convert string to int

Projects Programming
1

I need to use a browser to set and change a variable in my arduino pool control. I am using the variable as the set point for the temperature to turn the heat on and off. The water temperature is monitored and stored in a variable as well.

My issue is this. The variable i need to assign a value to is called int setTemp. When i use a form and pass the value of a text box using GET, I index the string the extract the value of the text box, (the temperature i want to set the heat to)

The trouble i am having is that since the setTemp value is being passed in the URL as a string, I can not assign the value to int setTemp since that variable will only take an integer. What i need to do is convert the value first but I can’t seem to do this.

my URL is xxx.xxx.x.xx:88/settemp.htm?setTemp=85

I am able to extract the 85 but it is being interoperated as a string and not an integer so i need a way to convert it.

I have looked up the use of atoi but the examples are quite involved and a little over my newbie head.

Any help?

2

What is difficult about atoi?

char myCharInt[] = "49";
int myInt = atoi(myCharInt);
3

You need to check what atoi( ) returns if it gets a defective input.

atoi( ) supposedly causes "undefined behaviour" !

You need to check on your implementation what it does.

4

To respond to your question, Im not sure what is so difficult other than to say it isn’t working for me.LOL

	String getReq = HTTP_req;

	int tempSetting = getReq.indexOf("setTemp=");


	setTemp =  atoi(getReq.substring(tempSetting)) ;

My compiler error comes back as:
poolv1.ino:181: error: cannot convert 'String' to 'const char*' for argument '1' to 'int atoi(const char*)'
Error compiling

5

There is a difference between string (character array) and String (data type). atoi accepts only the character array type.

6

so is there any way to do this?

7

Sure. You can identify the correct part of the string, extract it into a substring, and then convert that substring to a character array, and then use atoi( ).

You can also identify the actually numerical digit chars and convert them to a number yourself.

8

Try this

String s = HTTP_req ;
int i=s.lastIndexOf('=') ;
i++ ;
int res=0 ;
while ( true )
{
    char c=s.charAt(i) ;
    if ( isdigit(c) )
    {
         res = 10*res + ( c-'0') ;
         i++ ;
    }
    else break ;
}

If your number is right at the end of the url string, you will need to see for yourself what happens when charAt( i ) is at the end of the string. I don't use arduino strings, so I am not sure what it does.

If isdigit( ) doesn't exist on the arduino..

bool isdigit( char c )
{
    if ( c >= '0' && c <= '9' ) return true ;
   return false;
}
9

needahobby:
so is there any way to do this?

You can access the internal string using the subscript operator, or the c_str method.

String s = "12345";

int num1 = atoi( &s[ 0 ] );

//Or

int num2 = atoi( s.c_str() );

The first method is useful if you need to start the conversion part way through the string ( or you could do c_str() + offset ).