 # C Exercises: Check whether a number can be express as sum of two prime numbers

## C For Loop: Exercise-56 with Solution

Write a program in C to Check Whether a Number can be Express as Sum of Two Prime Numbers.

Pictorial Presentation: Sample Solution:

C Code:

``````#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
int num,i,j,temp1,temp2,ctr=0;
printf("input the number:\n");
scanf("%d",&num);
for(i=2;i<=num/2;i++){
temp1=i;
temp2=num-i;
for(j=2;j<=i/2;j++){
if(i%j==0){ctr++;break;}
}
if(ctr==0){
for(j=2;j<=(num-i)/2;j++){
if((num-i)%j==0){ctr++;break;}
}
if(ctr==0) printf("%d can be written as %d + %d.\n ",num,i,num-i);
}
ctr=0;
}
return 0;
}
```
```

Sample Output:

```input the number: 16
16 can be written as 3 + 13.
16 can be written as 5 + 11.
```

Flowchart: C Programming Code Editor:

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## C Programming: Tips of the Day

You are seeing the ffffff because char is signed on your system. In C, vararg functions such as printf will promote all integers smaller than int to int. Since char is an integer (8-bit signed integer in your case), your chars are being promoted to int via sign-extension.

Since c0 and 80 have a leading 1-bit (and are negative as an 8-bit integer), they are being sign-extended while the others in your sample don't.

```char    int
c0 -> ffffffc0
80 -> ffffff80
61 -> 00000061
```
Here's a solution:
```char ch = 0xC0;
printf("%x", ch & 0xff);
```

This will mask out the upper bits and keep only the lower 8 bits that you want.

Ref : https://bit.ly/3vOLizM