How to count Up and Down using For Loop - c

Question

I want to get output like this using nested for loops:
0 0
0 1
0 2
0 3
1 3
1 2
1 1
1 0
2 0
2 1
2 2
2 3
3 3
3 2
3 1
3 0
So I came up with this solution:
for(i=0;i<4;i++) {
if(i == 1){
for(j=3;j>=0;--j) {
Serial.print(i);
Serial.print(" ");
Serial.println(j);
}
}else if(i == 3){
for(j=3;j>=0;--j) {
Serial.print(i);
Serial.print(" ");
Serial.println(j);
}
}else{
for(j=0;j<4;j++) {
Serial.print(i);
Serial.print(" ");
Serial.println(j);
}
}
}
This works but is there way to do this without if conditions inside the loops?

Answer 1

Here you are.
#include <stdio.h>
int main(void)
{
const int N = 3;
for ( int i = 0; i <= N; i++ )
{
for ( int j = 0; j <= N; j++ )
{
printf( "%d %d\n", i , i % 2 == 0 ? j : N - j );
}
}
return 0;
}
The program output is
0 0
0 1
0 2
0 3
1 3
1 2
1 1
1 0
2 0
2 1
2 2
2 3
3 3
3 2
3 1
3 0
Or the loops can be written like
const int N = 4;
for ( int i = 0; i < N; i++ )
{
for ( int j = 0; j < N; j++ )
{
printf( "%d %d\n", i , i % 2 == 0 ? j : N - j - 1 );
}
}

Answer 2

I am not the Arduino guy, but as its similar to C and this question is more about the algorithm let me sketch an approach in C:
#define LEFT_START (0)
#define LEFT_END (3)
#define RIGHT_START (0)
#define RIGHT_END (3)
enum DIRECTION {
DOWN = -1,
UP = 1
};
int main(void)
{
for (int direction = UP, i = LEFT_START, j = RIGHT_START - sign;
i <= LEFT_END;
++i, direction *= -1)
{
for (j += direction;
j >= RIGHT_START && j <= RIGHT_END;
j += direction)
{
printf("%d %d\n", i, j);
}
}
}
No if/then, not explicitly, nor implicitly via ternary-operator, BTW. ;)

Answer 3

I don't know what your purpose was, but you need to use if statement. This code will do the thing you want
int n =3;
void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
}
void loop() {
// put your main code here, to run repeatedly:
for (int i=0; i<=n; i++){
if(i%2 == 0){
for (int j=0; j<=n; j++){
Serial.print(i);
Serial.print(" ");
Serial.println(j);
}
}
else{
for (int j=n; j>=0; j--){
Serial.print(i);
Serial.print(" ");
Serial.println(j);
}
}
}
}
The output looks like this,
Defining variables/constant n, i, j is entirely up to you based on your project. This code is just a simple demonstration.