Generating a random vector of strings in C++

Question

I've got a problem with my function that's supposed to generate a vector of random strings. However, all I get is the first element in the vector is random and the rest is empty. Here's the code:

void losuj(vector <string> &vec, int ile){
    int i, j;
    string str;
    stringstream ss;
    char c;
    for(i=0; i<ile; i++){
        for(j=0; j<5; j++){
            c= rand()%25 + 97;
            ss << c;
        }
        ss >> str;
        vec.push_back(str);
        ss.str().clear();
        str.clear();
    }
}

Any thoughts on what could be wrong?

doesn't ss.str() just return a string, instead of anything else? calling clear on that wouldn't do too much, I think? — TankorSmash
– TankorSmash, Commented Jan 28, 2015 at 18:49
Lose the string stream. it isn't needed. a local str inside outer for-loop using str.append(c) would be sufficient. And <random> would make this much cleaner, btw. — WhozCraig
– WhozCraig, Commented Jan 28, 2015 at 18:50
Please use character literals, such as 'A', instead of numbers like 97. — Thomas Matthews
– Thomas Matthews, Commented Jan 28, 2015 at 18:52
My understanding is that there are 26 letters in the English alphabet, but you are generating up to 25 different possibilities. Am I missing something? — Thomas Matthews
– Thomas Matthews, Commented Jan 28, 2015 at 18:53

Daerst · Accepted Answer · 2015-01-28 19:05:17Z

1

First of all, as WhozCraig stated, you don't need to use a stringstream here. std::string provides everything you need.

The reason that your code didn't work is that your eof bit was set through the operator>>. Calling stringstream::clear() resets the bit and makes the stream usable again.

Here is your code with slight modifications, working alright. I also added the fixes suggested by Thomas Matthews:

void losuj(vector <string> &vec, int ile)
{
    int i, j;
    string str;
    char c;
    for(i=0; i<ile; i++)
    {
        for (j=0; j<5; j++)
        {
            c = rand() % 26 + 'A';
            str += c;
        }
        vec.push_back(str);
        str.clear();
    }
}

edited Jan 28, 2015 at 19:05

answered Jan 28, 2015 at 18:56

Daerst

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5 Comments

eilchner Over a year ago

Works like a charm, thanks a lot. Any idea why my version didn't work?

Daerst Over a year ago

I added some info and a link to the answer for clarification.

David Haim Over a year ago

don't forget about srand(time(NULL)) to seed your generator

eilchner Over a year ago

Thanks again. I found that ss.str().clear(); would clear the stringstream in a different question on stackoverflow. @user3613500 I've got that in main, no worries.

WhozCraig Over a year ago

@eilchner fyi, it would have worked had the stream been inside the first for-loop. And the string str in your case would be needless if you really wanted to use streams. Just an ostringstream ss inside the first loop, then pushing ss.str() after the inner loop completes would also have accomplished what you sought.

Karoly Horvath · Accepted Answer · 2015-01-28 18:57:06Z

0

You have to reset the state of the stringstream because you've reached eof:

ss.clear();

answered Jan 28, 2015 at 18:57

Karoly Horvath

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