Expected expression error in C - c

Question

I'm trying to create two functions. The first function accepts integer inputs from the user until they are between 0 and 100. The seconds function display the validation to the stdOut.
However, it keeps giving me an error saying "Expected expression" when I call the function.
#include <stdio.h>
// function prototype for the function input
int input(int);
// function prototype for the function validate
int validate(int);
//main function
int main(void)
{
//calling the function input
input(int x)
//calling the function validate
validate(int y)
return 0;
}
// Function definition for input
int input(int a)
{
int r;
printf("Enter the int value of r\n");
scanf("%d",&r);
}
// Function definition for validate
int validate(int b)
{
int r;
if(r>= 0 && r<= 100)
printf("Valid number");
else
printf("Invalid");
}

Answer 1

There is at least one bug on almost every line of this program.
This is a standard problem for which there is a whole lot of incorrect advice out there (most importantly, only the strtol/strtoul/strtod family of functions should be used to convert strings to numbers; never use the atoi family and never use scanf) so I am going to give a complete worked example of how to write this program correctly, including proper use of comments.
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
long read_number_in_range(const char *prompt, long lo, long hi)
{
// a signed 64-bit number fits in 21 characters, +1 for '\n', +1 for NUL
char buf[23], *endp;
long rv;
for (;;) {
puts(prompt);
if (!fgets(buf, sizeof buf, stdin)) {
perror("stdin");
exit(1);
}
errno = 0;
rv = strtol(buf, &endp, 10);
if (endp != buf && (*endp == '\0' || *endp == '\n')
&& !errno && rv >= lo && rv <= hi) {
return rv;
}
// if we get here, fgets might not have read the whole line;
// drain any remainder
if (!strchr(buf, '\n')) {
int c;
do c = getchar();
while (c != EOF && c != '\n');
}
puts("?Redo from start");
}
}
int main(void)
{
long val = read_number_in_range("Enter the int value of r", 0, 100);
// do something with val here
return 0;
}
Read on for line-by-line nitpicking of the original program.
#include <stdio.h>
Correct.
// function prototype for the function input
Comment redundant with code.
int input(int);
Function signature incorrect (see comments on body of function).
// function prototype for the function validate
Comment redundant with code.
int validate(int);
Function signature incorrect (see comments on body of function).
//main function
Comment redundant with code.
int main(void)
{
Correct.
//calling the function input
Comment redundant with code.
input(int x)
Variables cannot be declared inside function call expressions.
Return value of function is ignored.
Missing semicolon at end of line.
//calling the function validate
Comment redundant with code.
validate(int y)
Value returned from input should be passed to validate, presumably, instead of a new uninitialized variable.
Variables cannot be declared inside function call expressions.
Return value of function is ignored.
Missing semicolon at end of line.
return 0;
}
Correct.
// Function definition for input
Comment redundant with code.
int input(int a)
{
Parameter a is unnecessary.
int r;
Correct.
printf("Enter the int value of r\n");
Minor: use puts when there is nothing to format.
scanf("%d",&r);
Never use scanf.
}
Missing return r;.
// Function definition for validate
Comment redundant with code.
int validate(int b)
{
Function has no return value, so should be void validate(int b).
int r;
Unnecessary variable.
if(r>= 0 && r<= 100)
r should be b on this line.
printf("Valid number");
else
printf("Invalid");
Minor: again, puts.
}
Correct.

Answer 2

You have some stray ints in your calls, they need to go.
The calls should probably be:
x = input();
validate(x);
You can't pass an integer to a function and expect it to change in the caller's context, that is not how C's pass-by-value semantics work. You should just return the number from input() instead, i.e. its prototype should be int input(void);.

Answer 3

You need to add semicolons at the end of each line of code in your main() function, and also remove the type specifier in the function calls. Also, don't forget to declare the variables x and y somewhere:
int main(void)
{
int x=0;
int y=0;
//calling the function input
input(x);
//calling the function validate
validate(y);
return 0;
}

Answer 4

I bet the compiler asks you for an expression in some place where you write a function declaration with a missing semicolon.
//calling the function input
input(int x)
//calling the function validate
validate(int y)
Neither of these is a function call.

Answer 5

#include <stdio.h>
int input(int*);
int validate(int);
int main(void){
int x;
input(&x);
if(validate(x))
printf("Valid number");
else
printf("Invalid");
return 0;
}
int input(int *r){
printf("Enter the int value of r\n");
scanf("%d", r);
return *r;
}
int validate(int r){
return r>= 0 && r<= 100;
}