Getting calculation into struct field type of char

I have declared following struct

typedef struct STRUCT{
    char calculation[30];
}STRUCT;

I have a function:

int add_number(int num1, STRUCT *pointer) {
   int integer;
   int sum;
   printf("\nGive an integer to be added: ");
   scanf("%d", &integer);
   sum = num1 + integer;
   printf("%d+%d=%d\n", num1, integer, sum);
   scanf("%s", pointer->calculation);   // here I would want to get the %d+%d=%d stored into pointer->calculation 
   num1 = sum;
   return num1;
}

I would want to store this:

"%d+%d=%d\n", num1, integer, sum

into this:

pointer->calculation

(So for example, if num1 = 1 and integer = 2 I would want to have 3 = 1 + 2 stored into pointer->calculation)

How could it be done? I don't get it.

You can use e.g. snprintf to print a formatted string into a buffer (similar to what printf does but instead of directly to stdout).

For example:

#include <stdio.h>

int main()
{
	int num1 = 1;
	int integer = 2;
	int sum = num1 + integer;
	char buf[30];
	snprintf(buf, 29, "%d+%d=%d\n", num1, integer, sum);
	printf(buf);
}

Output:

1+2=3

The size passed to snprintf is 29 instead of 30, because we need to keep one element for the zero termination of the string.

Note: you wrote that you would like the buffer to contain 3 = 1 + 2 which does not match your string format "%d+%d=%d\n", so I assume you meant you'd like 1+2=3.