1. Programming with Matlab
Day 3: Functions

2. Solving problems: Functions
Function: Code that performs a specific task
FUNCTION
(Suma)
INPUT
(vector)
OUTPUT
(number)

3. Difference between functions and scripts
Command Window
>> vector=[ ];
>> n_elems=length(vector);
>> s=0;
>> for c_elems=1:n_elems
s=s+vector(c_elems);
end
>>
Command Window =
>> vector=[ ];
>> suma
>>
n_elems=length(vector);
s=0;
for c_elems=1:n_elems
s=s+vector(c_elems);
end
Editor
The script uses previously defined variables (for example, vector).
All variables defined in the script (i.e. n_elems, s, c_elems) remain in the workspace after the script is executed.

4. Difference between functions and scripts
Command Window
>> vector=[ ];
>> n_elems=length(vec);
>> s=0;
>> for c_elems=1:n_elems
s=s+vec(c_elems);
end
>>
Command Window ≠
>> vector=[ ];
>> s=suma(vector);
>>
function s=suma(vec)
n_elems=length(vec);
s=0;
for c_elems=1:n_elems
s=s+vec(c_elems);
end
Editor

5. Difference between functions and scripts
vec
Workspace
Command Window
>> vec=[ ];
>> s=suma(vec);
>>
vec
function s=suma(vector)
n_elems=length(vector);
s=0;
for c_elems=1:n_elems
s=s+vector(c_elems);
end
Editor
Function suma
Create variables
Delete variables
Change variables
All the mess remains inside s
A function can only access those workspace variables that are given to it as input arguments.
A function only adds to the workspace those variables specified as output arguments.
vec s
Workspace

6. Function definition … Function's name. BUT: This name has no effect.
The filename is the true name of the function (the one we will use to call and execute it).
It is better that both names match.
function
function [output1,output2,…] = function_name (input1,input2,…)
output1=0;
for c=1:10
output1=output1+input1(c);
end
output2=‘Pues sí’;
Editor - C:\Directorio\function_name.m …
Output arguments
From zero to many
Between square brackets []
Separated by commas
Code
Input arguments
From zero to many
Between parenthesis ()
Separated by commas
The values of output variables (output1, output2…) when the program finishes, are sent to the workspace.

7. Use of functions Editor - C:\Directorio\sumadorl.m
function s = sumadorl (sumando1,sumando2)
s = sumando1 + sumando2;
Editor - C:\Directorio\sumadorl.m
>> vector=[ ];
>> suma=sumadorl(vector,10)
suma =
>>
Command Window

8. Use of functions Editor - C:\Directorio\sumadorl.m
function s = sumadorl (sumando1,sumando2)
s = sumando1 + sumando2;
Editor - C:\Directorio\sumadorl.m
Variable names do not need to match inside and outside the function (but they may match).
>> vector=[ ];
>> suma=sumadorl (vector,10)
suma =
>>
Command Window
Input arguments may be either variables or just numbers/vectors typed directly.

9. Local Variables
function s = sumadorl(sumando1,sumando2)
s = sumando1 + sumando2;
Editor - C:\Directorio\sumadorl.m
Local variables: Used inside a function. Each function has its own Workspace, and they never get mixed.
>> vector=[ ];
>> suma=sumadorl(vector,10)
suma =
>>
Command Window
Base Workspace: Used from Command Window.
There may be variables with the same name in different workspaces, but they are completely different variables.

10. Turns out, we have been using functions all the time
>> s = sin(2);
Command Window
Input
Output

11. Logical operators AND, OR, NOT
They may be nested.
Examples: (a==b & c==d) | e>f
a==b & (c==d | e>f)
a==b & ~(c>d)

12. Efficiency: Preallocating variables
>> a(1)=5; >> a(2)=3; >> a(3)=7;
Reserve space for one number
Three space requests
Reserve space for another number
Reserve space for another number
>> a=zeros(1,3);
>> a(1)=5;
>> a(2)=3;
>> a(3)=7;
Reserve space for three numbers
One space request
MUCH FASTER
Space was already reserved
Specially important in loops
If you do not know the final length from the beginning, preallocate plenty of space and cut the vector in the end