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====== Math 445 lecture 7: fprintf, for loops, functions ====== ===== Formatted printing with fprintf ===== **fprintf** is Matlab's **formatted print function**. It's used for printing variables to the screen (or files) in a format that you specify. The basic syntax is <code matlab> fprintf('format-string', variable, variable, variable, ...) </code> but it is really best illustrated by example. <code matlab> >> w = 'perambulate'; >> n = length(w); >> fprintf('The word %s has %d letters.\n', w, n) The word perambulate has 11 letters. </code> The format string contains two //slots// ''%s'' and ''%w''. The percent sign is a special marker that signals a slot, and the character following it specifies the type of variable that goes in the slot, ''%s'' for string and ''%d'' for decimal. The values of the variables $w$ and $n$ are placed in those slots when the fprintf function is executed. The ''\n'' is a special //control character// for "new line." The most important slot types are * ''%d'' decimal (integer), e.g. 5 * ''%f'' floating-point, e.g. 2.75 * ''%s'' string (a sequence of characters), e.g. '' 'banana'''. * ''%c'' character, e.g. '' 'q' '' ===== for loops ===== **for** loops are used to repeat a sequence of commands a specified number of times. A **for** loop has an index variable (often $n$ or $i$) whose value changes over every iteration of the loop. For example, this Matlab **for** loop <code matlab> for n=1:4 fprintf('The value of n is %d.\n',n); end </code> produces the output <code> The value of n is 1. The value of n is 2. The value of n is 3. The value of n is 4. </code> Note how the //body// of the loop (the fprintf) statement is executed four times, once for each value of $n$ from 1 to 4. ===== functions ===== The Matlab **function** command lets you define your own functions. For example, putting the following code in a file named ''quadratic.m'' <file matlab quadratic.m> function y = quadratic(x) y = x^2 + 3*x -7; end </file> defines a function named ''quadratic'' that computes the polynomial $y(x) = x^2 +3x -7$. The filename must match the name of the function. If the ''quadratic.m'' function is in Matlab's current working directory, you'll be able to execute the function just like a regular Matlab built-in function, for example <code> >> quadratic(0) ans = -7 >> quadratic(1) ans = -3 >> quadratic(12.74) ans = 193.5276 </code>
