sstream not working properly?

Question

I am using sstream to parse a string. The string contains an arbitrary number of integers separated by spaces. But sstream is not parsing the input properly. Here's my code-

#include<cstdio>
#include<cstring>
#include<vector>
#include<cstdlib>
#include<sstream>
#include<iostream>

using namespace std;

vector<int> val[10010];

int main(){

    int n,i,j,temp;
    stringstream stream;
    string s;
    scanf("%d",&n);
    vector<int>::iterator it;

    for(i=0; i<n; i++){
        getline(cin,s);
        stream.str(s);

        while(1) {
            stream >> temp;
            val[i].push_back(temp);
            if(!stream)
                break;
        }

        for(it=val[i].begin(); it!=val[i].end(); it++)
            printf("%d ",*it);

        printf("\n");
    }
    return 0;
}

Here are the test cases : Code

Why are *crossing the streams*? If you are going to use C++ streams for parsing, you can also use them for input and output. — Thomas Matthews, Mar 07 '14 at 17:13

marcinj · Answer 1 · 2014-03-07T17:43:56.473

1

Not sure if this is your main error, but you have a \n - new line after scanf("%d",&n);, scanf is only grabbing integer, new line is left and getline is not reading your input.

You can ignore this new line with below, just before calling getline:

if ( isspace(cin.peek()) )
  cin.ignore();

also, its better to:

if ( stream >> temp ) {
   // do something with temp
}

now you always add string with previously parsed value as last element.

Also, you should clear stringstream before using it again, otherwise it stays in error state and you are not able to use it. Add this: stream.clear(); before stream.str(s);.

edited Mar 07 '14 at 17:43

answered Mar 07 '14 at 17:25

marcinj

48,511
9
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Yeah. Got the error thanks. What difference does if( stream >> temp ) make? – Abhishek Modi Mar 07 '14 at 17:39
it prevents you from inserting temp to vector if stream failed to parse it. Currently it will contain previous parsed value, so you will have duplicated values. – marcinj Mar 07 '14 at 17:43
Still having problems. Here are some values I tried http://ideone.com/vfLQhl – Abhishek Modi Mar 07 '14 at 18:04
your code looks fine, have you tried testing it outside ideaone? Maybe in ideaone there are actualy \r\n and not \n chars? – marcinj Mar 07 '14 at 18:19
Yes, it works fine. Some spaces crept in while copy pasting. That gave the error. – Abhishek Modi Mar 07 '14 at 21:37

score 0 · Answer 2 · answered Mar 07 '14 at 17:59

Add cin.get (); after your scanf -- this consumes the \n at the end of your line of input where you're reading in n. After that, you'll stop getting nonsense in your vector. cin.ignore(SOME_BIG_NUMBER) might also work, skipping not just the next char (likely \n) but also spaces preceding it.

There's one other fix you need: in the innermost loop, if(!stream) break; must come before val[i].push_back(temp);, or you'll get an extra copy of the last int on your input line.

I further recommend that you either use stream I/O or C-style I/O (printf, scanf) and not both -- but that's not your problem here.

This is incorrect on several levels. First, `cin.get()` doesn't consume the `'\n'`; it consumes the next character (which might be a trailing space). You absolutely need to use the ignore. Except, of course, there's no point in the first input anyway. And there's no point in putting the read from the stream in the loop, and then using `while`; there are much better ways of writing this. — James Kanze, Mar 07 '14 at 18:38

score 0 · Accepted Answer · answered Mar 07 '14 at 18:35

What's happening is clear. The first line you read is empty (because you've left the '\n' in the stream when reading the count), then you parse it with stream, ending with stream in a failed state. And once stream is in a failed state, it stays that way, and all further operations are no-ops.

More generally, there are a number of problems with your code:

Don't mix C style IO and iostream, especially not on the same file. Sometimes it's unavoidable, when you have to deal with legacy libraries, but don't do it intentionally. In fact, there's never any reason to use C style IO in C++.
Don't define all of your variables at the top. Don't define them until you need them. In particular, there's no point in declaring the string stream until you're in the loop (which would avoid the problem you're seeing).
Do check that std::getline has succeeded before using the values you have read.

The idiomatic way of doing something like this would use a vector of vectors, with push_back:

std::vector<std::vector<int>> val;

//   and for the loop...
std::string line;
while ( std::getline( std::cin, line ) ) {
    val.push_back( std::vector<int>() );
    std::istringstream parse( line );
    int i;
    while ( parse >> i ) {
        val.back().push_back( i );
    }
}

Except, perhaps, with more error checking. (Did the parse >> i in the while fail because it has read all of the line, or because some one slipped an "abc" in as a number?)

sstream not working properly?

3 Answers3