Converting from string to int with a function - c++

Question

I'm fairly new to programming and I'm trying to get a function working that converts a string to an int. My idea with this function was to collect every number in the string and store it in another string, then convert it to an int.
The function returns the value 0.
What this function is supposed to do is return the converted number. Which should not be 0.
int getNumberFromString(int convertedNumber, string textToConvert)
{
for (int i = 0; i < textToConvert.size(); i++)
{
string collectNumbers;
int j = 0;
if (textToConvert[i] == '1' || textToConvert[i] == '2' || textToConvert[i] == '3' ||
textToConvert[i] == '4' || textToConvert[i] == '5' || textToConvert[i] == '6' ||
textToConvert[i] == '7' || textToConvert[i] == '8' || textToConvert[i] == '9' || textToConvert[i] == '0')
{
collectNumbers[j] = textToConvert[i];
j++;
}
if (collectNumbers.size() == 0)
{
return false;
}
else if (collectNumbers.size() > 0)
{
stringstream convert(collectNumbers);
if (!(convert >> convertedNumber))
{
convertedNumber = 0;
}
return convertedNumber;
}
}
}

Answer 1

Maybe you should just use library function ?
int stoi (const string& str, size_t* idx = 0, int base = 10);

Answer 2

You want somehting more like:
int getNumberFromString(int convertedNumber, string textToConvert) {
int retval = 0;
for (auto c: textToConvert) {
retval *= 10;
retval += c - '0';
}
return retval;
}
if you need to code it, or simply use stoi()

Answer 3

Your MAIN problem is that you are trying to convert the number before you have collected all the digits. You should loop over all the digits (use isdigit or if (x >= '0' && x <= '9') to avoid long list of individual digits - or, if you really like to list all digits, use switch to make it more readable).
Once you have collected all the digits, then convert AFTER the loop.
The statement return false, will be the same as return 0; since false will get converted to an integer with the value zero. So you won't be able to tell the difference between reading the value zero from a string and returning false (this is not PHP or JavaScript where type information is included in return values).