How can we restrict user to enter characters and also integers greater than specified length in C?

Question

I'm trying to create a C program on OTP (One Time Pad) encryption. The program takes a string to be encrypted from the user. Then it has to ask for the length of the key before taking the key (in integer). Since I want the program to be 100% correct, I'm putting a constraint on the length of key. As we know, in OTP the length of the key has to be in integers can't exceed the length of the text. So, how can we implement such a filter? I've tried creating the code but it is not working.

Here's the code:

//An Under-Construction program to implement OTP (One time Pad) encryption     
#include "stdio.h"
#include "cstring"
#include "ctype.h"
int main()
{
    char str[10000], key[10000];
    printf("Enter the string:- ");
    scanf("%[^\n]", str); //stops scanning when user presses return

    int len /* stores the number of digits of the OTP key*/, isalphabet=0;
        do
        {
            printf("What is the length of the key you are entering?:-");
            scanf("%d", &len);

            if( isalpha(len) ) // Checks if len is a character
            {
                isalphabet=NULL; //isalphabet becomes NULL if it is a character and not an integer
            }


        } while (len > strlen(str) || isalphabet == NULL); //reiterate the loop until the conditions are satisfied

        for(int i = 0; i < len; i++)
    {
        printf("%dth digit of Key= ", i+1);
        scanf("%d", &key[i]);
    }
    return(0);  
}

I want the program to take only integer value from the user while scanning 'len' and also the value of 'len' shouldn't exceed the length of the string to be encrypted, and reiterate the loop if these conditions aren't satisfied. Can someone point out my mistake and show the the solution? Also, please explain me the reason as to why isn't the code working as it should.

Answer 1

check scanf("%d", len); is there somewhere a & missing? also initialize len

Update:- sorry for being late, okk, actually there's few things you need to correct, sorry i didn't point them in first post.

1) correct the header inclusion delimiter. Use <stdio.h> instead of "stdio.h"

2) initialize len = 0

3) flush stdin after you have read. If you really enter a char, the scanf(%d,&len) is not going to clear it/read it. So you will be stuck in an infinite loop as because of a char stucking in stdin. same goes for the last for loop. Fortunately here you will not be stuck, but the loop will finish prematurely.

4) isalphabet=NULL, isalphabet is int, NULL basically a void pointer with 0x0 value, you should assign 0 to an int.

5) So you came inside the loop with isalphabet=0, the number is not a alphabet, you did not touch the isalphabet, and end of while loop, isalphabet==NULL, here perhaps it will run as true and again the while loop starts. So when is the loop breaking condition getting set to isalphabet?

6) Also why use the cstring in c code? why not string.h (but this is more of a personal choice :))

7) correcting the len > strlen( to len >= strlen

I edited a little bit, check if it works

//An Under-Construction program to implement OTP (One time Pad) encryption     
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main()
{
    char str[10000], key[10000];
    printf("Enter the string:- ");
    scanf("%[^\n]", str); //stops scanning when user presses return

    int len /* stores the number of digits of the OTP key*/, isalphabet=0;
        do
        {
            printf("What is the length of the key you are entering?:-");
            scanf("%d", &len);
            fflush(stdin);

            if( isalpha(len) ) // Checks if len is a character
            {
                isalphabet=1; //isalphabet becomes NULL if it is a character and not an integer
            }


        } while (len >= strlen(str) || isalphabet == 1); //reiterate the loop until the conditions are satisfied

        int i;
        for(i = 0; i < len; i++)
    {
        printf("%dth digit of Key= ", i+1);
        scanf("%d", &key[i]);
    }
    return(0);  
}

Answer 2

First, scanf("%d", len); should be

scanf("%d", &len);

The strlen(str) returns the length, and the comparison should be

len > (strlen(str) -1)

and the comparisons for if is an alpha or not, could be done inside the while loop. Check this code.

#include "stdio.h"
#include "string.h"
#include "ctype.h"

#define TRUE 1
#define FALSE 0

int main()
{
    char str[10000];
    char key[10000];

    printf("Enter the string:- ");
    scanf("%[^\n]", str); //stops scanning when user presses return

    int len = 0;
    int isalphabet= TRUE;
    int apply = 1;
    do
    {
        printf("What is the length of the key you are entering?:-");
        scanf("%d", &len);

        isalphabet= isalpha(len);
        apply = (len > (strlen(str) -1)) ;

    } while (!apply || isalphabet); //reiterate the loop until the conditions are satisfied

    int i;
    for(i = 0; i < len; i++)
    {
        printf("%dth digit of Key= ", i+1);
        scanf("%c\n", &key[i]);
    }

    for(i = 0; i < len; i++){
        printf("key[%d] = %c\n", i, key[i]);
    }
    return(0);  
}