Loop condition, the Peek method of the StreamReader determines whether there are any additional characters. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.
Privacy policy. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Contents Exit focus mode. Note The Do Is this page helpful? Yes No. Any additional feedback? Skip Submit. Submit and view feedback for This product This page. View all page feedback. In the previous lecture we learnt about logical statements that determine whether or not code gets run.
Here we learn about loops which allow sections of code to run zero or more times, with a controlling logical expression. The while loop repeats as long as the condition is true non-zero. This may sound slightly unintuitive if there's nothing to do, why have a loop?
In this rather contrived example, x is halved every time the loop is executed until at last the test condition is false and the loop quits. If x had started with a value of less than or equal to one the loop would not have executed even once. If the condition is false the body of the loop never executes at all. NB: if you have problems with this see the min-exercise in the section "Debugging loops". Write a simple while loop that reads in an integer. To demonstrate a while loop.
Create a new on-line program in a new window, with a suitable title and opening comment. Declare an int variable and initialise it to zero. Write a simple while whose test is that the variable is greater than or equal to zero and whose body: Prints a simple message asking for an integer Then has a call to scanf that reads the value of your variable from the keyboard. Now modify your program so that the initial value of your variable is -1 minus one. Do you think your program will ask you for any input?
Check the output is correct.. The increment In one sense while loops are a bit more complicated and subtle than they first appear. Whilst our initial pseudo-example looks simple, it's clear that the controlling expression can't go on being true forever or else the loop would never stop. To avoid accidental "infinite loops" that never stop the loop must do something to change the value of the controlling expression.
We shall refer to this as the increment. Another example of an increment might be a function call covered in the next lecture that returns a value to say if the loop should continue, or a call to scanf for example a loop that reads in an integer and loops until it's within a certain allowed range of values.
Although it's very unusual, it's possible for the controlling expression to contain its own increment although whether this is wise is another matter altogether.
Understanding the increment is critical to understanding the loop. We can use the while loop to write a "poor man's power function" which calculates value to the power of exponent where exponent is a positive integer.
We show the core loop of program first, with the complete program following below:. Although most control expressions are fairly simple we are, of course, allowed to use all the logical operators we can use with the if expression:. Then follow 2, 3, 4… — the values show up one after another. So 5 is not shown.
But the alert call is separate. So it increments i to 5 , but returns the old value. Rewrite the code changing the for loop to while without altering its behavior the output should stay same.
Write a loop which prompts for a number greater than If the visitor enters another number — ask them to input again. Here we can assume that the visitor only inputs numbers.
Both checks are required. An integer number greater than 1 is called a prime if it cannot be divided without a remainder by anything except 1 and itself. For example, 5 is a prime, because it cannot be divided without a remainder by 2 , 3 and 4. Write the code which outputs prime numbers in the interval from 2 to n. For instance, we could look for the divisors from 2 to square root of i. But anyway, if we want to be really efficient for large intervals, we need to change the approach and rely on advanced maths and complex algorithms like Quadratic sieve , General number field sieve etc.
We want to make this open-source project available for people all around the world. Tutorial map. We often need to repeat actions. Loops are a way to repeat the same code multiple times. Curly braces are not required for a single-line body. Inline variable declaration. The continue directive helps decrease nesting.
A conditional branch with more than two possible flows of execution. In Python chained conditionals are written with if A Python statement that has two parts: a header and a body. The header begins with a keyword and ends with a colon :.
The body contains a series of other Python statements, all indented the same amount. A statement that controls the flow of execution depending on some condition.
In Python the keywords if , elif , and else are used for conditional statements. A variable used to count something, usually initialized to zero and incremented in the body of a loop.
A loop where we have an upper bound on the number of times the body will be executed. Definite iteration is usually best coded as a for loop. A sequence of one or more characters used to specify the boundary between separate parts of text. A loop where we just need to keep going until some condition is met.
A while statement is used for this case. To initialize a variable is to give it an initial value. In other programming languages this is not the case, and variables can be created without being initialized, in which case they have either default or garbage values. A statement or group of statements that execute repeatedly until a terminating condition is satisfied.
One program structure within another, such as a conditional statement inside a branch of another conditional statement. Thus when you can establish that two of the relations are false, you can assume the remaining one is true. To follow the flow of execution of a program by hand, recording the change of state of the variables and any output produced. Chapter 4 exercise set 0: Chapter Review.
Chapter 4 exercise set 1. Chapter 4 exercise set 2. Conditionals and loops. Flowchart of an if statement. I want spam! Flowchart of a if else statement. Python terminology Python documentation sometimes uses the term suite of statements to mean what we have called a block here. We could have written the previous example as follows: Flowchart of this nested conditional. Before we look at those, we need to review a few ideas. Each item in turn is re- assigned to the loop variable, and the body of the loop is executed.
Please come to my party on Saturday! Tip As with all the examples you see in this book, you should try this code out yourself and see what it does. Our trace so far would look something like this: name guess pos output 'Harrison' 'Maribel' 0. Nope, that's not it! Hint: letter 1 is 'H'. Guess again:. Guess again: 'Harrison' 'Henry' 1 Nope, that's not it!
Hint: letter 2 is 'a'. Guess again: 'Harrison' 'Hakeem' 2 Nope, that's not it! Hint: letter 3 is 'r'. Guess again: 'Harrison' 'Harold' 3 Nope, that's not it! Hint: letter 4 is 'r'. Guess again: 'Harrison' 'Harry' 4 Nope, that's not it! Hint: letter 5 is 'i'. Guess again: 'Harrison' 'Harrison' 5 Great, you got it in 6 guesses!
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