Error: expected expression in else if statement - c

Question

This is the error I am getting:
test.c:110:21: error: expected expression
else if(isupper(p_i))
^
1 error generated.
In an else if statement towards the end of the code—"else if(isupper(p_i))"—the error is generated.
I have commented above this 'else if' statement. Please let me know what's wrong. Thank you.
#include <stdlib.h> // The library which contains the 'atoi()' function
#include <stdio.h> //
#include <cs50.h> // typedef char *string; and GetString()
#include <string.h> //
// 'argv[]' is an array of strings. (Fun fact: A string is an array of characters.)
// 'argc' is the integer variable which stores the number of strings that are in 'argv[]'.
int main(int argc, string argv[])
{
// VARIABLE DECLARATIONS
int k; // Integer variable for the non-negative, encryption key
string plaintext; // Variable to store the information to be encrypted
int n; // Integer variable for the string length
string ciphertext = NULL; // Variable to store the encrypted information
// This loop analyzes the command-line argument(s): We need exactly one argument (i.e. argc = 2)
if (argc > 2)
{
printf("\n");
printf("Too many arguments. Please try again.\n");
return 1;
}
else if (argc < 2)
{
printf("\n");
printf("This program requires that you provide an argument. Please try again.\n");
return 1;
}
else if (argc == 2)
{
k = atoi(argv[1]);
if (k == 0 || k < 0)
{
printf("\n");
printf("Invalid input: encryption key needs to be a non-negative integer.\n");
return 1;
}
}
// Prompt the user for a string input to be encrypted:
printf("\n");
printf("Please enter the information to be encrypted:\n");
plaintext = GetString();
n = strlen(plaintext);
printf("n = %d \n", n);
// We need to implement Caesar's Cipher algorithm:
// But first, we select for alphabets only by using the 'isalpha()' function:
for (int i = 0; i < n; i++)
{
int p_i = plaintext[i];
int isalpha(int p_i);
if (isalpha(p_i))
{
int islower(int p_i);
if (islower(p_i))
{
printf("Caesar's algorithm for the lower case goes here.\n");
}
int isupper(int p_i);
//-----------------------------------------------------------
// THE FOLLOWING else if STATEMENT IS THE SOURCE OF THE ERROR
//-----------------------------------------------------------
else if(isupper(p_i))
{
printf("Caesar's algorithm for the upper case goes here. \n");
}
}
else
{
for (int j = 0; j < n; j++)
ciphertext[i] = plaintext[i];
}
}
// Program terminates
return 0;
}

Answer 1

You have a function prototype between the if and the else:
if (islower(p_i))
{
printf("Caesar's algorithm for the lower case goes here.\n");
}
int isupper(int p_i);
//-----------------------------------------------------------
// THE FOLLOWING else if STATEMENT IS THE SOURCE OF THE ERROR
//-----------------------------------------------------------
else if(isupper(p_i))\
The else block must follow immediately after the if block. If you had put the line int isupper(int p_i); pretty much anywhere else (before the first time you use it), you would not have this error. Even better, you should load this prototype via #include <ctype.h> at the top of your file.

Answer 2

int isupper(int p_i);
//-----------------------------------------------------------
// THE FOLLOWING else if STATEMENT IS THE SOURCE OF THE ERROR
//-----------------------------------------------------------
else if(isupper(p_i))
Remove this declaration: int isupper(int p_i);.
Use the correct #include directive at the top of your source file to declare isupper function:
#include <ctype.h>

Answer 3

You cannot use any statement between if and else statement. Example -
if()
{
//some code
}
//some Code ---> This is wrong //this should not be in between if and else
else
{
//some code
}

Answer 4

Your function prototypes:
int islower(int p_i);
int isupper(int p_i);
do not belong within the main() function (or any function) - though that's technically legal which is why the first is not causing a problem.
However, they cannot live within an 'if / else' construct - you have the second sandwiched between an if clause and an else clause.
The best solution is to put them at the head of the file, or better just use:
#include <ctype.h>
to get the relevant header file with the prototypes in included, then remove the prototypes from the function.