Sum and average for n numbers with -1 as exit integer - c

Question

Hello I have a small problem with my code and I want to understand it. My task is to write a program that take the sum and average for n numbers with while loops or nested while loops with if conditions, when you want to exit the code you should enter -1. The code is down below. The problem I have is that I can't get it to exclude the -1 from the calculation. What am I doing wrong. And it is suppose to be a simple code.
int main(void) {
int count;
float sum, average, number;
sum = 0;
count = 0;
printf("Calculate sum and average (-1 to quit)\n");
while (number!=-1) {
scanf("%f", &number);
sum = sum + number;
count = count + 1;
}
average = sum / count;
printf("Sum=%f", sum);
printf(" Average=%f", average);
}

Answer 1

in the while block, you read the number and then add it to your average calculation, and only then after the iteration repeats you're checking if it's different than -1 to stop the iteration:
there are obviously different ways to solve it:
one way can be to use while(true) and in the while block after you read the number add an if statement to compare it with -1 and break out of the loop
while (true) {
scanf("%f", &number);
if (number == -1) break;
sum = sum + number;
count = count + 1;
}

Answer 2

Reordering the different steps could solve this:
int main(void) {
int count;
float sum, average, number=0.0f;
sum = 0;
count = -1;
printf("Calculate sum and average (-1 to quit)\n");
while (number!=-1) {
sum = sum + number; // first time no effect with 0
count = count + 1; // first time "no effect" counting to 0
scanf("%f", &number); // will not be counted or summed if -1
}
average = sum / count;
printf("Sum=%f", sum);
printf(" Average=%f", average);
}
Think it through with immediatly entered -1:
sum is 0+0, suitable for no numbers being added up
count is -1+1==0, suitable for no numbers
dividing by 0 should be avoided, but that is a separate issue
Think it through with one number:
number is read in with sum==0 and count==0
is not -1, so loop iterates again
sum and count are updated meaningfully
-1 one is entered
loop ends without processing -1
By the way, comparing a float for identity is risky. You would be safer if you could compare more "generously", e.g. while (number>-0.5).

Answer 3

I think you should take the input at the last of the loop as it will get check in the next iteration. And when -1 comes up it will not be added.
#include <stdio.h>
int main(){
int count=0;
float sum=0, average=0, number=0;
printf("Calculate sum and average (-1 to quit)\n");
while(number!=-1){
sum = sum + number;
count = count + 1;
scanf("%f", &number);
}
average = sum / count;
printf("Sum=%f", sum);
printf(" Average=%f", average);
}

Answer 4

Code needs to test number as -1 before including it in the summation. OP's code test almost does this right. OP's code test for -1 before number is even assigned leading to undefined behavior (UB).
float number;
while (number!=-1) { // UB!
Also good to test the return code of scanf() for success.
while (scanf("%f", &number) == 1 && number != -1) {
sum = sum + number;
count = count + 1;
}