Program flow of displaying output in reverse - c

Question

1.I have this sample program in C that accepts positive inputs from users and stops when the user inputs negative numbers. It accepts a maximum of ten inputs. It then displays the result in reverse order. I'm wondering how does that happen because when I try to map it the value of c is negative, the second loop will only work if the condition (c>=0) is satisfied.
Example if I input only one positive number and I will input a negative number next.
input[0]=2
i=0+1
c=1-2
input[1]=-4
i=1+1
c=2-2
num=2*input[-1]?
#include<stdio.h>
int main()
{
float in;
float input[10];
int i=0;
int c;
printf("Please input a number:");
do{ scanf("%f", &in);
input[i] = in;
i=i+1;
if(i>10)
break;
c=i-1;
}while(in>=0);
do{ float num;
num= 2*input[c];
printf("Input = %.2f. Num= %.2f.\n", input[c], num);
c=c-1;
}while(c>=0);
return 0;
}
The input using the do-while code:
2
3
-4
The output:
Input:3.00 Output:6.00
Input:2.00 Output: 4.00
And When I try to implement it using for loop, it compiled in gcc but when I try to execute it displays segmentation fault (core dumped).
#include<stdio.h>
int main()
{ float in;
float input[10];
int i,c;
printf("Please input a number:");
for(i=0; in>=0; ){
scanf("%f", &in);
input[i] = in;
i=i+1;
if(i==10)
break;
c=i-2;
}
for(; c>=0;){
float num;
num= 2*input[c];
printf("Input = %.2f. Num= %.2f.\n", input[c], num);
c=c-1;
}
return 0;
}

Answer 1

I'm wondering how does that happen because when I try to map it the value of c is negative
I don't think c does go negitive, when i try to map it with 2 & -4 as inputs i get..
When you enter 2;
it makes input[i] = 2 (i is equal to 0) then after that i is incremented to 1.
and then it makes c = i - 1 or c = 1 - 1 or c = 0.
doesn't quit because in >= 0 (2).
When you enter -4;
i makes input[i] = -4 (i is equal to 1, c is equal to 0) then after i is incremented to 2
and then it makes c = i - 1 or c = 2 - 1 or c = 1.
this time quits because in < 0 & not in >= 0.
do{ scanf("%f", &in);
input[i] = in;
i=i+1;
if(i>10)
break;
c=i-1;
} while (in >=0)