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--- gibson:teaching:fall-2014:math445:lecture7-diary [2014/10/07 11:09]
gibson created
+++ gibson:teaching:fall-2014:math445:lecture7-diary [2014/10/07 11:44] (current)
gibson [A function with an if statement and fprintf]
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 ====== Math 445 lecture 7 diary: functions, if statements ======
+=====Today's topics=====
+  * functions
+  * ''if'' statements
+  * ''fprintf'': the formatted printing function
+=====Today's vocabulary=====
+<code>
+function
+if-end
+fprintf
+argument
+return value
+function definition
+function declaration
+function body
+</code>
+=====Lecture synopsis=====
+====An exceedingly simple function====
 The simplest way to define you own function in Matlab is to write it in a text file whose filename matches the function name, with a '.m.' suffix. For example, we can define a ''faren2celsius'' function by creating a text file ''faren2celsius.m'' with the following contents
 <code matlab>
 function tempC = faren2celsius(tempF)
-% convert celsius temperature to farenheit ('help string')
+% convert celsius temperature to farenheit
 % input argument tempC
 % output value   tempF
-   tempC = 5/9*(tempF-32);  % semicolon to suppress printing
+   tempC = 5/9*(tempF-32);
 end
 </code>
+Note the following:
+  -The //function definition// begins with the word ''function'' and ends with ''end''.
+  -The //function declaration// in the first line specifies that the function ''faren2celsius'' will take one //argument// (or input variable) ''tempF'' and produce one //return value// (or output variable) ''tempC''.
+  -The comment lines immediately following the function declaration are a //help string//, which Matlab will print in response to ''help faren2celsius''.
+  -The //body// of the function lies between the help string and the ''end''. Here the function body is a single line of code that calculates ''tempC'' from ''tempF''.
+====A function with an if statement and fprintf====
+That was an exceedingly simple function. Let's write the inverse function ''celsius2faren'' with a few more bells and whistles. Specifically, let's print a warning message if the input temperature is below absolute zero, -273.15 C.
+<code matlab>
+function tempF = celsius2faren(tempC)
+% convert temperature in Celsius to Farenheit
+  if tempC < -273.15
+    fprintf('%d C is below -273.15, i.e. absolute zero', tempC);
+  end
+  tempF = 9/5*tempC + 32;
+end
+</code>
+This code includes an ''if'' statement that checks for temperatures below absolute zero. If the condition of the ''if'' statement evaluates to true, then the body of the ''if'' statement will be executed. If it evaluates to false, the body will be skipped, and execution will resume immediately after the ''end''. Thus here the warning message will be printed only if ''tempC'' is less than -273.15.
+The warning message is printed using the //formatted print function// ''fprintf''. ''fprintf'' takes a variable number of arguments. The first argument is always the ''format string''. The format string is simply a message to be printed with a number of slots into which values of variables will be substituted. The slots are marked with percent signs ''%'', and following the percents signs is a letter code that tells Matlab what type of variable to expect for the slot. Here ''%d'' tells Matlab to expect a decimal-valued variable (i.e. a floating-point number). After the format string, the remaining arguments to ''fprintf'' are the variables whose values you want substituted into those slots.

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