how can i convert c++ str to int?

Question

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c++ converting string to int

I have the user input 9 numbers in sequence. I need to convert the string numbers to an int

string num;
int num_int, product[10];

cout << "enter numbers";
cin >> num;

for(int i =0; i<10; i++){
   product[i] = num[i] * 5; //I need the int value of num*5
}

If you want the user to enter a number, just have them enter a number. — Jerry Coffin, Feb 05 '13 at 19:11
Are you sure you want `num[i]`? That accesses just a single character from `num`. — Barmar, Feb 05 '13 at 19:19

score 5 · Answer 1 · answered Feb 05 '13 at 19:12

5

Why don't you just read immediately to an integer?

int num;
cin >> num;

answered Feb 05 '13 at 19:12

this is a very good answer but i need both variables, a string and an int – user1082764 Feb 05 '13 at 19:13
2

@user1082764 Why? Why do you need a string? – James Kanze Feb 05 '13 at 19:14

score 4 · Answer 2 · edited Feb 05 '13 at 19:37

You do not need to have two variables. It's usual in C++ to convert on the fly, in the input stream, without ever seeing the text as a string. So you can simply write:

int num;
std::vector< int > product( 10 );

std::cout << "enter number: ";
std::cin >> num;

...

Note that I've corrected the way you've declared the array as well. You wouldn't normally use int product[10]; in C++. (And you'd almost never define two variables on the same line, even if the language allows it.)

score 3 · Accepted Answer · answered Feb 05 '13 at 19:42

3

The easiest way by far to convert to a string and back again is to use the conversion functions.

 std::string s="56";
 int i=std::stoi(s);

http://en.cppreference.com/w/cpp/string/basic_string/stol

and back

 int i=56;
 std::string s=std::to_string(i);

http://en.cppreference.com/w/cpp/string/basic_string/to_string

of course if you are reading input you might as well do it then and there too

 int i;
 std::cin >> i;

answered Feb 05 '13 at 19:42

111111

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Something to note is that std::to_string is a C++11 feature. Not all compliers have it. – Caesar Feb 05 '13 at 19:54
@Caesar how long are we going to have to keep saying that for? – 111111 Feb 05 '13 at 20:17
1

Until the average C++ program uses it. – Caesar Feb 05 '13 at 21:00
@Caesar Nah, until free compilers on each prevalent OS support the majority of it, which is more or less now. – 111111 Feb 06 '13 at 10:57

score 1 · Answer 4 · answered Feb 05 '13 at 19:13

1

Here is a full sample:

//library you need to include
    #include <sstream>
    int main()
    {
        char* str = "1234";
        std::stringstream s_str( str );
        int i;
        s_str >> i;
    }

answered Feb 05 '13 at 19:13

tomaytotomato

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Aniket Inge · Answer 5 · 2013-02-05T19:23:41.707

if you absolutely must use a std::string (for any other reason.. perhaps homework?) then you can use the std::stringstream object to convert it from a std::string to int.

std::stringstream strstream(num);
int iNum;
num >> iNum; //now iNum will have your integer

Alternatively, you can use the atoi function from C to help you with it

std::string st = "12345";
int i = atoi(st.c_str()); // and now, i will have the number 12345

So your program should look like:

vector<string> num;
string holder;
int num_int, product[10];

cout << "enter numbers";
for(int i = 0; i < 10; i++){
    cin >> holder;
    num.push_back(holder);
}
for(int i =0; i<10; i++){
   product[i] = atoi(num[i].c_str()) * 5; //I need the int value of num*5
}

how can i convert c++ str to int?

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