C Program - Do - while loops not working - c

Question

My target output is after users entering a number >2 & <20 (result show) then program continue asking users enter another number. Or if users enters number <=2 or >=20, it will not show result but just re-asking users to enter number.
My Current Output: If I input number <=2 || >=20, it will re-ask. but if I enter number between 2 and 20. It will just stops which suppose to be keep asking for entering new numbers.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define MAX 20
int main(void)
{
unsigned int random_array[MAX][MAX];
unsigned int r, c, x, y;
do {
printf("Number Matrix in array ? ");
scanf("%d", &c);
system("cls");
r = c;
if (c>2 && c<20) {
r = c;
for (x = 0; x <= r - 1; ++x)
{
for (y = 0; y <= c - 1; ++y)
{
random_array[x][y] = -1;
}
}
for (x = 0; x <= r - 1; ++x)
{
for (y = 0; y <= c - 1; ++y)
{
if (x == y)
random_array[x][y] = 0;
else
if (x<y)
random_array[x][y] = 1;
printf("%4d", random_array[x][y]);
}
puts("");
}
system("pause");
}
} while (c<=2 || c >=20);
return 0;
}

Answer 1

Since you want to prompt the user for input regardless of what they last input, you probably need an infinite loop. For this, replace your line with the while condition to this:
} while (1);
This basically tells your program to loop infinitely.

Answer 2

Your codes
do
{
// show something
} while (c<=2 || c >=20);
means that it will stop after showing something if (c > 2 && c < 20), and that is exactly why your program quits after the condition is met.
To achieve your goal, consider using an infinite loop, and do different things using if-else inside the loop.
printf("Number Matrix in array ? ");
scanf("%d", &c);
system("cls");
if (c<=2 || c >=20)
{
continue;
}
else
{
// show something
}

Answer 3

The while condition should be while(c>2&c<20). But if you enter a number <=2 or >=20 the program will end and will not ask you for an input anymore. So the solution would be to use an infinite while loop and use break to end the loop when you want by using a condition.

Answer 4

A do-while loop will run the block of code once, then will repeat until the while conditional evaluates to false.
do{
//Stuff
}while (c > 20 || c < 2);
That would do the //Stuff part once, then it would do it again until c is either greater than 20 or less than 2.
What you want to do is surround the entire thing in an infinite loop, (either for(;;) or while(1)) so that it continues regardless.
But you also want to validate the input, so that's when you could use a do-while loop. When you're getting the scanf, you could do something like:
do{
printf("Enter c: ");
scanf("%d", &c);
}while (c > 20 or whatever);
Then you could make him keep putting in c until it's the desired input!
Hope this helps.
EDIT: Here's an example of putting the do-while inside the while:
while(1){
do{
printf("Enter a positive number: ");
scanf("%d", &aNum);
}while(aNum < 0);
printf("Your positive number is %d.\n", aNum);
}
That would ask a user for input, and if he puts in a negative number it would ask him again. If it's a positive number it would print, then go back to the start and ask him for an input again.