Although these operations are probably familiar to you, the Julia symbols are different from the mathematical symbols. The semantics meaning of these operators is similar to their meaning in English. Finally, the!
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In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly. Conditional statements give us this ability. The simplest form is the if statement:. The boolean expression after if is called the condition.
If it is true, the indented statement runs. If not, nothing happens. Statements like this are called compound statements.
There is no limit on the number of statements that can appear in the body. The syntax looks like this:. If the remainder when x is divided by 2 is 0, then we know that x is even, and the program displays an appropriate message. If the condition is false, the second set of statements runs.
Since the condition must be true or false, exactly one of the alternatives will run. The alternatives are called branches , because they are branches in the flow of execution. 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 run. There is no limit on the number of elseif statements. 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 runs and the statement ends. Even if more than one condition is true, only the first true branch runs. One conditional can also be nested within another. We could have written the example in the previous section like this:. The outer conditional contains two branches. The first branch contains a simple statement. The second branch contains another if statement, which has two branches of its own. Those two branches are both simple statements, although they could have been conditional statements as well.
Although the non-compulsory indentation of the statements makes the structure apparent, nested conditionals become difficult to read very quickly. 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:.
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It is legal for one function to call another; it is also legal for a function to call itself. It may not be obvious why that is a good thing, but it turns out to be one of the most magical things a program can do. For example, look at the following function:. If n is 0 or negative, it outputs the word, "Blastoff! A function that calls itself is recursive ; the process of executing it is called recursion. For simple examples like this, it is probably easier to use a for loop.
But we will see examples later that are hard to write with a for loop and easy to write with recursion, so it is good to start early. In Stack Diagrams , we used a stack diagram to represent the state of a program during a function call. The same kind of diagram can help interpret a recursive function. For a recursive function, there might be more than one frame on the stack at the same time.
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As usual, the top of the stack is the frame for Main. It is empty because we did not create any variables in Main or pass any arguments to it. The four countdown frames have different values for the parameter n. It does not make a recursive call, so there are no more frames.
If a recursion never reaches a base case, it goes on making recursive calls forever, and the program never terminates. This is known as infinite recursion , and it is generally not a good idea. Here is a minimal program with an infinite recursion:. In most programming environments, a program with infinite recursion does not really run forever. Julia reports an error message when the maximum recursion depth is reached:.
This stacktrace is a little bigger than the one we saw in the previous chapter. When the error occurs, there are recurse frames on the stack! If you encounter an infinite recursion by accident, review your function to confirm that there is a base case that does not make a recursive call. And if there is a base case, check whether you are guaranteed to reach it.
The programs we have written so far accept no input from the user. They just do the same thing every time. Julia provides a built-in function called readline that stops the program and waits for the user to type something. Before getting input from the user, it is a good idea to print a prompt telling the user what to type:. A semi-colon ; allows to put multiple statements on the same line. In the REPL only the last statement returns its value. If you expect the user to type an integer, you can try to convert the return value to Int64 :.
When a syntax or runtime error occurs, the error message contains a lot of information, but it can be overwhelming. The most useful parts are usually:.
Syntax errors are usually easy to find, but there are a few gotchas. In general, error messages indicate where the problem was discovered, but the actual error might be earlier in the code, sometimes on a previous line. The same is true of runtime errors. Suppose you are trying to compute a signal-to-noise ratio in decibels.
The formula is. To find the error, it might be useful to print the value of ratio, which turns out to be 0. The problem is in line 3, which uses floor division instead of floating-point division. A statement that consists of a header and a body. The body is terminated with the keyword end. Eventually, an infinite recursion causes a runtime error. Write a script that reads the current time and converts it to a time of day in hours, minutes, and seconds, plus the number of days since the epoch.
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If you are given three sticks, you may or may not be able to arrange them in a triangle. For example, if one of the sticks is 12 inches long and the other two are one inch long, you will not be able to get the short sticks to meet in the middle. For any three lengths, there is a simple test to see if it is possible to form a triangle:.
If any of the three lengths is greater than the sum of the other two, then you cannot form a triangle. Otherwise, you can. Write a function that prompts the user to input three stick lengths, converts them to integers, and uses istriangle to check whether sticks with the given lengths can form a triangle. What is the output of the following program? Draw a stack diagram that shows the state of the program when it prints the result. Write a docstring that explains everything someone would need to know in order to use this function and nothing else.
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