Difference between while loop and do while loop

Question

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  quot Word length     quot            scanf  quot  d quot    amp wdlen         while wdlen lt 2    This code works perfectly  It prints word length and tascans the input  But when I changed it to while wdlen lt 2           printf  quot Word length     quot            scanf  quot  d quot    amp 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

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While Loop   while test-condition          statements        increment decrement       Lower Execution Time and Speed Entry Conditioned Loop No fixed number of iterations   Do While Loop   do         statements        increment decrement   while test-condition      Higher Execution Time and Speed Exit Conditioned Loop Minimum one number of iteration   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

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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

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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 lt 2 condition is checked for the first time  so the printf scanf is executed at least once

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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     While- your condition is at the begin of the loop block  and makes possible to never enter the loop  In While loop  the condition is first tested and then the block of code is executed if the test result is true

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while test the condition before executing statements in the while loop   do while test the condition after having executed statement inside the loop

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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

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do while in an exit control loop  while is an entry control loop

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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 lt 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 lt 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

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The do while loop executes the content of the loop once before checking the condition of the while    Whereas a while loop will check the condition first before executing the content   In this case you are waiting for user input with scanf    which will never execute in the while loop as wdlen is not initialized and may just contain a garbage value which may be greater than 2

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while wdlen lt 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  lt  2  the loop executes  otherwise it doesn t    do        while wdlen lt 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

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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

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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

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do       printf  Word length            scanf   d    amp wdlen     while wdlen lt 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 lt 2       printf  Word length            scanf   d    amp 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

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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

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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

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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  lt  5    Here is detail explaination  Explanation Visit  coderforevers

[c] Difference between "while" loop and "do while" loop

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