#include <stdio.h>
#include <math.h>

int c;
float string[1024];
int i=0,b=0;

void main(){
    while((scanf("%d",&c)) != EOF){
        string[b]=c;
        b++;
        }
    for (i=b-1;i>-1;i--)printf("%f \n",sqrt(string[i]));
}

Try double instead of float because the numbers can be as large as 10^18.
Also the 1024 limit of the array is small, try 1000000.

I try, but I have got the same result

1) Sure float is not enough, use double.
2) Also the numbers in input can be about 10^18, and int is also not enough! Use long long.
3) Also there is about 256*1024 = 262 144 bytes in input, so there could be about 131 072 numbers!

I edited your code a little and got AC.

#include <stdio.h>
#include <math.h>

long long c;
double string[1000000];
long i=0, b=0;

void main(){
    while((scanf("%d",&c)) != EOF){
        string[b]=c;
        b++;
        }
    for (i=b-1;i>-1;i--)printf("%f \n",sqrt(string[i]));
}

?

Now problem with scanf/printf parameters
 - for long long "lld" or "I64d";
 - for double "lf"

So correct to scanf("%lld"... and printf("%lf... and get AC =)

Thank you :)