Lecture 6: Output 1.Presenting results in a professional manner 2.semicolon, disp(), fprintf() 3.Placeholders 4.Special characters 5.Format-modifiers 1

Input() input('Enter your age'); input = ('Enter height of triangle: '); Units = input('Enter unit system chosen: '); base = input('Enter base of triangle: ', 's'); 2

Where we left off… % Collect inputs from the user Base = input(‘Enter base of triangle: ’) Height = input(‘Enter height of triangle: ’) Units = input(‘Enter unit system chosen: ’, ‘s’) % Compute the area of the triangle Area_tri = 0.5 * Base * Height % Display the answer on the screen % ??? How is the output displayed? 3

Um… isn’t the result being displayed already? 4

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Several Approaches to Display Results There are multiple ways to display the value of a variable 1.Use the semicolon appropriately 2.use the disp() built-in function 3.use the fprintf() built-in function 6

For example: 1.Use the semicolon appropriately 2.use the disp() built-in function 3.use the fprintf() built-in function Several Approaches to Display Results 7

There are multiple ways to display the value of a variable 1.Use the semicolon appropriately 2.use the disp() built-in function 3.use the fprintf() built-in function Each is used for specific reasons 1.Debugging – finding problems with the code 2.Simple programs, simple results (the programmer’s use) 3.Formatted (“professional”) output 8

Approach #1 – the semi-colon % Collect inputs from the user Base = input(‘Enter base of triangle: ’); Height = input(‘Enter height of triangle: ’); Units = input(‘Enter unit system chosen: ’, ‘s’); % Compute the area of the triangle Area_tri = 0.5 * Base * Height Not very professional (nothing indicates where and what the output is). The number of decimal places cannot be controlled, and it generally defaults to 4 in MATLAB. 9

Approach #2 – disp() % Collect inputs from the user Base = input(‘Enter base of triangle: ’); Height = input(‘Enter height of triangle: ’); Units = input(‘Enter unit system chosen: ’, ‘s’); % Compute the area of the triangle Area_tri = 0.5 * Base * Height; % Display the answer on the screen disp(Area_tri); WORSE!!! The number of decimal places cannot be controlled, and it generally defaults to 4 in MATLAB. 10

Approach #3 - Using fprintf() 1.fprintf(...) % is the built-in function 1.Basic form: fprintf('format String with placeholders', list of variables) % The format string allows you to put words and specify a format (UPPER CASE vs. lower case, and punctuation only) 3.Examples: fprintf('pi has a value of %f. ', pi) fprintf(’%f is the value for pi. ', pi) 11

Approach #3 - Using fprintf() 1.fprintf(...) % is the built-in function 1.Basic form: fprintf('format String Interted HeRe', list of variables) % The format string allows you to put words and specify a format (UPPER CASE vs. lower case, and punctuation only) 3.fprintf('format string with placeholders', list of variables to print are inserted here). %If more than one variable is to be printed, each is separated by a comma from the previous one) % Placeholders allow a specific format to be set (aligned right, and 2 decimal places for example) 12

Placeholders – why and how Placeholders are codes used in a format string which let the programmer use values stored in variables (without knowing the actual value) Why are placeholders needed? – Suppose a variable, result, holds a value. Let’s further suppose it holds a float. – How does the program print the value? Remember – the programmer may not know what value is in the variable. It may be computed during the running of the program. 13

Using placeholders The fprintf() function should print the value stored in the variable Area_tri. 14 Result: The area of the triangle is >> fprintf('The area of the triangle is %f.', Area_tri); “placeholder” (part of format string) 6 decimal places by default.

Most common placeholders – %f floating-point numbers, 6 decimals by default – %d integers – %s strings – %c a single character Vocabulary 15 fprintf('The area of the triangle is %f.', Area_tri); “function call”‘format string’ placeholder (part of format string) variable to be printed Arguments

Printing multiple variables When more than one variable must be printed to the screen, match each variable with its placeholder, and place the list of variables in order of the placeholders. Example age = input('Your age? '); %ask for age name = input('Your name? ', 's'); %ask for name fprintf('%s is %d years old. ', name, age); %display Sample run: Your age? 47 Your name? Fred Fred is 47 years old.>> 16

Special Characters Escape sequences can also be used within the format string: \n - this will create a new line when printing the string \t - tab (tabs the text to the right) '' - this will place one apostrophe in the final sentence displayed % - this will display a single % sign within the final sentence Example of all three: >> fprintf('%s''s age:\t\t%d years old\n\n', name, age); Fred's age:47 years old >> 17

Format Modifiers Once the base placeholder is ready, modifiers further change how the values are displayed. Complete Format Modifier form: 18 %-7.2f Left-justify the value TOTAL width to occupy Number of decimal places

Format Modifiers To display a floating point value to 3 decimal places: fprintf('The value of pi: %-7.3f.', pi); Output: The value of pi: _ _ The value of pi: Underscores indicate whitespace – they will not actually show in the output. There are 7 spaces occupied

Format Modifiers When debugging, it can be helpful to “delimit” the output (using dashes and > < symbols) – this lets you see where the “white space” is: fprintf('The value is:\t-->%9.3f<--\n\n', pi); Output: The value is:--> 3.142<-- >> 20 The delimiters make it easy to notice the white space: spaces, tabs, newlines

Format Modifiers Normally we don’t use the decimal place portion of format modifiers for strings, and it doesn’t work at all for integers – but the other portions still work! Example name = ‘Fred’; age = 47; fprintf(‘%-6s is %4d years old!\n’, name, age); Output: Fred is 47 years old! 21 Note the spaces

Common Mistakes: Do not print a value Can this be the solution? fprintf('The value in result is 23.4\n'); Result: >> fprintf('The value in result is 23.4\n'); The value in result is 23.4 >> 22

Common Mistakes: Do not print the variable name Can we just say this? fprintf('The value in result is: result\n'); Result: >> fprintf('The value in result is: result\n'); The value in result is: result >> 23

Common Mistake: Forgetting the placeholder! How about this? fprintf('The value in result is: ', result); Result: >> fprintf('The value in result is: ', result); The value in result is: >> 24

Conclusion: Approach #3 – fprintf() % Collect inputs from the user Base = input(‘What is the base in inches? ’); Height = input(‘What is the height in inches? ’); Units = input(‘Enter unit system chosen: ’, ‘s’); % Compute the area of the triangle Area_tri = 0.5 * Base * Height; % Display the answer on the screen fprintf(‘\nWith this data, the area is %.2f %s^2.\n’, Area_tri, Units); Best ever! The programmer controls every detail exactly! The PROGRAMMER decides the number of decimals. 25

Key Ideas Display strings to the screen to: – Give an introduction/welcome screen to the software – Give error messages when invalid inputs – Terminate the program nicely And of course… To display results Omit the semicolon (debugging purposes) Use disp() – debugging purposes as well Use fprintf() – specify a very specific format to display from 0 to an unlimited amount of variables fprintf(…) requires placeholders, with or without any format modifiers: %d, %f, %s, %c, %-10.2f, %-5s, %2d 26