Branches, Conditional Statements

Transcription

1 Branches, Conditional Statements

2 Branches, Conditional Statements A conditional statement lets you execute lines of code if some condition is met. There are 3 general forms in MATLAB: if if/else if/elseif/else

3 The if- Statement The if statement s basic form is if conditional expression code line 1 code line 2 Can be any number of lines of code If the conditional expression is true, MATLAB runs the lines of code between the if and the. Then it continues with the code after the.

4 The if- Statement The if statement s basic form is if conditional expression code line 1 code line 2 Can be any number of lines of code If the conditional expression is false, MATLAB skips the lines of code between if and. Then it continues with the code after the.

5 if- as a flowchart diagram

6 Example output a warning if the variable x is negative: in editor blue color & auto indent x = { computed somehow }; if x < 0 disp('warning: negative value')

7 The if-else- Statement if-else- structure lets you execute one section of code if a condition is true and a different section of code if it is false. if logical expression statement group 1 else statement group 2

8 if-else- as a flowchart diagram

9 Example output 1 message if x is greater than zero and another message if x is not: x = { computed somehow }; if x > 0 disp('the value is positive') else disp('the value is negative or zero')

10 IMPORTANT POINT!!! If the conditional expression is an array, it is true only if all the elements of the array are true!

11 For example, if we fail to recognize how the test works, the following statements do not perform the way we might expect. x = [4,-9,25]; if x < 0 disp( Some of the elements are negative. ) else y = sqrt(x) When this program is run it gives the result The result of the conditional expression is Logical vector [0,1,0] y = i i i

12 Remember x<0 means Are all the elements less than 0? Alternative: x = [4,-9,25]; if x > 0 % Test for all positive y = sqrt(x) else disp( Some of the elements are negative. ) When executed, this produces the following message: Some of the elements are negative.

13 if elseif else If gpa >=90 letter_grade = A ; elseif gpa >=80 letter_grade = B ; elseif gpa >= 70 letter_grade = C ; elseif gpa >= 60 else letter_grade = D ; letter_grade = F ; Many elseifs are allowed, but only 1 else This is like the nested IF in Excel else does not have a relational expression. It catches everything else.

14 If / elseif / else example hour = input ( Enter hour of the day (0-24): ); if hour < 10 greeting = Good morning ; elseif hour < 20 greeting = Good day ; else greeting = Good evening ; fprintf ( %s\n, greeting);

15 if-elseif-else- as a flowchart diagram

16 The if-elseif-else- structure If the first conditional expression (if statement) is true, the program executes the commands between the if and the elseif statement and then skips to the. If false, the program skips to the elseif statement. If the conditional expression (elseif statement) is true, the program executes the commands between the elseif and the else statement and then skips to the. If false, the program skips to the else statement and executes the commands between the else and the. The last else statement is optional.

17 For example, suppose: y = log(x) x > 10, y = sqrt(x) 0 <= x <= 10, y = exp(x)-1 x < 0. The following statements will compute y. Assume x already has a scalar value. if x > 10 y = log(x) elseif x >= 0 y = sqrt(x) else y = exp(x) - 1 note you do not have to check for x <= 10 No relational expression with else

18 Practice Exercise Create a script called what_to_wear that inputs one argument which is a temperature in degrees F: temp = input ( Enter Temperature: ); If the temperature is 70 o F or higher, print a message saying Wear shorts. If the temperature is between 50 o F and 70 o F print a message saying Wear long sleeves. If the temperature is below 50 o F, print a message saying Wear a jacket. Test your program by running the script and entering different temperatures.