unwanted proximity in dividing two float numbers

[EnjoyDeSilence]

Hello,
I was writing a parser from strings to positive float numbers.
While debugging the part of the code that deals with the fraction, I saw that the result of 4/100.0 is 0.3999999 rather than 0.04, which obviously ruins the parser.
Could anyone please explain why it happens and how do I solve this problem?

The code I added describe only handling the fraction part of the number:

  newFractionToBeAdded = (str[i]-'0'/divisor);
			result += newFractionToBeAdded;
			divisor = divisor*10.0;
			

[helios]

It seems you misplaced a parenthesis: (str[i]-'0')/divisor
I don't have enough context to see any other errors.

[EnjoyDeSilence]

Tried it with the parenthesis, it doesn't seem to be the problem, still returns 0.03999999.

Here's the full code:

float toNum(char* str){
	int flag = 0;
	int len = strlen(str);
	int i=0;
	float result=0;
	float divisor = 10.0;
	float newFractionToBeAdded = 0;
	
	do{
		if (str[i] == '.'){
			flag = 1;
			i++;
			continue;
		}
		if (flag==0){
		result = (result*10) + str[i]-'0';
		i++;
		}else {
			
			newFractionToBeAdded = ((str[i]-'0')/divisor);
			result += newFractionToBeAdded;
			divisor = divisor*10.0;
			i++;
		}

	
	}
	while (len>i);
	return result;
}

[Chervil]

Looks like it is simply the properties of floating point numbers which are important, rather than a problem with the code.

int main()
{
    char decimal[] = "0.04";
    float test     =  0.04;

    cout.precision(10);
    
    cout << "toNum(decimal) " << toNum(decimal)          << '\n';
    cout << "test           " << test                    << '\n';
    cout << "difference     " << (test - toNum(decimal)) << '\n';
}

Output:

toNum(decimal) 0.03999999911
test           0.03999999911
difference     0

Changing everything from float to double won't make the problem go away, but it gives a few more decimal places before things are visibly awry:

toNum(decimal) 0.040000000000000000833
test           0.040000000000000000833
difference     0

[EnjoyDeSilence]

Chervil - That does the work! :)
Thank you!