I am trying to learn C++ by my self. I looked up a sample question after going through some text. Though i would like someone to review my code .. Basically asking you to break it to show some flaws or some thing i missed, as a beginner I tried exhaustively to improve it and now hit a wall to analyse robustness of the code

Problem statement:

In this challenge, write a program that takes in three arguments, a start temperature (in Celsius), an end temperature (in Celsius) and a step size. Print out a table that goes from the start temperature to the end temperature, in steps of the step size; you do not actually need to print the final end temperature if the step size does not exactly match. You should perform input validation: do not accept start temperatures less than a lower limit (which your code should specify as a constant) or higher than an upper limit (which your code should also specify). You should not allow a step size greater than the difference in temperatures

#include <iostream>
#include <cstdlib>
#include <string>

#define LOWER_LIMIT 23.3
#define UPPER_LIMIT 256.3

using namespace std;

bool isnum(string s)
{
    //check if the string is a number
    //48 & 57
    int len = s.length();
    for(int i = 0; i < len; i++)    
    {
        //cout << s[i] << "\t" << int(s[i])<< endl;
        if(int(s[i])>=48 && int(s[i])<=57)
            return true;
        else
        {
            return false;
            break;
        }
    }
}

int main(int argc, char** argv)
//The int argc holds the argument count and the argv is a 2-D array4
// of characters
{
    double start,end,step_size;
    if(argc!=4)
    {
        cout<<"Please enter three intigers"<<endl;
        cout<<"celcius <start_temprature> <end_temprature> <step_size>"<<endl;
        cout<<"Last step may not be printed"<<endl;
    }
    else
    {
        //check if all the arguments are intigers
        if(isnum(argv[1]) && isnum(argv[2]) && isnum(argv[3]))
        {   
            cout<<argv[1]<<endl;
            cout<<argv[2]<<endl;
            cout<<argv[2]<<endl;
            start = atof(argv[1]);
            end = atof(argv[2]);
            step_size = atof(argv[3]);
            //calculate the table and print.
            if(start < LOWER_LIMIT || start >UPPER_LIMIT)
            {
                cout<<"The <start_temprature> does not meet the limit requirement ("<<LOWER_LIMIT<<"\u00B0"<<"C - "<<UPPER_LIMIT<<"\u00B0"<<"C)"<<endl;
                // The degree symbol to be printed on command line requires UTF-8 characters which has the degree symbol and the location is \u00B0
                return -1;
            }

            if(end >UPPER_LIMIT || end <LOWER_LIMIT)
            {
                cout<<"The <end_temprature> does not meet the requirement ("<<LOWER_LIMIT<<"\u00B0"<<"C - "<<UPPER_LIMIT<<"\u00B0"<<"C)" <endl;             
                return -1;
            }
                
            if (step_size  <1 || step_size >=(UPPER_LIMIT - LOWER_LIMIT))
            {   //zero or negetive stepsize checking
                cout<<"The step_size cannot be negetive, zero or greater than or equal to step_size"<<endl;
                return -1;
            }

            if(end<start) //swapping variables if start is greater than end
            {
                cout <<"Swapped! end and start values for simplicity" <<endl;
                double tmp = start;
                start = end;
                end = tmp;
            }
            cout << "start "<<start<<endl;
            cout << "end "<<end<<endl;
            cout << "step_size "<<step_size<<endl;
            int nend = (int)((end-start)/step_size);
            cout << nend <<"number of iterations"<<endl;
            for(int i = 0; i < nend ;i++)
            {
                cout << start << "\u00B0"<<"C  =  " << ((start*(9/5))+32) << "\u00B0"<<"F" <<endl;
                start += (double)step_size;
            }

        }
        else
        cout <<"All three input arguments must be positive numbers!" <<endl;
    }
}