c++: Iteration of vector<string>

Question

#include <iostream>
#include <vector>
#include <string>

using namespace std;

int main()
{
    vector<string>  a;
    a.push_back("1 1 2 4");
    a.push_back("2 3 3 3");
    a.push_back("2 2 3 5");
    a.push_back("3 3 3 3");
    a.push_back("1 2 3 4");
    for (int i=0;i<a.size();i++)
        for(int j=0;j<a[i].length();j++)
            cout<<a[i].at[j];
    return 0;
}

Hi,when I run the code above,there is an error as below:

error C2109: subscript requires array or pointer type

Please help me and tell me why,thanks!

Or don't use at at all. I've never found a situation where its semantics are appropriate (especially with std::string). — James Kanze
– James Kanze, Commented Oct 17, 2013 at 8:44
@JamesKanze: With vector, sometime at() is useful and is needed. — Sarfaraz Nawaz
– Sarfaraz Nawaz, Commented Oct 17, 2013 at 8:51
@JamesKanze: what is the specific point of semantic on std::string that worries you ? I usually recommend using at() rather than [] to protect against out of bounds aspects. — Matthieu M.
– Matthieu M., Commented Oct 17, 2013 at 8:54
@Nawaz Yes. If you plan to catch the exception, and do something with it, then by all means, at is the solution. But beware in actual applications. If a bounds check error is an actual error (violation of a precondition), you don't want to unwind the stack (which may be corrupted); you want to get out of the process executing as little code as possible. If the internal state is corrupted, even executing a destructor could make things even worse. — James Kanze
– James Kanze, Commented Oct 17, 2013 at 11:17

Sarfaraz Nawaz · Accepted Answer · 2013-10-17 08:49:41Z

7

at is a function, need to be called with () not []

update

cout<<a[i].at[j];
//           ^^^

to

a[i].at(j)
//   ^^^^^

To output string, you don't need to cout each char, just do

for (int i=0; i<a.size(); i++)
{
   std::cout << a[i] << "\n";
}
std::cout << std::endl;

Or if C++11:

for(auto const & s : a)
{
   cout << s << "\n";
}

edited Oct 17, 2013 at 8:49

Sarfaraz Nawaz

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answered Oct 17, 2013 at 8:43

billz

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Comments

Vlad from Moscow · Accepted Answer · 2013-10-17 08:59:53Z

0

It is more simpler to use the range-based for statement. For example

for ( const std::string &s : a )
{
    for ( char c : s ) std::cout << c;
    std::cout << std::endl;
}

answered Oct 17, 2013 at 8:59

Vlad from Moscow

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