Reject unwanted values from command line - c++

Question

In this code the value needed in the code is given by the user via command line. But I need the code to reject any numerical value <0 and every alphanumeric code or string (-23 is not accepted, h42b is not accepted,hello is not accepted, 10 is accepted)
int main(int argc, char* argv[])
{
int S = argc > 1 ? std::atoi(argv[1]) : 100;
if (S < 0) {
S = 100;
std::cout << "S value must be >0, value set to 100" << '\n';
}
If the user puts any value that's not a number>0 I want the code to print "S value must be >0, value set to 100".
I tried using a for loop, but I don't know how to reject alphanumerical codes or strings.
Can you tell me what can I use? It shouldn't be an advanced tool, just something that gets the job done nicely

Answer 1

I need the code to reject any numerical value <0 and every alphanumeric code or string
means you cannot assume numerical input, which means you cannot use atoi().
If the user puts any value that's not a number
... then you need to detect that. You can only detect if you can access the input.
Which means you need to read it in as input without assumptions on syntax or formatting. I.e. you need to store in a string.
We are however not looking at normal input, you are working on commandline. Which means you need to parse enough to decide when to give up. As far as I understand, that is when even the first parameter is not a number. For that a loop is not needed - if the program fails there is nothing which you can process in additional loop iterations (assuming that you do not want to ignore the first, non-numerical, parameter and process the second, i.e. whatever is after the first blank).
So all in all
verfiy that there is at least one paratemer (your code already does this)
get the first parameter
as a string, not as a number
try whether it is a number, with something NOT expecting it to be a number; i.e. not atoi()
if you select sscanf() make sure to interpret the return value (not only the scanned value)
if you detect a number use it
if you do not detect a number output the useage message

Answer 2

You can convert char array to std::string for convenience and then iterate over it, using std::isdigit to check whether it is a digit in range (0-9). If not, assign 100 to S, print error message and break the loop.
Quick solution looks might look like this:
#include <cctype>
#include <iostream>
#include <string>
int main(int argc, char *argv[]) {
int S{0};
bool correct_input{true};
std::string argument(argv[1]);
for (auto character : argument) {
if (!std::isdigit(character)) {
S = 100;
std::cout << "S value must be >0, value set to 100" << '\n';
correct_input = false;
break;
}
}
if (correct_input) {
S = std::stoi(argument);
}
}

Answer 3

If it is not too advanced you can use a regex to get the job done:
#include <iostream>
#include <regex>
#include <sstream>
int main(int argc, char* argv[]) {
std::regex regex("^(\\+)?([1-9][0-9]*)$");
for (int i = 0; i < argc; i++) {
std::string argument(argv[i]);
std::uint32_t value = 100;
// has argument the desired pattern, i.e. is it a positive number above 0?
if (std::regex_match(argument, regex)) {
std::stringstream strValue;
strValue << argument;
strValue >> value;
std::cout << "value: " << value << "\n";
} else
std::cout << "Input should be a number > 0 (value setted to 100)! provided: " << argument << "\n";
}
return 0;
}
Input:
1 +2 100 0 -1 h23b h3ll0 test
Output:
value: 1
value: 2
value: 100
Input should be a number > 0 (value setted to 100)! provided: 0
Input should be a number > 0 (value setted to 100)! provided: -1
Input should be a number > 0 (value setted to 100)! provided: h23b
Input should be a number > 0 (value setted to 100)! provided: h3ll0
Input should be a number > 0 (value setted to 100)! provided: test
Using regex has the benefit that this approach is highly customizable and you will only encounter numbers in your for-loop that are positive. You don't have to check if the current argument is a number or is above 0. The regex takes care of that. All you have to do is to check if the current argument matches with the regex. However, I don't know how "advanced" this feature is for someone starting with c++ or someone starting with programming. To convert your string you can use any method. I used a stringsteam. Simply write your value into the stream and then read it back into your desired variable. Hope it helped!
If you want to take a closer look at the regex: https://regexr.com/61bjl

Answer 4

I will try to implement this in the different ways as suggested. In the meantime I wrote this:
bool string_is_numeric(char* string) {
size_t length = strlen(string);
for(size_t i = 0; i < length; i++) {
if(!isdigit(string[i]) && static_cast<int> (string[i]) != 46 ) {
return false; }
}
return true;
}
int main(int argc, char* argv[])
{
int S = argc > 1 ? std::atoi(argv[1]) : 99;
if (argc > 1 && (S < 0 || !string_is_numeric (argv[1]))) {
S = 99;
std::cout << "S value not valid. Only numerical values > 0
are
allowed. Value set to 99" << '\n';
}
}
This code as far as I tried works. And let the user write the '.' for decimal numbers.