Char to Int in C++?

Question

Possible Duplicate:
How to convert a single char into an int

Well, I'm doing a basic program, wich handles some input like:

2+2

So, I need to add 2 + 2.

I did something like:

string mys "2+2";
fir = mys[0];
sec = mys[2];

But now I want to add "fir" to "sec", so I need to convert them to Int. I tried "int(fir)" but didn't worked.

score 5 · Accepted Answer · answered Sep 06 '11 at 16:15

5

There are mulitple ways of converting a string to an int.

Solution 1: Using Legacy C functionality

int main()
{
    //char hello[5];     
    //hello = "12345";   --->This wont compile

    char hello[] = "12345";

    Printf("My number is: %d", atoi(hello)); 

    return 0;
}

Solution 2: Using lexical_cast(Most Appropriate & simplest)

int x = boost::lexical_cast<int>("12345");

Solution 3: Using C++ Streams

std::string hello("123"); 
std::stringstream str(hello); 
int x;  
str >> x;  
if (!str) 
{      
   // The conversion failed.      
}

answered Sep 06 '11 at 16:15

Alok Save

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@MGZero Somebody is being a jerk and is downvoting the answers. – Kiril Sep 06 '11 at 16:18
Seems like the order of the days here in SO, recently. – Alok Save Sep 06 '11 at 16:19
For this particular case there's also this option `fir = mys[0] - '0';` – Praetorian Sep 06 '11 at 16:25
Thanks, but I prefer @Preatorian way, is it canonical ? – dsocolobsky Sep 06 '11 at 16:28
@dysoco The code I posted should work as long as you're trying to convert a single character into an int. If the string representation of the integer is longer than 1 character use one of the methods shown above. – Praetorian Sep 06 '11 at 16:36
@Praetorian : I supposed that, but for now I only need a single character. – dsocolobsky Sep 06 '11 at 16:41

score 2 · Answer 2 · answered Sep 06 '11 at 16:34

Alright so first a little backround on why what you attempted didn't work. In your example, fir is declared as a string. When you attempted to do int(fir), which is the same as (int)fir, you attempted a c-style cast from a string to an integer. Essentially you will get garbage because a c-style cast in c++ will run through all of the available casts and take the first one that works. At best your going to get the memory value that represents the character 2, which is dependent upon the character encoding your using (UTF-8, ascii etc...). For instance, if fir contained "2", then you might possibly get 0x32 as your integer value (assuming ascii). You should really never use c-style casts, and the only place where it's really safe to use them are conversions between numeric types.

If your given a string like the one in your example, first you should separate the string into the relevant sequences of characters (tokens) using a function like strtok. In this simple example that would be "2", "+" and "2". Once you've done that you can simple call a function such as atoi on the strings you want converted to integers.

Example:

string str = "2";
int i = atoi(str.c_str()); //value of 2

However, this will get slightly more complicated if you want to be able to handle non-integer numbers as well. In that case, your best bet is to separate on the operand (+ - / * etc), and then do a find on the numeric strings for a decimal point. If you find one you can treat it as a double and use the function atof instead of atoi, and if you don't, just stick with atoi.

score 0 · Answer 3 · answered Sep 06 '11 at 16:14

0

Have you tried atoi or boost lexical cast?

answered Sep 06 '11 at 16:14

Kiril

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Char to Int in C++?

3 Answers3