Problem with isalnum,isdigit,isalpha etc conditionals

[Humar]

First of all, i apologize if this sort of question has already been asked and if i'm being a nuisance but i just don't know what to do.

Alright, so i've been trying to make a car number registration program. I will confess, this is my homework, and i don't expect solutions right away, just some tips and a general direction to what i should do.
It's basically like this:

if the number length is 5/6 where the first 2/3 letters are digits and the rest are letters, it's a valid code (for example "123ABC" or "12ABC") (Estonian car number).

And then there are specially manufactured car numbers with a length limit of 9. And it has to have at least 1 number. (for example "ABCDE12")

Everything else is undefined and are non-compliant with current car number rules etc.


I tried using isalpha, isnum conditionals but i just can't seem to tell the program that i want just the first particular digits etc.

I also tried doing it with for loops, but that didn't work so well.
Then i tried doing it with a while function, that didn't work so well either.

Here's an example of what i tried to do:

#include <iostream>
#include <stdio.h>
#include <string>
#include <ctype.h>

using namespace std;

int main()
{
    int i=0;
    const int j=3;


    cout << "Please enter a car number";
    string carnum;
    getline(cin, carnum);

    if ( (i<carnum.length()) && (carnum.length()<=6) && (isdigit(carnum[i<=3])&&(isalpha(carnum[i>3]))))
    printf("This car number is valid (Traditional car number)");
    
    return 0;
}

All and any help is appreciated

[writetonsharma]

What exactly you are trying in the above code ? let me try the specially manufactured car function.

bool isSMCN(string num)
{
	if(num.length() != 9)
	{
		return false;
	}

	bool flag = false;
	for(int i = 0; i < num.length();i++)
	{
		if(isdigit(num[i])
		{
			return true;		//if any number is digit, its true
		}
	}
	
	return false;			//we are here which means there were no digits.
}

hope you can try to write other functions like this.

[Humar]

Thank you for responding so quickly, i am currently trying to use this boolean function you posted.

Though i would really like to know, how would i go about telling my if statement to check only 3 first digits and then 3 last digits.
For example:

string carnumber=123ABC;
for(int i=0;i<carnumber.length;i++){
if( (carnumber.length<=6) && (isdigit(carnumber[i<=3]) && (isalpha(carnumber[i>3])

 // here i'm trying to tell the if function 
 // check the first letters of carnumber and then check the last 3 letters of 
 //Though i don't know if this is the right way to go about it

printf("Valid car number");}

[writetonsharma]

i <= 3 will give either 1 or 0. So, you are actually doing this:

isdigit(carnumber[0]) or isdigit(carnumber[1])

do the same like I did. do two loops, one will run for first three index and other will run for last three index's. See if both the loops satisfy your conditions.