Extra values when printing an array(converting from %s to %c) - c

Question

I am trying to create a simple program where the user will have to enter a series of numbers and the program should output the square and the cube of the given number. However, when I try to use an array, it prints some random numbers I didn't even input. Any help would be appreciated to eliminate the unecessary input. Thank you.
#include <stdio.h>
int main()
{
char *value;
value = malloc(sizeof(20));
float answer;
int x;
int y;
scanf("%s" , value);
for(x=0; x < 20; x++)
{
y = value[x] - '0';
printf("\nThe square of %d is: %d" , y , y*y);
printf("\nThe cube of %d is: %d \n" , y , y*y*y);
}
return 0;
}

Answer 1

You are taking input in char and doing arithmetic operations on it.
Use this code, it will give you correct output.
#include <stdio.h>
int main()
{
int *value;
value = (int *)malloc(20 * sizeof(int));
//float answer;
int x;
int y;
for(x=0; x < 20; x++)
{
scanf("%d" , value + i);
}
for(x=0; x < 20; x++)
{
y = value[x];
printf("\nThe square of %d is: %d" , y , y*y);
printf("\nThe cube of %d is: %d \n" , y , y*y*y);
}
return 0;
}

Answer 2

The problem is with your malloc statement.
sizeof is used to determine the parameter size - in your case a hard-coded integer. The generated array is of size 4, which is exactly sizeof(20) instead of 20 integers which is 20*sizeof(int). It will be best to allocate the array statically if you know what size you need, see code below:
#include <stdio.h>
int main()
{
// This line sets value to an array of 20 ints
int value[20];
// Another, less favorable option, but still works:
// char *value = malloc(20 * sizeof(int))
float answer;
int x;
int y;
scanf("%s" , value);
for(x=0; x < 20; x++)
{
y = value[x] - '0';
printf("\nThe square of %d is: %d" , y , y*y);
printf("\nThe cube of %d is: %d \n" , y , y*y*y);
}
return 0;
}

Answer 3

The expression sizeof(20) returns the size of an int (the literal 20 is an int), which is typically only 4 bytes. In other words, you are only allocating a single integer for your array. All access outside of that single integer will result in undefined behavior.
You need to allocate sizeof(int) times the number of elements, if you want to dynamically allocate the memory. Or (which I recommend) use a normal array:
int value[20];
There is also another problem, in that you only read a single value from the user. You should probably be reading in the loop too.
But if you read in the loop, then there is really no need to even have an array to begin with, only a single int Variable which you read into, and then print its value as squared and cubed.
So the code could be simplified as
#include <stdio.h>
int main(void)
{
int value;
for (unsigned i = 0; i < 20 && scanf("%d", &value) == 1; ++i)
{
printf("The square of %d is: %d\n", value, value * value);
printf("The cube of %d is: %d\n", value, value * value * value);
}
return 0;
}
You also need to be careful of overflows when multiplying.