Validating a string to be all digits - c++

Question

Hi I'm having trouble validating this string to be all decimals, even if I type in 9999 it still tell me my if statement comes out false. I think it's a typo but I don't know where.
cout<<"Enter a very large number"<<endl;
cin>>In1; //inputs a string
for(int i=0; 0<In1.length();i++){ //the loop that goes thru each index
if (!(isdigit(In1[i]))){ //validates each index
//tells the user to try again
cout<<"You did not enter a valid input, please try again"<<endl;
In1="";
cin>>In1;
i=0;//starts the loop over if reached
}
}
I keep receiving the "You did not enter a valid input, please try again" regardless of whether I type it right or wrong.

Answer 1

for(int i=0; 0<In1.length();i++){
See what you did? Change to
for(int i=0; i<In1.length();i++)
In your loop condition you need to compare i with In1.length().

Answer 2

You might want to change
0<In1.length()
to
i<In1.length()

Answer 3

Using
#include<algorithm>
if ( std::find_not_if( in1.begin(), in1.end(), isdigit ) != in1.end() ){ ...
might have prevented this unfortunate incident, and is also quite clear on the intent. The dual _not/!= muddles it slightly but still.
There are quite a few convenience algorithms, replacing common uses for simple for- statements. Most of them are on the form
do_this( where_to_start, where_to_end, do_this_operation )
There is usually nothing special or dramatic with these function, they apply the operation to each element in the start-end sequence.
You have find, count, copy, and generate to mention a few. Their purpose is to clarify the intent of your for-statement. You can find a complete list at http://en.cppreference.com/w/cpp/algorithm

Answer 4

You will almost certainly find that, over time, you become more adept at seperating different parts of code into the functionality that they each provide. Making debugging and later modification considerably easier.
It also makes, as Captain Giraffe points out, the intent of the code considerably more clear - something that can only make reading the code easier & quicker.
I've not used std::find_not_if, opting instead to use the method that you've chosen (based on the assumption that the important thing is knowing how to get the right answer, as opposed to simply supplying the right answer - well, that and me not knowing of find_not_if's existence :grin:) You'll see that I've chucked it into it's own function, which I call from main. The function also only performs a single task - that of checking the validity of the string. Any attempt to prompt the user for this text, re-prompt in the case of error and finally, take action on the correct input is the sole responsibility of the code that calls isValidNumericalString - there's no reason you couldn't throw those functions into their own functions, as opposed to having a single, large body of main.
#include <iostream>
using namespace std;
// returns true if all characters in string are numerical (0-9)
bool isValidNumericalString(string inputString)
{
int i, n = inputString.length();
for (i=0; i<n; i++)
if ( !isdigit(inputString[i]) )
return false;
return true;
}
int main()
{
string In1;
cout << "Enter a very large number (digits 0-9 only. 10e1 is unacceptable): ";
cin >> In1;
while (!isValidNumericalString(In1))
{
cout << "You did not enter a valid input, please try again :p" << endl;
cout << "Enter a very large number (digits 0-9 only. 10e1 is unacceptable): ";
cin >> In1;
}
cout << "Congratulations - '" << In1 << "' is a valid string representation of a number" << endl;
return 0;
}