pset2 caesar function to read argv[1] not working

Question

So I am still struggling to get my Caesar program to read argv1[1]. my program compiles and returns a value of "1" if the incorrect number of arguments are entered. The problem seems to be that it is not reading argv[1] at all. I feel like i have set up a function that should look at every index in argv[1] to determine if they are decimals or not. BUT, it appears that this is not happening.

The evidence is that I am not returning the "Usage: key" message when i enter to many arguments at the command line. Also, no matter what I enter whether it is "2" or "2blab" i still get prompted to enter "plaintext".

my code might look cluttered with comments but I tried to comment every area to show my thinking. Can someone help me out?

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <stdbool.h>

//function that takes in argv[1], reads it, and determines if it meets criteria
//criteria defined as only decimal digits between 0 and 9.  letters etc are not accepted.
int verifyinput(string b);

int main (int argc, string argv[])
{
    //if correct number of arguments are entered check argv[1] for compliance with entry critera.
    if (argc==2)
    {
        //sends argv[1] to verify its contents
        //returns an integer that is the size of the number of acceptable digits
        int verify = verifyinput(argv[1]);
        //if the number of acceptable digits is different then the length of argv[1]
        //the user is given the following error message and the program ends
        if (verify != strlen(argv[1]))
        {
        printf("Usage: ./caesar key \n");
        }
        //if the number of acceptable digits is the same as the length of the argv[1]
        //the user is prompted to enter text to encipher
        else if (verify == strlen(argv[1]))
        {
        string k = get_string("PlainText: ");
        }
    }
    //if user arguments are not entered correctly
    //error message 1 is displayed
    if (argc!=2)
    {
    printf("1 \n");
    }
}


int verifyinput(string b)
{
    int length = strlen(b);
    int check = 0;

        // loop that reads each digit of string b
        for (int i = 0; i<=length; i++)
        {
            //ascii code for zero through nine are 0 through 9.  any digit larger then ascii 9 is outside accepted range
            //check is added to in the event that b[i] is less then or equal to 9
            if (b[i]<=9)
            {
                check++;
            }
            //check is subtracted from if b[i] is greater then 9
            if (b[i]>9)
            {
                check--;
            }
        }
 //returns value to  int verify in main 
return check;
}

Answer 1

When compare a char with a int you have two options. For the int use the ascii value for the letter or number you are comparing to, or you can use the ' symbol ie.: '9'. Also when checking if a char is inside a range maybe you find useful something like: ( c >= 'a' && c <= 'z') only goes true if c is a lowercase letter.