如何将矢量转换为字符串并转换回矢量

Question

----------------- EDIT -----------------------

Based on juanchopanza's comment : I edit the title

Based on jrok's comment : I'm using ofstream to write, and ifstream to read.

I'm writing 2 programs, first program do the following tasks :

1. Has a vector of integers
2. convert it into array of string
3. write it in a file

The code of the first program :

vector<int> v = {10, 200, 3000, 40000};
int i;
stringstream sw;
string stringword;

cout << "Original vector = ";
for (i=0;i<v.size();i++) 
{
     cout << v.at(i) << " " ;
}
cout << endl;

for (i=0;i<v.size();i++) 
{
    sw << v[i];
}
stringword = sw.str();
cout << "Vector in array of string : "<< stringword << endl;

ofstream myfile;
myfile.open ("writtentext");
myfile << stringword;
myfile.close();

The output of the first program :

Original vector : 10 200 3000 40000
Vector in string : 10200300040000
Writing to File ..... 

second program will do the following tasks :

1. read the file
2. convert the array of string back into original vector

----------------- EDIT -----------------------

Now the writing and reading is fine, thanks to Shark and Jrok,I am using a comma as a separator. The output of first program :

Vector in string : 10,200,3000,40000,

Then I wrote the rest of 2nd program :

string stringword;

ifstream myfile;
myfile.open ("writtentext");
getline (myfile,stringword);
cout << "Read From File = " << stringword << endl;

cout << "Convert back to vector = " ;
for (int i=0;i<stringword.length();i++)
{
    if (stringword.find(','))
    {
        int value;
        istringstream (stringword) >> value;
        v.push_back(value);
        stringword.erase(0, stringword.find(','));
    }
}
for (int j=0;j<v.size();i++) 
{
    cout << v.at(i) << " " ;
}

But it can only convert and push back the first element, the rest is erased. Here is the output :

Read From File = 10,200,3000,40000,
Convert back to vector = 10

What did I do wrong? Thanks

Answer 1

The easiest thing would be to insert a space character as a separator when you're writing, as that's the default separator for operator>>

sw << v[i] << ' ';

Now you can read back into an int variable directly, formatted stream input will do the conversion for you automatically. Use vector's push_back method to add values to it as you go.

Answer 2

Yes, this question is over a year old, and probably completely irrelevant to the original asker, but Google led me here so it might lead others here too.

When posting, please post a complete minimal working example , having to add #include and main and stuff is time better spent helping. It's also important because of your very problem.

Why your second code isn't working is all in this block

for (int i=0;i<stringword.length();i++)
{
    if (stringword.find(','))
    {
        int value;
        istringstream (stringword) >> value;
        v.push_back(value);
        stringword.erase(0, stringword.find(','));
    }
}

1. istringstream (stringword) >> value interprets the data up to the comma as an integer, the first value, which is then stored.

2. stringword.find(',') gets you the 0-indexed position of the comma. A return value of 0 means that the character is the first character in the string, it does not tell you whether there is a comma in the string. In that case, the return value would be string::npos .

3. stringword.erase deletes that many characters from the start of the string. In this case, it deletes 10 , making stringword ,200,3000,40000 . This means that in the next iteration stringword.find(',') returns 0.

4. if (stringword.find(',')) does not behave as wished. if(0) casts the integer to a bool , where 0 is false and everything else is true . Therefore, it never enters the if-block again, as the next iterations will keep checking against this unchanged string.

And besides all that there's this:

for (int j=0;j<v.size();i++) 
{
    cout << v.at(i) << " " ;
}

it uses i . That was declared in a for loop, in a different scope.
The code you gave simply doesn't compile, even with the added main and includes. Heck, v isn't even defined in the second program.

It is however not enough, as the for condition stringword.length() is recalculated every loop. In this specific instance it works, because your integers get an extra digit each time, but let's say your input file is 1,2,3,4, :

• The loop executes normally three times
• The fourth time, stringword is 4, stringword.length() returns 2, but i is already valued 3, so i<stringword.length() is invalid, and the loop exits.

If you want to use the string's length as a condition, but edit the string during processing, store the value before editing. Even if you don't edit the string, this means less calls to length() .

If you save length beforehand, in this new scenario that would be 8. However, after 4 loops string is already empty, and it executes the for loop some more times with no effect.

Instead, as we are editing the string to become empty, check for that.

All this together makes for radically different code altogether to make this work:

while (!stringword.empty())
{
    int value;
    istringstream (stringword) >> value;
    v.push_back(value);
    stringword.erase(0, stringword.find(',')+1);
}
for (int i = 0; i < v.size(); i++) 
{
    cout << v.at(i) << " " ;
}

A different way to solve this would have been to not try to find from the start, but from index i onwards, leaving a string of commas. But why stick to messy stuff if you can just do this.

And that's about it.