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Another key concept in programming is the ability to test a conditional statement and make decisions about the flow of the program based on the truth value of the statement. Examples of such statements are "Is A equal to B?

To formulate the above questions, we will use binary operators that take two arguments 'A' and 'B' in the above examples and returns a value of "true" or "false". In Matlab, "true" is integer 1 and "false" is the integer 0. The binary operators that we will find use in comparing numeric values are called "relational operators", and are given here. Matlab interprets this by evaluating from left to right. Writing the same expression using parenthesis, it is perhaps clearer how Matlab is evaluating this expression.

The expression in the parenthesis evaluates to true , which Matlab stores are the integer value 1. The next conditional statement checks to see if m is less than 0, and of course it is not. These relational tests, when used with Matlab arrays, produce another array whose entries are '0' where the relational test is false, and '1' where the relational statement is true. These arrays in turn can be used to index into the original array and mask certain elements. For example, we can pull out only the positive entries of x:.

We can use this array to selectively set entries in x to different values depending on the masking array. Where these logical statements become useful is when they can be used to control the flow of a program. For example, suppose you wanted to divide one number by another number. You might first check to see that the divisor is not equal to 0. As a more complex example, we determine whether a quadratic polynomial has 0, 1 or two real roots, and whether the parabola opens up or down.

In the above, we had conditional statements checking to see whether a real number is equal to 0. In general, it is not a good idea to check for exact equality when compare the values of two real numbers.

Consider the following example. In finite precision arithmetic, we cannot compute the exact value of the function cos x. We can also use conditional statements in arrays. In this case, we have to come up with a single truth value for the logical array, and as a result, we must be very careful about how we use logical arrays in conditional statements.

For example, consider the following code fragment:. You might think that this sets negative entries in x to and positive entries to But in fact, all it does is set the array x to the scalar value The first statement x will only be true if all entries of x are negative. Since we probably have a mix of both positive and negative entries, the code will set x to the scalar value Sometimes, you may want to know the indices of the entries in an array that meet some criteria. For example, suppose we wanted to know where the first non-zero number in an array occurs.

The Matlab find function will return for us an array of indices at which some logically array is set to true. The array idx contains the indices of the positive entries. If no entries are found meeting the criteria, then find returns an empty array. In this lab, you will practice using conditional statements. Use a mask to find all the entries in the vector x that are between Plot the following discontinuous function over the interval [-5,5].

Be sure to include any end point conditions. Use only an array 'mask' to select points to include in each piece. Do you want to try the above code fragments on your own? Download the Matlab script that produces this page here.

Back to tutorial index Using logical operators and conditional statements Topics in this lab Logical operators Using logical operators with arrays Conditional statements Using arrays in conditional statements Using the Matlab 'find' function Lab exercises.

Logical operators Another key concept in programming is the ability to test a conditional statement and make decisions about the flow of the program based on the truth value of the statement. A Here are some simple examples illlustrating the use of relational or "logical" operators.

Back to the top Using logical operators with arrays These relational tests, when used with Matlab arrays, produce another array whose entries are '0' where the relational test is false, and '1' where the relational statement is true. For example, we can pull out only the positive entries of x: Or, we can pull out only the negative entries of x: Back to the top Conditional statements Where these logical statements become useful is when they can be used to control the flow of a program.

The real roots are: Back to the top Using arrays in conditional statements We can also use conditional statements in arrays. For example, consider the following code fragment: Back to the top Using the Matlab 'find' function Sometimes, you may want to know the indices of the entries in an array that meet some criteria. Back to the top Lab exercises In this lab, you will practice using conditional statements.