Arduino String to int: How to convert a string into an integer. Find out the standard function for this operation and learn how to code it yourself.

Arduino string to int - find out:

• The standard function for conversion.
  
  
• The algorithm used for the process.
  
  
• Exactly how to code it yourself.
  
  
• Code for conversion of hexadecimal or binary strings.
  

You probably need to do some mathematical calculations using the "numerical value" of the number string in your string, and the function below allows you to do just that.

Arduino string to int: Standard form

The function you need is atoi() which has the prototype:

    int atoi( char *str );

Where:

str - is a string buffer that stores the string.

The function takes the above arguments and returns an integer using base 10 (or decimal).

The name of the function is an acronym for ASCII to int  - atoi().

Where ASCII is another acronym standing for American Standard Code for Information Interchange. It defines is a table of characters.

Arduino string to int: Example

This example allows you to "add" two string numbers together

sketch: atoi_string_to_int_test.ino

Arduino: String to int Algorithm

The Arduino string to int code code is a lot simpler than itoa() since the string length is fixed (set by user as the input string) and the output type size is also fixed (integer). In addition the base used is,...  you guessed it...  fixed at base 10 for decimal.

One complication is that the first character could be a minus sign. So first store that information and move to the next character.

int sign = 0;

    if (str[0]=='-') {
      sign = 1;
      str++'
    }

To convert a string to a number start at the least value digit and work backwards.

char *e = str + strlen(str)-1; // At end character.   

Now build up the output value:

    int v = 0;
    int radix = 1;

    while(e>=str) { // Has pointer moved before string start?
     
       if (!isdigit(*e)) return 0; // In ctype.h

       v += (*e - '0') * radix;
       e--; // Prev character.
       radix *= 10;
    }

If the value of the character is not a digit then bail out returning zero

The code(*e - '0') gets the character value and changes it to a number (0-9). This number us multiplied by the current position radix value or decimal multiplier that increases x10 each time round the loop. When pointer e

Now account for the stored sign value.

    if (sign) v=-v;

    return v;

Arduino String to int: Complete code

Arduino String to int: Output

 
Arduino string to int...
Num 1 123
 123
Num 2 12345
 12345
Convert and add val 12468
Should match 12468
Check bad chars: 0
Check negative: -543
Check(2) negative: -500

Arduino String to int: Adding radix conversion

This code allows you to convert numebers of differnent radix e.g. Hex or binary (or more).

Arduino String to int: Radix Output

 
Arduino string to int with RADIX...
binary 101 op should be 5
5
hex 11 o/p should be 17
17
dec -27 o/p should be -27
-27
hex 3f2 o/p should be 930
930
dec 12345 o/p should be 12345
12345
Bin 1010010110100101 = 0xa5a5 o/p should be -23131
-23131

Conclusions

The built in version - atoi() - works as expected; You can easily obtain an integer directly from a string.

You have learned, from this page, how to code the Arduino string to int function - _atoi() - from first principles.

On this page you have also found out how to code a non standard function that converts a string of different radix (i.e.different to 10) into an integer - that can convert a hexadecimal or binary string to a base-10 number:

    atoir()

 

Written by John Main, who has a degree in Electronic Engineering.

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