i don't undestand the task, please someone explain me the logic

Domanda sul compito

Risolta

at firs read i though about a loop, but i couldn't figure out how to do it, so i do it with paper so i think as it follow double a = (t/5.00); divide insert time for 5 double b = 0; b= t-(a*5); insert ime - (correct numer of ciclo) /* if (b>=0 && b,3) System.out.println("yellow"); else if(b>=4 && b<5) System.out.println("red"); else System.out.println("green");* but it was't working, a friend give me the attached solution, but i guess till i don't undesand it i can't solve it.

The pedestrian traffic light is programmed as follows:
at the beginning of each hour, the green signal is on for three minutes,
then the signal is yellow for one minute,
and then it is red for one minute.
Then the light is green again for three minutes, etc.
Use the keyboard to enter a real number t that represents the number of minutes that have elapsed since the beginning of the hour.
Determine what color the traffic light is at the specified time.
Display the result as follows:
"green" if the light is green,
"yellow" if the light is yellow, and
"red" if the light is red.

Example for 2.5:
green
Example for 3:
yellow
Example for 4:
red
Example for 5:
green

Requisiti:

The program should read a real number from the keyboard.
The program should display text on the screen.
If the light is green, display: "green"
If the light is yellow, display: "yellow"
If the light is red, display: "red"

package com.codegym.task.task04.task0416; /* Crossing the road blindly */ import java.io.*; import java.util.*; public class Solution { public static void main(String[] args) throws Exception { //write your code here Scanner s = new Scanner(System.in); double t = s.nextDouble(); double a = (t/5.00); double b = 0; b= t-(a*5); //Se divisibile per 3 è yellow, se divisibile per 4 è red e in tutti gli altri casi green if (t%3==0) System.out.println("yellow"); else if(t%4==0) System.out.println("red"); else System.out.println("green"); } }

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Thomas

Livello 41 , Bayreuth, Germany

4 ottobre 2022, 20:04

OK, you've managed the hardest part and have seen that you need to 'reduce' the input to a number between 0 and 5. There are several ways to do it. The logically easiest one would probably be to deduct 5 from the input as long as it is > 5.

double input = 21.8; // sample input
while (input >= 5)
    input -= 5;
System.out.println(input); // you probably will see rounding issues, that's the downside of using double

you also tried some other variant so I do not need to explain that one. I just used int instead of double to get a correct result

double input = 21.8;
input = input - (int) (input / 5) * 5;
System.out.println(input);

input / 5 results in 4.36, the cast to int (int) will cut off the .36 and the term will end in input - 4 * 5 The last possibility is to use the modulus operator. It describes the rest of an int division. Example: 23 % 5 = 3 cause 23 / 5 is 4 and the rest of that division is 3 and that's what modulus means. You can use it with int and double divisions.

double input = 21.8;
double result = input % 5;
System.out.println(result);

That's already a lot.... but that task has a lot more to offer ;) For example you're calculating with doubles. But why? The task is to output eg. green if a number is < 3. It does not matter if the number is 2.99 2.11 or 2. So we could just use ints instead.

Scanner console = new Scanner(System.in);
int input = (int) Double.parseDouble(console.nextLine());
int time = input % 5;
System.out.println(time);

Thomas

Livello 41 , Bayreuth, Germany

4 ottobre 2022, 20:13

Now you just use if - else if or switch case (cause there are only a handful possibilities and no doubles)

Scanner console = new Scanner(System.in);
int time = (int) Double.parseDouble(console.nextLine()) % 5;

// if - else - if
if (time < 3)
    System.out.println("green");
else if (time < 4)
    System.out.println("yellow");
else
    System.out.println("red");

// switch - case
switch (time) {
    case 3:
        System.out.println("yellow");
        break;
    case 4:
        System.out.println("red");
        break;
    default:
        System.out.println("green");
}

// new switch-case ... probably does not validate as it requires Java 14+
System.out.println(switch (time) {
    case 3 -> "yellow";
    case 4 -> "red";
    default -> "green";
});

EvelynSt

Livello 10 , Italy

5 ottobre 2022, 04:46

Wow this is so helpful!!!! I want try use while and inside it a if else, and i should convert double to int, we will see if it works. i got this in my mind but i couldn't figure out how to do it, specially the conversion, and build of loop, that for some strange reason i wanted with for, but you are right with while is much easier way to build it. Finally a light in the darkness on my brain. Thank you i will try

Thomas

Livello 41 , Bayreuth, Germany

5 ottobre 2022, 08:38

Perfect ;) So just a little explanation to your for loop problem. If you can use a while loop then you always can use a for loop as well. There is just one small difference. If you use the for loop and declare a counter variable in its header, then that variable is only valid inside the loop. If you use a while loop, then that variable has to be initialized outside and it's scope is different. Like here it would be inside the entire main method.

double input = 21.8; // scope of input is the entire main method
while (input >= 5)
    input -= 5;
System.out.println("input: " + input); // input is valid here

int justACounter = 0;
for (double input2 = 21.8; input2 >= 5; input2 -=5)
    System.out.format("input2 - iteration %d: %.2f%n", ++justACounter, input2);
// System.out.println(input2); // compiler does not know of that variable here anymore

// that's exactly the same as the while loop above
double input3 = 21.8;
for (; input3 >= 5;)
    input3 -=5;
System.out.println("input3: " + input3);

// or, you just need to initialize the counter var outside if you need its value outside later
double input4 = 21.8;
for (; input4 >= 5; input4 -=5);
System.out.println("input4: " + input4);

EvelynSt

Livello 10 , Italy

5 ottobre 2022, 15:14

you are a great at explaining, i make the task, understanding it, i understand what i was doing wrong, i want try on eclipse all the loop version that you post . really thank you, can you explain input -=5

Thomas

Livello 41 , Bayreuth, Germany

5 ottobre 2022, 19:14

That's just a short form for:

input = input - 5;

EvelynSt

Livello 10 , Italy

7 ottobre 2022, 13:51

Thanks you, you are great