Prompt user to re-enter number - c

Question

I'm doing a practice 'do while' program where I want the user to enter three numbers and the program will print the sum on screen. After receiving the answer, the program will ask the user if he wants to enter another three numbers to get another answer, and so on. I'm fine with this.
If user enters anything other than an integer ("A,!,%") etc, I want the program to prompt the user to re-enter a number. See comments in program.
#include <stdio.h>
/*
Do - While
This program shall ask the user to enter
three numbers and print out the sum. Entering letters
or special characters will ask the user to re-enter that
number.
example:
Enter Number 1: 2
Enter Number 2: 5
Enter Number 3: 9
Answer is 16
...
Enter Number 1: 2
Enter Number 2: yum here user incorrectly enters letters
Enter Number 2: 8 re-prompted to enter number to continue
Enter Number 3: 9
Answer is 19
*/
int main(void) {
int a,b,c,ans,n;
do{
do{
printf("Enter Number 1: ");
scanf("%d", &a);
printf("\n");
}
while((a>0) || (a<0)|| (a==0));
do{
printf("Enter Number 2: ");
scanf("%d", &b);
printf("\n");
}
while((b>0) || (b<0)|| (b==0));
do{
printf("Enter Number 3: ");
scanf("%d", &c);
printf("\n");
}
while ((c>0) || (c<0)|| (c==0));
ans = a+b+c;
printf("Answer is %d\n\n", ans);
printf("Press 1 to start over, or 0 to quit...");
scanf("%d",&n);
printf("\n");
}while (n!=0);
return 0;
}

Answer 1

Your program contains multiple repeated sections. Since you are gathering three numbers, you should use a for loop which runs three times.
for (int i = 0; i < 3; i++) {
/* ... */
}
Inside the for loop, you're gathering the i+1-th number each time. If the user doesn't enter a valid number, you keep trying to gather it. So the do-while loop containing the printf and scanf will go inside the for loop.
for (int i = 0; i < 3; i++) {
do {
printf("Enter Number %d: ", i+1);
scanf( /* ... */ );
} while ( /* ... */ );
}
scanf returns the number of input items successfully read as an int. So the do-while loop should repeat as long as the return value of scanf is zero. We could store the return value in a variable and check the variable in the while (...);, but we can just move the scanf itself into the while (...);. We also need an array to store the three input numbers.
int n[3];
for (int i = 0; i < 3; i++) {
do {
printf("Enter Number %d: ", i+1);
} while (scanf("%d", n+i) == 0);
}
The rest of the program would loop over the array and store the sum of the elements. You would then output the sum. This approach is robust and maintainable as changing the amount of input numbers is easy and repeated or similar code sections are eliminated.

Answer 2

You can use fgets to get the input and use strtol() to cast the string the user input into an int. If strtol returns 0 when the user input does not start with a number or have a number in it. From there you can check if the user input is 0 and then reprompt the user until a is not 0.
*instead of scanf()*
char num1[5];
char *end;
fgets(num1, 5, stdin);
a = strtol(num1, &end, 10);
while( a = 0){
fgets....
}