Here, the first print statement will execute if food is equal to 'spam' , and the print statement indented under the else clause will get executed when it is not. The syntax for an if else statement looks like this:. Each statement inside the if block of an if else statement is executed in order if the boolean expression evaluates to True.
The entire block of statements is skipped if the boolean expression evaluates to False , and instead all the statements under the else clause are executed. There is no limit on the number of statements that can appear under the two clauses of an if else statement, but there has to be at least one statement in each block. In that case, you can use the pass statement, which does nothing except act as a placeholder. Python documentation sometimes uses the term suite of statements to mean what we have called a block here.
Notice too that else is not a statement. The if statement has two clauses , one of which is the optional else clause. The Python documentation calls both forms, together with the next form we are about to meet, the if statement. Sometimes there are more than two possibilities and we need more than two branches.
One way to express a computation like that is a chained conditional :. Again, exactly one branch will be executed. There is no limit of the number of elif statements but only a single and optional final else statement is allowed and it must be the last branch in the statement:. Each condition is checked in order. If the first is false, the next is checked, and so on. If one of them is true, the corresponding branch executes, and the statement ends.
Even if more than one condition is true, only the first true branch executes. One conditional can also be nested within another. It is the same theme of composibility, again! We could have written the previous example as follows:. The outer conditional contains two branches. The second branch contains another if statement, which has two branches of its own.
Those two branches could contain conditional statements as well. Although the indentation of the statements makes the structure apparent, nested conditionals very quickly become difficult to read.
In general, it is a good idea to avoid them when you can. Logical operators often provide a way to simplify nested conditional statements. For example, we can rewrite the following code using a single conditional:. The print function is called only if we make it past both the conditionals, so we can use the and operator:. Computers are often used to automate repetitive tasks. Repeating identical or similar tasks without making errors is something that computers do well and people do poorly.
Repeated execution of a set of statements is called iteration. Python has two statements for iteration — the for statement, which we met last chapter, and the while statement.
As we saw back in the Variables are variable section, it is legal to make more than one assignment to the same variable. A new assignment makes an existing variable refer to a new value and stop referring to the old value. With reassignment it is especially important to distinguish between an assignment statement and a boolean expression that tests for equality.
Unlike mathematics, it is not! Note too that an equality test is symmetric, but assignment is not. Furthermore, in mathematics, a statement of equality is always true. The third line changes the value of a but does not change the value of b , so they are no longer equal. When an assignment statement is executed, the right-hand-side expression i. Then the result of that evaluation is written into the variable on the left hand side, thereby changing it. One of the most common forms of reassignment is an update, where the new value of the variable depends on its old value.
So after executing the two lines above, n will have the value Before you can update a variable, you have to initialize it, usually with a simple assignment:.
This second statement — updating a variable by adding 1 to it — is very common. It is called an increment of the variable; subtracting 1 is called a decrement. The general form of a for loop is:.
This is another example of a compound statement in Python, and like the branching statements, it has a header terminated by a colon : and a body consisting of a sequence of one or more statements indented the same amount from the header. The loop variable is created when the for statement runs, so you do not need to create the variable before then.
Each iteration assigns the the loop variable to the next element in the sequence, and then executes the statements in the body. The statement finishes when the last element in the sequence is reached. This type of flow is called a loop because it loops back around to the top after each iteration.
Running through all the items in a sequence is called traversing the sequence, or traversal. As with all the examples you see in this book, you should try this code out yourself and see what it does. You should also try to anticipate the results before you do, and create your own related examples and try them out as well. If you get the results you expected, pat yourself on the back and move on. This is the essence of the scientific method, and is essential if you want to think like a computer programmer.
Often times you will want a loop that iterates a given number of times, or that iterates over a given sequence of numbers. The range function come in handy for that. We start by declaring the while loop, setting a condition, and then the code that we want to execute which goes inside. In the example above, as long as x is less than 4, the while loop will continue to execute. The code that is being executed increases the value of x with each iteration as well as adds the value of x to y.
A similar way to set up a while loop is with a do…while. A do…while statement is similar to a while loop in the fact that it will continue to run until the condition becomes false. The only difference is the order in which the loop runs. As you can see, i starts at 0 and is incremented by 1 with each passing iteration.
The new value of i is then also logged to the console. In this example, the console will print 1 2 3 4 5. Something that you need to be careful of when dealing with loops is creating one where a stopping condition is never met. This loop will run forever infinitely and can have negative consequences on your computer such as having it freeze and become unresponsive. If you would like to know more about loops, you can read about them here:.
Computer Science sophomore at the University of Mississippi. Coding tutorials and news. The developer homepage gitconnected. Sign in. All loops in C are logic controlled whatever that means : the value of the expression controlling the loop is evaluated in a logic true or false or 1 or 0 context. Add a comment. Active Oldest Votes. Improve this answer. Barmar Barmar k 49 49 gold badges silver badges bronze badges.
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