C++ Numerical Input Validation - c++

Question

I'm used to Python and I'm now learning C++ which is a bit more complicated to me. How can I modify the input so that it fits the description in the comment at the top of my code? I tried to include if (input[0]!='.' &&...) but it just returns 0. I want it to be included as part of the number. Same with the characters after the first character of the input.
I also don't know how I can separate numbers with more than three digits (starting from the end of the number obviously) with a comma (so 1000000 should be returned as 1,000,000).
/*
* The first character can be a number, +, -, or a decimal point
* All other characters can be numeric, a comma or a decimal point
* Any commas must be in their proper location (ie, separating hundreds from thousands, from millions, etc)
* No commas after the decimal point
* Only one decimal point in the number
*
*/
#include <iostream>
#include <cmath>
#include <climits>
#include <string>
int ReadInt(std::string prompt);
int ReadInt(std::string prompt)
{
std::string input;
std::string convert;
bool isValid=true;
do {
isValid=true;
std::cout << prompt;
std::cin >> input;
if (input[0]!='.' && input[0]!='+' && input[0]!='-' && isdigit(input[0]) == 0) {
std::cout << "Error! Input was not an integer.\n";
isValid=false;
}
else {
convert=input.substr(0,1);
}
long len=input.length();
for (long index=1; index < len && isValid==true; index++) {
if (input[index]==',') {
;
}
else if (isdigit(input[index]) == 0){
std::cout << "Error! Input was not an integer.\n";
isValid=false;
}
else if (input[index] == '.') {
;
}
else {
convert += input.substr(index,1);
}
}
} while (isValid==false);
int returnValue=atoi(convert.c_str());
return returnValue;
}
int main()
{
int x=ReadInt("Enter a value: ");
std::cout << "Value entered was " << x << std::endl;
return 0;
}

Answer 1

Writing parsing code is tricky. When parsing, it is easy to make a mess of the control flow. I suggest dividing up the I/O from the validation code: make a separate function bool IsVaildInt(const std::string& s) that returns whether s is a valid input, and do the prompts outside in a calling function.
It helps to think through systematically for every character what constitutes a valid input. If you are familiar with regexes, like cigien suggested, that may be a good way of organizing your approach even if you end up writing the parsing code by hand instead of using a regex library.
Here are the requirements that you posted:
* The first character can be a number, +, -, or a decimal point
* All other characters can be numeric, a comma or a decimal point
* Any commas must be in their proper location (ie, separating hundreds
from thousands, from millions, etc)
* No commas after the decimal point
* Only one decimal point in the number
That's a lot of logic, but it's doable. It sounds like you are working on this as an exercise to master C++ basics, so I won't post any code. Instead, here's an outline for how I would approach this:
Test that the first character is either 0-9, +, -, or decimal point. If it's not, return invalid.
Search the string for whether it has a decimal point. If it does, remember its position.
Loop over the remaining characters, in reverse starting from the last character.
Separate from the loop index, make a counter that says what the current digit place is (... -1 for tenths, 0 for ones, 1 for tens, 2 for hundreds, ...). If the string has a decimal point, use that vs. the string length to determine the digit place of the last character.
If a character is a comma, check that it is in a valid location compared to the current digit place. If not, return invalid.
Otherwise, if the character is a decimal point, it must be the one identified earlier. If not, that means there are multiple decimal points, so return invalid.
Otherwise, the character must be a digit 0-9, and the digit place counter should be incremented. If the character is not a digit, return invalid.
Finally, if the loop makes it all the way through without hitting an error, return that the string is valid.