C++ string.at(0)

[Maria2]

Hi, guys, new girl here :-) needs your help.

In one of programs I have, sentence.at(0) cout first character, in the other doesn't. Confused at first place that .at() can be applied at string at all. Thank you in advance for your help.

// it works - string.at(0) for first character
#include <iostream>
#include <string>

using namespace std;

int main()
{
string sentence;
getline(cin, sentence);

for (int i = 0; i < sentence.length(); i++)
    {
    sentence.at(i) = toupper(sentence.at(i));

    cout << sentence.at(i);
     }
return 0;
}
///here works - string.at(1) for first character
#include <iostream>
#include <string>

using namespace std;

int main(void)
{
    int n;
    cin >> n;

    string text;
    cin >> text;
//input this:     H*YAECTANENRHMPEADRRAPWR*YISY**M*S*

    int m; 
    m = text.length()/n;

    char mytable[m][n + 1];

    for(int i = 0; i < m; i++)
        mytable[i][n]='\0';

    int k(0);	// int k = 0;

    for(int j = 0; j < n; j++)
        for(int i = 0; i < m; i++)
            mytable[i][j] = text.at(k++);     //text.at(1) in first step

    for(int i = 0; i < m; i++)
        for(int j = 0; j < n; j++)
            if(mytable[i][j]!='*')
                cout << mytable[i][j];
            else
                cout<<" ";
    return 0;
}

[jonnin]

use [] instead of at most of the time.
sentence[i] = toupper(sentence[i]);

You use of at() seems correct; I would validate the LOGIC around line 44. Print to the screen in that double loop what you are loading to mytable... something must be wrong in there.

[E0p9LyTq]

Don't expect a post-fix increment to actually be evaluated AFTER the variable use in a function call (it can be undefined behavior until C++17):

   int k(0);

   for (int j = 0; j < n; j++)
   {
      for (int i = 0; i < m; i++)
      {
         mytable[i][j] = text.at(k);
         k++;
      }
   }

Recommend always using braces for statement blocks, makes it easier to add statements without messing up the logic.

What is your compiler? A modern C++ compiler won't allow variable length arrays (VLAs) as you did with your 2D char array at line 38. The size parameters have to be compile-time constants.

If you need a run-time container you should use a std::vector. Declaring a sized 2D vector could look something like this:
std::vector<std::vector<char>> mytable(m, std::vector<char>(n + 1, '\0'));

This creates your 2D container and automatically fills the elements with the null character, you can delete/comment out lines 40-41.

[Maria2]

Ok. braces and vectors. Tnx

The compiler was https://www.onlinegdb.com/online_c++_compiler

[E0p9LyTq]

I personally despise online compilers, I prefer using a compiler/IDE installed on my computer(s).

With that said, there are "better" on-line compilers that would at the very least give a warning using a VLA is against the standard.

There is one here at cplusplus: http://cpp.sh/
It only warns, it still compiles your 2nd program

http://coliru.stacked-crooked.com/ will show errors and halt compilation, as a good compiler should.

[jonnin]

A modern C++ compiler won't allow variable length arrays (VLAs)

modern/current g++ supports them if not instructed to complain explicitly. Actually g++ supports a great many things unless told not to, its quite forgiving.

[Maria2]

I'm using cpp.sh as well, but it has no debugger. As far as I see, nor Coliru have debugger. I'm not so good in debugging, but can manage elementary staff. Actually, I combine compilers and when one of them freeze, just jump to another. Got installed the code::blocks as well... and thinking about dev cpp also.

[jlb]

modern/current g++ supports them if not instructed to complain explicitly. Actually g++ supports a great many things unless told not to, its quite forgiving.

When properly configured g++ will only support standard C++ and will be quite pedantic. However if you fail to properly configure your compiler (any compiler) you will probably be able to use quite a few compiler specific hacks to aid in your self destruction.

[jonnin]

Exactly. Its not the age, its the settings, for anything that is new enough to support '17

[E0p9LyTq]

Got installed the code::blocks as well... and thinking about dev cpp also.

If you are using Windows consider Visual Studio 2017. It is free in the Community edition and has a decent built-in debugger.

It takes a while to download, and Visual C++ is not installed as part of the default setup.

Orwell's Dev-C++ was outdated when it was last released back in April 2015, the compiler was not the latest then.

When I installed VS2017 I created an offline install instead of using the web install. It required a bit of HD space, but was worth the time and effort to have the offline install.

The latest web installer allows for downloading an offline installation, that wasn't available when I installed 2017:

https://docs.microsoft.com/en-us/visualstudio/install/create-an-offline-installation-of-visual-studio?view=vs-2017

[Maria2]

If you are using Windows consider Visual Studio 2017. It is free in the Community edition and has a decent built-in debugger.

Thank you very much :-)