Comparison between pointer (char*) and integer error - c

Question

So basically the user inputs how many countries they have visited in a variable, then I take it as the size of the array.
After that I use a for loop to list out all the countries visited. But I want to make my code a little smarter and put and at the end of the sentence for the final country.
For example 3 countries:
You visited Japan, Korea and Canada.
^ ^^^
#include <stdio.h>
#include <cs50.h>
int main(void)
{
int number_of_p = get_int("How many countries did you visit?\n");
string countries[number_of_p];
for (int x = 0; x < number_of_p; x++)
{
countries[x] = get_string("What countries did you visit?\n");
}
printf("You visited %i countries, including: ", number_of_p);
for (int x = 0; x < number_of_p; x++)
{
printf("%s, ", countries[x]);
/* looks for last element in arrays, inserts 'and'
to make it look grammatically correct. */
if (countries[x] == number_of_p - 1 ) // ERROR HERE
{
printf(" and ");
}
}
printf(".\n");
}
I'm getting a comparison between pointer (char*) and integer error.
What does char* mean?
How do I access the last element in an array?

Answer 1

countries[x] is a string (which in CS50 is a typedef of char*), number_of_p is an int, you can't compare them, they are different types, you may have wanted to compare the index x, a possible (and quick) fix for your code, including the punctuation could look like this:
Live demo
#include <stdio.h>
#include <cs50.h>
int main(void)
{
int number_of_p = get_int("How many countries did you visit?\n");
string countries[number_of_p];
for (int x = 0; x < number_of_p; x++)
{
countries[x] = get_string("What countries did you visit?\n");
}
printf("You visited %i countries, including: ", number_of_p);
for (int x = 0; x < number_of_p; x++)
{
printf("%s", countries[x]);
if(x < number_of_p - 2){
printf(", ");
}
if (x == number_of_p - 2)
{
printf(" and ");
}
}
printf(".\n");
}
Input:
3
Japan
Korea
Canada
Output:
You visited 3 countries, including: Japan, Korea and Canada.

Answer 2

The condition in the if statement
if (countries[x] == number_of_p - 1 )
does not make a sense. The left operand countries[x] has the type char * while the right operand has the type int.
That is the type specifier string is an alias for the type char * and this declaration of an array
string countries[number_of_p];
is the same as
char * countries[number_of_p];
So you have an array of pointers that point to strings.
The alias string for the type char * in C defined the following way
typedef char * string;
The loop can look like
for (int x = 0; x < number_of_p; x++)
{
if ( x != 0 )
{
putchar( ',' );
putchar( ' ' );
if ( x == number_of_p - 1 )
{
printf( "%s ", "and " );
}
}
printf("%s", countries[x]);
}
Here is a demonstrative program
#include <stdio.h>
int main(void)
{
enum { number_of_p = 3 };
char *countries[number_of_p] =
{
"Japan", "Korea", "Canada"
};
printf( "You visited %i countries, including: ", number_of_p );
for (int x = 0; x < number_of_p; x++)
{
if ( x != 0 )
{
putchar( ',' );
putchar( ' ' );
if ( x == number_of_p - 1 )
{
printf( "%s ", "and" );
}
}
printf("%s", countries[x]);
}
return 0;
}
Its output is
You visited 3 countries, including: Japan, Korea, and Canada

Answer 3

You can do this sort of thing with one printf, using a predefined string for the separator. Using a single printf rather than special casing the tail of the list is generally easier to maintain. Many people find the ternary operator confusing; here's the approach in two different ways, using a switch first and the ternary operator second:
#include<stdio.h>
int main(void)
{
char *countries[] = { "foo", "bar", "baz" };
int number_of_p = sizeof countries / sizeof *countries;
/* First loop, using a switch */
for( int x = 0; x < number_of_p; x++ ) {
char *div = ", ";
switch( x ){
case sizeof countries / sizeof *countries - 2: div = ", and "; break;
case sizeof countries / sizeof *countries - 1: div = ".\n"; break;
}
printf("%s%s", countries[x], div);
}
/* Same loop as above, with if/else instead of the switch */
for( int x = 0; x < number_of_p; x++ ) {
char *div = ", ";
if( x == number_of_p - 2 ) {
div = ", and ";
} else if( x == number_of_p - 1 ) {
div = ".\n";
}
printf("%s%s", countries[x], div);
}
/* Same loop as above, written less verbosely */
for( int x = 0; x < number_of_p; x++ ) {
printf("%s%s", countries[x], x == number_of_p - 2 ? ", and " :
x == number_of_p - 1 ? ".\n" : ", ");
}
}