What is the difference between while loop and do while loop. I used to think both are completely same.Then I came across following piece of code:
do {
printf("Word length... ");
scanf("%d", &wdlen);
} while(wdlen<2);
This code works perfectly. It prints word length and tascans the input. But when I changed it to
while(wdlen<2){
printf("Word length... ");
scanf("%d", &wdlen);
}
It gives a blank screen. It do not work. So there is some functional difference between both loops. Can anybody explain it?
Is there any other difference in these two?
while test the condition before executing statements within the while loop.
do while test the condition after having executed statement within the loop.
source: let us C
While : your condition is at the begin of the loop block, and makes possible to never enter the loop.
Do While : your condition is at the end of the loop block, and makes obligatory to enter the loop at least one time.
do {
printf("Word length... ");
scanf("%d", &wdlen);
} while(wdlen<2);
A do-while loop guarantees the execution of the loop at least once because it checks the loop condition AFTER the loop iteration. Therefore it'll print the string and call scanf, thus updating the wdlen variable.
while(wdlen<2){
printf("Word length... ");
scanf("%d", &wdlen);
}
As for the while loop, it evaluates the loop condition BEFORE the loop body is executed. wdlen probably starts off as more than 2 in your code that's why you never reach the loop body.
The difference between a while constructs from Step 1 versus a do while is that any expressions within the do {} will be running at least once regardless of the condition within the while() clause.
println("\nStep 2: How to use do while loop in Scala")
var numberOfDonutsBaked = 0
do {
numberOfDonutsBaked += 1
println(s"Number of donuts baked = $numberOfDonutsBaked")
} while (numberOfDonutsBaked < 5)
Here is detail explaination: Explanation Visit: coderforevers
Probably wdlen starts with a value >=2, so in the second case the loop condition is initially false and the loop is never entered.
In the second case the loop body is executed before the wdlen<2 condition is checked for the first time, so the printf/scanf is executed at least once.
The difference between do while (exit check) and while (entry check) is that while entering in do while it will not check but in while it will first check
The example is as such:
Program 1:
int a=10;
do{
System.out.println(a);
}
while(a<10);
//here the a is not less than 10 then also it will execute once as it will execute do while exiting it checks that a is not less than 10 so it will exit the loop
Program 2:
int b=0;
while(b<10)
{
System.out.println(b);
}
//here nothing will be printed as the value of b is not less than 10 and it will not let enter the loop and will exit
output Program 1:
10
output Program 2:
[nothing is printed]
note:
output of the program 1 and program 2 will be same if we assign a=0 and b=0 and also put a++; and b++; in the respective body of the program.
The difference is in when the condition gets evaluated. In a do..while loop, the condition is not evaluated until the end of each loop. That means that a do..while loop will always run at least once. In a while loop, the condition is evaluated at the start.
Here I assume that wdlen is evaluating to false (i.e., it's bigger than 1) at the beginning of the while loop, so the while loop never runs. In the do..while loop, it isn't checked until the end of the first loop, so you get the result you expect.
The most important difference between while and do-while loop is that in do-while, the block of code is executed at least once, even though the condition given is false.
To put it in a different way :
do while in an exit control loop. while is an entry control loop.
While Loop:
while(test-condition)
{
statements;
increment/decrement;
}
Do While Loop:
do
{
statements;
increment/decrement;
}while(test-condition);
Find out more on this topic here: Difference Between While and Do While Loop
This is valid for C programming, Java programming and other languages as well because the concepts remain the same, only the syntax changes.
Also, another small but a differentiating factor to note is that the do while loop consists of a semicolon at the end of the while condition.
Do while loop will be executed atleast once.......but while loop will check the condition first and then it may or may not get executed depending on the condition.
In your example wdlen may assume any garbage value which is > 2 so while loop will never get executed.
whereas do while loop will be ececuted and will tell u to enter the value and check that value in terminating condition
While:
entry control loop
condition is checked before loop execution
never execute loop if condition is false
there is no semicolon at the end of while statement
Do-while:
exit control loop
condition is checked at the end of loop
executes false condition at least once since condition is checked later
there is semicolon at the end of while statement.
while test the condition before executing statements in the while loop.
do while test the condition after having executed statement inside the loop.
while(wdlen<2){
...
}
If wdlen (assuming it's a stack variable) is not initialized or assigned a value before the while loop is entered, it will contain whatever was in that space in memory before (i.e. garbage). So if the garbage value is < 2, the loop executes, otherwise it doesn't.
do{
...
}while(wdlen<2)
will execute once and then checks on condition to run loop again, and this time it might succeed if by chance wdlen which is uninitialized is found to be less than 2.
In WHILE first check the condition and then execute the program In DO-WHILE loop first execute the program at least one time then check the condition
Source: Stackoverflow.com