cs50 pset2 substitution problem

functions.c

int calculate_key_size(char argv[]); // returns size of key 
int is_alphabet_arr(char argv[]); // checks whether all elements of an array are alphabets
int is_alphabet_char(char a); // checks whether a character is an alphabet
int repeat_letter_check(char argv[]); // checks whether a letter repeats in an array
int alphabet_case_check(char a); // returns 0 for lowercase and 1 for uppercase

int calculate_key_size(char argv[]){
	int i = 0;
	int key_size = 0;
	while(argv[i] != '\0'){
		key_size += 1;
		i++;
	}
	return key_size;
}

int is_alphabet_arr(char argv[]){
	int a = 0;
	while(argv[a] != '\0'){
		if(!((argv[a] >= 'A' && argv[a] <= 'Z') || (argv[a] >= 'a' && argv[a] <= 'z'))){
			return 1;
		}
		a++;
	}
	return 0;
}

int is_alphabet_char(char a){
	for(int j = 'A', k = 'a'; j < ('Z'+1); j++, k++){
		if(a == j || a == k)
			return 0;
	}
	return 1;
}
int repeat_letter_check(char argv[]){
	char used_alphabets[26];
	for(int b = 0; b < 26; b++){
		for(int j = 'A', k = 'a'; j < ('Z'+1); j++, k++){
			if((int)argv[b] == j || (int)argv[b] == k){
				used_alphabets[j-'A'] = '!'; // we want to map A to first index of array so substacting 65 or simply 'A' works
				break;
			}
		}
	}
	for(int c = 0; c < 26; c++){
		if(used_alphabets[c] != '!'){
			return 1;
		}
	}
	return 0;
}
int alphabet_case_check(char a){
	for(int j = 'A', k = 'a'; j < ('Z'+1); j++, k++){
		if(a == j)
			return 1;
		else if(a == k)
			return 0;
	}
	return -1; //can return any value as such because execution will never reach here
}