Multi tool use

How to repeat input using a while loop with a sentinal

I need help with regards to where to put a while loop on this code. I just began studying Java a few weeks ago. I would like to write in a sentinel value of -1, if entered, the program shall exit. As long as user input is not -1, keep asking repeating the program.

When I put the while loop, "while(currentPop != -1)", below the first question, "Enter current population", the program runs its first course successfully. However, it does not loop back to the first question. Instead, it goes directly to the second question, "Enter birth rate:".

How shall I proceed and make sure that the first question keeps getting asked after running through the loops?

Thank you all!

import java.util.Scanner;

public class Population
{
public static void main(String args)
{
Scanner scan = new Scanner(System.in);
double birthRate, deathRate;
double currentPop = 0;
double newPop;
long years;

System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

System.out.print("Enter birth rate: ");
birthRate = scan.nextDouble();

System.out.print("Enter death rate: ");
deathRate = scan.nextDouble();
newPop = 0;
years = 0;

System.out.println("===================");
System.out.println("YEAR POPULATION");
System.out.println("===================");

if (birthRate > deathRate) {
System.out.printf("0 %,15dn", (int)currentPop);
double growthRate = (birthRate - deathRate);
double doublingTime = Math.log(2) /
Math.log(1 +(growthRate/100));
for (years = 1; years <= (doublingTime+1); years++) {
newPop = ((growthRate/100) * currentPop) + currentPop;
currentPop = newPop;
System.out.printf("%,d %,15dn",years,(int)currentPop);
}
System.out.printf("nIt will take %,d years to reach double "
+ "the population of %,dnn",
(int)(doublingTime + 1),(int)currentPop);
} else if (birthRate < deathRate) {
System.out.printf("0 %,15dn", (int)currentPop);
double growthRate = (birthRate - deathRate);
double decreaseTime = Math.log(1/currentPop)
/ Math.log(1 + (growthRate/100));
for (years = 1; years < (1 + decreaseTime) ; years++) {
newPop = ((growthRate/100) * currentPop) + currentPop;
currentPop = newPop;
System.out.printf("%,d %,15dn",years,(int)currentPop);
}
System.out.printf("nPopulation will be zero in %,d years.n",
(int)decreaseTime + 1);
} else if(birthRate == deathRate) {
System.out.printf("0 %,15dn", (int)currentPop);
double growthRate = (birthRate - deathRate);
double decreaseTime = Math.log(1/currentPop)
/ Math.log(1 + (growthRate/100));
for (years = 1; years < (1 + decreaseTime) ; years++) {
newPop = ((growthRate/100) * currentPop) + currentPop;
currentPop = newPop;
System.out.printf("%,d %,15dn",years,(int)currentPop);
}
System.out.printf("nPopulation is stable.");
}
}


}

5 Answers
5

Simply return from the main method

System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

if (currentPop == -1.0) {
return;
}

System.out.print("Enter birth rate: ");


Use break or continue within the body of a looping construct

break

continue

while (true) {
System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

if (currentPop == -1) {
break;
} else if (currentPop <= 0) {
System.out.println("Population must be positive");
continue; // restart the loop
}

System.out.print("Enter birth rate: ");

...
}
System.out.println("Done!");


-1

You can abstract out a repeated task (multiple inputs) using a method

public static double getValue(Scanner s, String msg) {
double value = -1;
while (value == -1) {
System.out.print(msg);
value = s.nextDouble();
}
return value;
}


In the main method

currentPop = getValue(scan, "Enter current population: ");
birthRate = getValue(scan, "Enter birth rate: ");


While loops will repeat only the code they contain. So this code:

System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

while (currentPop != -1) {
System.out.print("Enter birth rate: ");
birthRate = scan.nextDouble();

// the rest of your code
}


will only repeat what's inside the while loop. In this case, it will only ask for the birth rate.

The actual code you want is:

while (true) {
System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();

if (currentPop == -1) {
break;
}

System.out.print("Enter birth rate: ");
birthRate = scan.nextDouble();

// the rest of your code
}


Let's break this down.

If you want the current population to be asked for again, you need to put that line within the while loop.

I don't know if you're familiar with break, but you said you were new to this. What break does is exits whatever loop it's in. This, along with the while (true) means the while loop will run forever, unless the if statement is called. Hope that helps!

break

break

while (true)

I think you should add while(currentPop != -1) { before first question and rest of code. After that line remember to set currentPop = 0 and on the end of code }.
edit:
And of course
after the first question
if (currentPop == -1) break;

while(currentPop != -1) {

currentPop = 0

}

if (currentPop == -1) break;

Here is the code that you want -

import java.util.Scanner;

public class Population
{
public static void main(String args)
{
Scanner scan = new Scanner(System.in);
double birthRate, deathRate;
double currentPop = 0;
double newPop;
long years;

while(true) {
System.out.print("Enter current population or -1 to exit: ");
currentPop = scan.nextDouble();
if(currentPop == -1)
break;

System.out.print("Enter birth rate: ");
birthRate = scan.nextDouble();

System.out.print("Enter death rate: ");
deathRate = scan.nextDouble();
newPop = 0;
years = 0;

System.out.println("===================");
System.out.println("YEAR POPULATION");
System.out.println("===================");

if (birthRate > deathRate) {
System.out.printf("0 %,15dn", (int)currentPop);
double growthRate = (birthRate - deathRate);
double doublingTime = Math.log(2) /
Math.log(1 +(growthRate/100));
for (years = 1; years <= (doublingTime+1); years++) {
newPop = ((growthRate/100) * currentPop) + currentPop;
currentPop = newPop;
System.out.printf("%,d %,15dn",years,(int)currentPop);
}
System.out.printf("nIt will take %,d years to reach double "
+ "the population of %,dnn",
(int)(doublingTime + 1),(int)currentPop);
} else if (birthRate < deathRate) {
System.out.printf("0 %,15dn", (int)currentPop);
double growthRate = (birthRate - deathRate);
double decreaseTime = Math.log(1/currentPop)
/ Math.log(1 + (growthRate/100));
for (years = 1; years < (1 + decreaseTime) ; years++) {
newPop = ((growthRate/100) * currentPop) + currentPop;
currentPop = newPop;
System.out.printf("%,d %,15dn",years,(int)currentPop);
}
System.out.printf("nPopulation will be zero in %,d years.n",
(int)decreaseTime + 1);
} else if(birthRate == deathRate) {
System.out.printf("0 %,15dn", (int)currentPop);
double growthRate = (birthRate - deathRate);
double decreaseTime = Math.log(1/currentPop)
/ Math.log(1 + (growthRate/100));
for (years = 1; years < (1 + decreaseTime) ; years++) {
newPop = ((growthRate/100) * currentPop) + currentPop;
currentPop = newPop;
System.out.printf("%,d %,15dn",years,(int)currentPop);
}
System.out.printf("nPopulation is stable.");
}
}
scan.close();
}
}


To implement a while, loop at first you've to determine the termination condition, and it works till the condition reaches the termination condition (i.e.false),its syntax could be as

while(true) { //your code}


By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.