Program to compute the average of values: if to test chars - c

Question

Please, take a look at a program that prompts the user to enter 3 values and computes the average of these values.
I decided to add a test condition that checks if the user has entered a number or a character.
// a program that calculates the average of an array of 3 floating-point values; with for loop
#include <stdio.h>
int main (void)
{
float values[3];
float element;
float sum = 0;
int i;
printf ("Please, enter 3 floating values: \n");
for ( i = 0; i < 3; i++)
{
if (scanf ("%f", &element) == 1)
{
if ( (element >= 'a' && element <= 'z') || (element >= 'A' && element <= 'Z') )
printf ("Sorry, you have entered letters\n");
else if (element <= 0)
printf ("Please, enter positive values\n");
else
values[i] = element;
sum += values[i];
}
}
printf ("The average of 3 values is %.2f\n", sum / 3);
return 0;
}
When I run the program, I get the following problems:
1) it seem to work with positive numbers;
2) when I try to enter negative numbers, the program does print the required statement, but the result is still incremented into i, hence lowering the number of further entries; also, even if the number is negative, the program still comes up with some average nonsense value.
Please, enter 3 floating values:
-75
Please, enter positive values
98
Sorry, you have entered letters
-786.9
Please, enter positive values
The average of 3 values is 2087205705198174400484999168
3) when I test the program with letters, it still returns some average, though a zero one:
Please, enter 3 floating values:
jklj
The average of 3 values is 0.00
I would be grateful for your help!

Answer 1

Your element will never contain letters, since you've declared it as float and scanned it as float. If you wish to know if you were given a letters, you should read a string, test it, and only then convert it to float (comparing element to 'a' is actually comapring it to the ASCII code of 'a' = 97).
About the wrong sum - you forgot the curly braces for your else block. Change it to:
else {
values[i] = element;
sum += values[i];
}

Answer 2

If you want your program to end if negative numbers are entered you can use the return statement or deincrement the value of i for more cases. As for letter it never enters the if block so you can use an else. Try the following:
#include <stdio.h>
int main (void)
{
float values[3];
float element;
float sum = 0;
int i;
printf ("Please, enter 3 floating values: \n");
for ( i = 0; i < 3; i++)
{
if (scanf ("%f", &element) == 1)
{
if (element <= 0)
{
printf ("Please, enter positive values\n");
i--; // or use the return statement
}
else
{
values[i] = element;
sum += values[i];
}
}
else
{
printf("You entered special characters");
return;
}
}
printf ("The average of 3 values is %.2f\n", sum / 3);
return 0;
}