My file looks something like that:
150 // this is an integer
4e-1 // this is a double
1.05 // another double
0 // this is a logical (Boolean) variable: 0 is false and 1 is true
A simple version of the code that I use is
int N;
double var_1, var_2;
bool inital;
ifstream read_parameters;
read_parameters.open("parameters.dat");
read_parameters >> N >> var_1 >> var_2 >> initial;
read_parameters.close();
The comments seem to ruin everything and even without them there seem to be some problems with reading the logical variables correctly. The file that I try to read is made by me, so I can substitute the '//' above with something else if necessary. Does anyone have any advice?
Thanks in advance!
Simple, cheesy way:
Read a token then read_parameters.ignore(numeric_limits<streamsize>::max(), '\n') to discard the rest of the line. eg:
read_parameters >> N;
read_parameters.ignore(numeric_limits<streamsize>::max(), '\n');
read_parameters >> var_1;
read_parameters.ignore(numeric_limits<streamsize>::max(), '\n');
...
This doesn't care if a comment exists or not, but requires modification if you have two or more tokens on a line.
Oh, and remember to test the state of the stream after reading. Plugging in "fubar" for one of the doubles will currently ruin things. read_parameters will be in an error state that needs to be cleared before you can read it again.
if (!read_parameters >> N)
{
std::cerr << "bad input for parameter N" << std::endl;
read_parameters.clear();
}
read_parameters.ignore(numeric_limits<streamsize>::max(), '\n');
Is better, but you probably want to handle an error with something better than a printline.
Take input in a Dynamic Character array then run a for loop in which if a character has ascii 65 - 97 - onward then it will be Alphabet and else if ascii represents integers like 1,2,3 then separte them in another array after calculating total integers as count++ .
I am writing a code that tally when an integer data type or character is entered.
int numero,
countInteger = 0;
countCharacter = 0;
while ( 1 ) {
try {
cin >> numero;
cout << numero;
throw numero;
} catch (...) {
countCharacter++;
cout << "Error";
}
}
If I entered Integer, counter in "countInteger" (but not show it in the code). If I enter a character, it is aa exception and recorded in "countCharacter".
But when I run the code generates an infinite loop and does not allow me to re-enter again. They could help me please. Guide me, you may have a bad concept.
When you try to read an integer, and you give something that's not an integer as input, there are two things happening: The first is that the stream gets its failbit set, the second things that happens is that the input is not extracted. So next iteration you read the same input again, and again and again...
I suggest another tactic: Read as a character, then see if it is a digit, an alphabetic character, or something else completely. Optionally, if you need the actual full number, read as a string, and try to convert to an integer.
A clarification: Input using std::cin is buffered. When you use the input operator >> then std::cin extracts characters from the buffer. If you try to read a number, but the first character in the buffer is not a digit, then the input operator will fail, and leave the character in the buffer.
Simple (hopefully) example:
Lets say you have this code
int number;
std::cin >> number;
std::cin >> number;
std::cin >> number;
As input for that part of the code, you enter
123abc
The first input will read 123 from the input, and stop at the letter, leaving the input as
abc
Now we come to the second input, and the code will see that the first character is not a digit, so it will set the failbit in the stream and leave the input as is:
abc
Then with the third input, the exact same thing as in the second happen.
Now imagine this was in a loop instead, the input operator >> will iteration after iteration see the non-digit input an promptly return, effectively giving you an infinite loop.
Now for a clarification of my suggestion... Depending on the goals and requirements of the program, you can instead read into a character and use the character classification functions to see what types you have.
Something like
int countDigit = 0;
int countCharacter = 0;
char ch;
while (std::cin >> ch)
{
if (std::isdigit(ch))
++countDigit;
else if (std::isalpha(ch))
++countCharacter;
else
{
// Not a digit or an alphabetic character
// I.e. newlines, spaces, control characters, etc.
}
}
Finally a note about using exceptions for this: Most people would consider it bad. Exceptions are for exceptions, exceptional cases, not as part of the normal flow of the program. Throwing an exception is expensive and disrupts the normal flow. Only use them for exceptional things, like errors.
I have to loops to gather input, the first gathers input into a vector of doubles...
double input;
while (cin >> input)
{
list.push_back(input);
}
and the second gathers input into a vector of ints...
int input;
while (cin >> input)
{
list.push_back(input);
}
The second loop keeps auto-exiting and so I added the following two lines...
cin.clear();
cin.ignore(INT_MAX,'\n'); // I've also tried cin.ignore()
However this has occurred in my output being discarded in the second loop. How can I get both of these to work the way they need to?
The first loop reads until the stream goes bad because there isn't any double to read from. When the stream has gone bad it won't become good again unless you do something, e.g., using std::cin.clear() to clear the state flags.
Of course, just clearing the state flags won't make much of a difference because it would have read all viable numbers: the format of valid doubles is a superset of the format of valid ints (well, OK, unless they are using base 16). That is, you'll need some sort of separator, probably a non-space, non-digit character. Your use of ignore() should skip over a separator and up to the next newline (although the magic value isn't INT_MAX but std::numeric_limits<std::streamsize>::max()).
It is unclear what you are trying to input but if you use something like
1 2 3 exit 4 5 6
the first three values would be read as double and everything else would be ignored. That is, you may want to be ignore characters a bit more careful, e.g., clear the input and keep trying to read an int and clear() and ignore() until this is successful:
// skip separator:
std::cin.clear();
int input;
while (!(std::cin >> input)) {
std::cin.clear();
std::cin.ignore(); // ignore the next character only
}
do {
list.push_back(input);
} while (std::cin >> input);
I'm trying to read in a file that should contain only numbers in it. I can successfully read in the entire file if it meets that criteria, but if it so happened to have a letter in it, I need to return false with an error statement.
The problem is I'm finding it hard for my program to error when it finds this character. It can find it no problem, but when it does, it decides to just skip over it.
My code to read in the file and attempt to read in only numbers:
bool compute::Read (ifstream& stream)
{
double value;
string line;
int lineNumber = 1;
if (stream)
{
while (getline(stream, line))
{
lineNumber++;
istringstream strStream(line);
while (strStream >> value)
{
cout << value << endl;
}
}
}
return true;
}
The input file which I use for this is
70.5 61.2 A8 10.2
2
Notice that there is a non-number character in my input file. It should fail and return false at that point.
Currently, all it does is once it hits the "A", it simply returns to the next line, continuing the getline while loop.
Any help with this would be much appreciated.
The stringstream does catch those errors, but you're doing nothing to stop the enclosing loop from continuing when an error is found. You need to tailor your main loop so that it stops when the stringstream finds an error, which you can't do if the stringstream is being reconstructed on each iteration. You should create a for() loop instead and construct the stringstream in the declaration part. And the condition to the loop should be "as long as the stringstream and stream do not catch an error". For example:
for (std::istringstream iss; iss && std::getline(stream, line);)
{
iss.clear();
iss.str(line);
while (iss >> value)
{
std::cout << value << '\n';
}
}
Futhermore, it doesn't look like you need to use std::getline() or std::istringstream if you just want to print each value. Just do:
while (stream >> value) {
std::cout << value << '\n';
}
The above will stop when it finds an invalid character for a double.
You need the code to stop streaming but return false if it hasn't yet reached the end of the "input".
One way, possibly not the most efficient but still one way, to do that is parse a word at a time.
If you read first into a std::string and if it works (so the string is not empty) create an istringstream from that string, or reuse an existing one, and try streaming that into a double value.
If that fails, you have an invalid character.
Of course you can read a line at a time from the file, then split that into words, so that you can output a meaningful error message showing what line the bad text was found.
The issue of reading straight into doubles is that the stream will fail when it reaches end of file.
However it is possible to workaround that too because the reason for failing has an error status which you can check, i.e. you can check if it eofbit is set. Although the f in eofbit stands for "file" it applies to any stream not just files.
Although this method may sound better than reading words into a string first, I prefer that method in normal circumstances because you want to be able to report the error so you'll want to print in the error what was read.
I'm dealing with a problem using eof().
using
string name;
int number, n=0;
while(!in.eof())
{
in >> name >> number;
//part of code that puts into object array
n++;
}
sounds normal to me as it whenever there are no more text in the file.
But what I get is n being 4200317. When I view the array entries, I see the first ones ats the ones in the file and other being 0s.
What could be the problem and how should I solve it? Maybe there's an alternative to this reading problem (having undefined number of lines)
The correct way:
string name;
int number;
int n = 0;
while(in >> name >> number)
{
// The loop will only be entered if the name and number are correctly
// read from the input stream. If either fail then the state of the
// stream is set to bad and then the while loop will not be entered.
// This works because the result of the >> operator is the std::istream
// When an istream is used in a boolean context its is converted into
// a type that can be used in a boolean context using the isgood() to
// check its state. If the state is good it will be converted to an objet
// that can be considered to be true.
//part of code that puts into object array
n++;
}
Why your code fails:
string name;
int number, n=0;
while(!in.eof())
{
// If you are on the last line of the file.
// This will read the last line. BUT it will not read past
// the end of file. So it will read the last line leaving no
// more data but it will NOT set the EOF flag.
// Thus it will reenter the loop one last time
// This last time it will fail to read any data and set the EOF flag
// But you are now in the loop so it will still processes all the
// commands that happen after this.
in >> name >> number;
// To prevent anything bad.
// You must check the state of the stream after using it:
if (!in)
{
break; // or fix as appropriate.
}
// Only do work if the read worked correctly.
n++;
}
in << name << number;
This looks like writing, not reading.
Am I wrong?
int number, n = 0;
You weren't initializing n, and you seem to have a typo.
This probably would be more correct
string name;
int number, n = 0;
while (in >> name && in >> number)
{
n++;
}
The eof is a bad practice.
Note that there is a subtle difference here from your code: your code ended when it encountered an eof or silently looped for infinite time if it found a wrong line (Hello World for example), this code ends when it encounters a non correctly formatted "tuple" of name + number or the file ends (or there are other errors, like disconnecting the disk during the operation :-) ). If you want to check if the file was read correctly, after the while you can check if in.eof() is true. If it's true, then all the file was read correctly.