Click here to Skip to main content
15,796,904 members
Articles / Desktop Programming / MFC

Validation for Decimal Number

Rate me:
Please Sign up or sign in to vote.
1.44/5 (4 votes)
17 Aug 2015CPOL1 min read 18K   3   5
Validate whether value in string is a valid decimal number or not.


Many times, we come across a situation when we require to validate whether the text in string is a valid decimal number or not.

In C/C++, there is no inbuilt function to do that, so the only thing we come across is to write a validation logic for it, covering all possible cases of validating as a decimal number.


This article explains how to validate a text for decimal number. For a decimal numer, we require to have that a number starts with a digit, dot ('.'), or minus ('-').

It is compulsory to have a dot ('.') in a text for a decimal value. Minus is used as a decimal number and can be a signed number also.

Sign decimal numbers, like -2.3, -0.3 can have minus sign at first position only.

A decimal number can start with dot also, like .34, -.12, so dot also can be at the first position. But a decimal number should have only one dot.

There should not be any alphabetic or special characters. Alphabetic characters can be checked using function isalpha() as shown below:

int isalpha ( int c );

For reference, the code snippet to validate a text as a decimal value, is as given below:

bool ValidateDecimalNumber(CString cstrValue)
    if(cstrValue.IsEmpty())  //String should not be empty
        return false;

    if(cstrValue.Find(_T(".")) == -1)  //String should contain '.' compulsory 
        return false;

    std::string strValue, strChar;
    int nDotNum = 0;

    //Validating first character
    strValue.assign ((std::string)(LPCSTR) cstrValue.GetString ());
    if(strValue.find_first_not_of("1234567890.-") != string::npos)
        return false;

    //Loop starts with second character, as first character is already validated above
    for(int i = 1; i < cstrValue.GetLength(); i++)
        //Checking against alpabetic characters
            return false;

        strChar = cstrValue.GetAt(i);
        //Checking if dot is there as first character, then should not be there again
        if((".") == 0) && ( == 0))
            return false;

        //Checking if dot is not there at first position, then should not be more than one
        if(".") == 0)
            if(nDotNum > 1)
                return false;

        //Checking against special and other characters
        if(strChar.find_first_of("~`!@#$%^&*()_+={}\\|':;?/><,\"-") != string::npos)
            return false;
    return true;

Points of Interest

The above code can be optimized further. Suggestions are always welcome.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
Software Developer
India India
This member has not yet provided a Biography. Assume it's interesting and varied, and probably something to do with programming.

Comments and Discussions

QuestionNot culture aware Pin
Pete O'Hanlon26-Aug-15 4:25
subeditorPete O'Hanlon26-Aug-15 4:25 
QuestionFormat Pin
Nelek17-Aug-15 3:54
protectorNelek17-Aug-15 3:54 
AnswerRe: Format Pin
Sweta Mittal26-Aug-15 4:17
Sweta Mittal26-Aug-15 4:17 
GeneralPlease update it. Pin
super17-Aug-15 3:22
professionalsuper17-Aug-15 3:22 
GeneralRe: Please update it. Pin
peterboulton18-Aug-15 9:15
professionalpeterboulton18-Aug-15 9:15 
.. not to mention internationalisation of decimal point character, thousands separator etc..

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.