Restarting a for loop after getting a wrong input [duplicate] - c++

Question

I am interested in discussing methods for using stringstream to parse a line with multiple types. I would begin by looking at the following line:
"2.832 1.3067 nana 1.678"
Now lets assume I have a long line that has multiple strings and doubles. The obvious way to solve this is to tokenize the string and then check converting each one. I am interested in skipping this second step and using stringstream directly to only find the numbers.
I figured a good way to approach this would be to read through the string and check if the failbit has been set, which it will if I try to parse a string into a double.
Say I have the following code:
string a("2.832 1.3067 nana 1.678");
stringstream parser;
parser.str(a);
for (int i = 0; i < 4; ++i)
{
double b;
parser >> b;
if (parser.fail())
{
std::cout << "Failed!" << std::endl;
parser.clear();
}
std::cout << b << std::endl;
}
It will print out the following:
2.832
1.3067
Failed!
0
Failed!
0
I am not surprised that it fails to parse a string, but what is happening internally such that it fails to clear its failbit and parse the next number?

Answer 1

The following code works well to skip the bad word and collect the valid double values
istringstream iss("2.832 1.3067 nana 1.678");
double num = 0;
while(iss >> num || !iss.eof()) {
if(iss.fail()) {
iss.clear();
string dummy;
iss >> dummy;
continue;
}
cout << num << endl;
}
Here's a fully working sample.
Your sample almost got it right, it was just missing to consume the invalid input field from the stream after detecting it's wrong format
if (parser.fail()) {
std::cout << "Failed!" << std::endl;
parser.clear();
string dummy;
parser >> dummy;
}
In your case the extraction will try to read again from "nana" for the last iteration, hence the last two lines in the output.
Also note the trickery about iostream::fail() and how to actually test for iostream::eof() in my 1st sample. There's a well known Q&A, why simple testing for EOF as a loop condition is considered wrong. And it answers well, how to break the input loop when unexpected/invalid values were encountered. But just how to skip/ignore invalid input fields isn't explained there (and wasn't asked for).

Answer 2

Few minor differences to πάντα ῥεῖ's answer - makes it also handle e.g. negative number representations etc., as well as being - IMHO - a little simpler to read.
#include <iostream>
#include <sstream>
#include <string>
int main()
{
std::istringstream iss("2.832 1.3067 nana1.678 x-1E2 xxx.05 meh.ugh");
double num = 0;
for (; iss; )
if (iss >> num)
std::cout << num << '\n';
else if (!iss.eof())
{
iss.clear();
iss.ignore(1);
}
}
Output:
2.832
1.3067
1.678
-100
0.05
(see it running here)

Answer 3

I have built up a more fine tuned version for this, that is able to skip invalid input character wise (without need to separate double numbers with whitespace characters):
#include <iostream>
#include <sstream>
#include <string>
using namespace std;
int main() {
istringstream iss("2.832 1.3067 nana1.678 xxx.05 meh.ugh");
double num = 0;
while(iss >> num || !iss.eof()) {
if(iss.fail()) {
iss.clear();
while(iss) {
char dummy = iss.peek();
if(std::isdigit(dummy) || dummy == '.') {
// Stop consuming invalid double characters
break;
}
else {
iss >> dummy; // Consume invalid double characters
}
}
continue;
}
cout << num << endl;
}
return 0;
}
Output
2.832
1.3067
1.678
0.05
Live Demo

Answer 4

If you like concision - here's another option that (ab?)uses && to get cout done only when a number's been parsed successfully, and when a number isn't parsed it uses the comma operator to be able to clear() stream error state inside the conditional before reading a character to be ignored...
#include <iostream>
#include <sstream>
#include <string>
int main()
{
std::istringstream iss("2.832 1.3067 nana1.678 x-1E2 xxx.05 meh.ugh");
double num = 0;
char ignored;
while (iss >> num && std::cout << num << '\n' ||
(iss.clear(), iss) >> ignored)
;
}
http://ideone.com/WvtvfU

Answer 5

You can use std::istringstream::eof() to validate input like this:
#include <string>
#include <sstream>
#include <iostream>
// remove white-space from each end of a std::string
inline std::string& trim(std::string& s, const char* t = " \t")
{
s.erase(s.find_last_not_of(t) + 1);
s.erase(0, s.find_first_not_of(t));
return s;
}
// serial input
std::istringstream in1(R"~(
2.34 3 3.f 3.d .75 0 wibble
)~");
// line input
std::istringstream in2(R"~(
2.34
3
3.f
3.d
.75
0
wibble
)~");
int main()
{
std::string input;
// NOTE: This technique will not work if input is empty
// or contains only white-space characters. Therefore
// it is safe to use after a conditional extraction
// operation >> but it is not reliable after std::getline()
// without further checks.
while(in1 >> input)
{
// input will not be empty and will not contain white-space.
double d;
if((std::istringstream(input) >> d >> std::ws).eof())
{
// d is a valid double
std::cout << "d1: " << d << '\n';
}
}
std::cout << '\n';
while(std::getline(in2, input))
{
// eliminate blank lines and lines
// containing only white-space (trim())
if(trim(input).empty())
continue;
// NOW this is safe to use
double d;
if((std::istringstream(input) >> d >> std::ws).eof())
{
// d is a valid double
std::cout << "d2: " << d << '\n';
}
}
}
This works because the eof() check ensures that only the double was entered and not garbage like 12d4.