# Matlab基础语法(二)

Posted by jjx on November 24, 2016

• 选择
• 循环

#### 选择

5.1 Selection

If-Statement:
Edit guess_my_number
function guess_my_number(x)
if x == 2
fprintf('Congrats! You guessed my number!\n');
end
end
x == 2中的==表示等于，一般的=表示赋值
If-else-statement
function guess_my_number(x)
if x == 2
fprintf('Congrats! You guessed my number!\n');
else
fprintf('Not right, but a good guess.\n');
end
end
If-elseif-else-statement
function guess_my_number(x)
if x == 42
fprintf('Congrats! You guessed my number!\n');
elseif x < 42
fprintf('Too small. Try again.\n');
else
fprintf('Too big. Try again.\n');
end
end
You can have how many “else-ifs" as you want.


5.2 If-Statements, continued

Return: Return control to invoking function
This MATLAB function forces MATLAB to return control to the invoking function before it reaches the end of the function. 在function里面可以将得到的结果重新运用于上一个if语句

5.3 Relational and Logical Operators

Relational Operators:
==: is equal to
~=: is not equal to
>: is greater than
<: is less than
>=: is greater than or equal to
<=: is less than or equal to

Logical operators:
&&: and (true only if both operants are true)
||: or (false only if both operants are false)
~: not (true->false; false->true)


5.4 Nested If-Statements

If-statements can contain other if-statements
More flexible to categorize & combine things in multiple ways.


5.5 Variable Number of Function Arguments

How do we make our functions polymorphic? — Use if-statments!
Two built-in functions:
nargin: returns the number of actual input arguments that the function was called with
nargout: returns the number of output arguments that the function caller requested


5.6 Robustness

A function declaration specifies:
Name of the function,
Number of input arguments, and
Number of output arguments
Function code and documentation specify:
What the function does, and
The type of the arguments
What the arguments represent
Robustness
A function is robust if it handles erroneous input and output arguments, and
Provides a meaningful error message
Extra text that is not part of the code
MATLAB disregards it
Anything after a % is a comment until the end of the line
Purpose:
Explain important or complicated parts of the program
Comments right after the function declaration are used by the built-in help function

isscalar(m): 判断m是否为scalar
error(‘…’): 输出错误提示并停止运算


5.7 Persistent Variables

Variables:
Local
Global
Persistent
Persistent variable:
It’s a local variable, but its value persists from one call of the function to the next.
Relatively rarely used
None of the bad side effects of global variables.

Example:
edit accumulate
function total = accumulate(n)
persistent summa;
if isempty(summa)
summa = n;
else
summa = summa + n;
end
total = summa;

Example: 重新修正multable（逗比报错版）
edit snarky_multable
function [table summa] = snarky_multable(n,m)


#### 循环

6.1 For-Loops

The loop is a new control construct that makes it possible to repeat a block of statements a number of times.
We have already used loops without knowing it:

matlab里矩阵的点运算（.* etc）都是implicit loop

edit fibo
function f = fibo(n)
if ( ~isscalar(n) || n < 1 || n ~= fix(n))
error('n must be a positive integer!');
end

f(1) = 1;
f(2) = 1;
for ii = 3:n
f(ii) = f(ii-2) + f(ii-1);
end

edit mul
[row col] = size(A);
for r = 1:row
fprintf('Working on row %d...\n',r);
for c = 1:col
P(r,c) = A(r,c) * A(r,c);
fprintf('(%d %d)\n',r,c);
end
end

edit asterisks
N = 7;
for mm = 1:N
for nn = 1:mm
fprintf('*');
end
fprintf('\n');
end



6.2 While-Loops

for-loops work well when we know the number of necessary iterations before entering the loop Consider this problem: Starting from 1, how many consecutive positive integers do we need to add together to exceed 50? The only way to solve this with a for-loop is to guess a large enough number for the number of iterations and then use a break statement. There is a better solution: a while-loop! Difference between while & if: while condition is evaluated repeatedly block is executed repeatedly as long as condition is true


**6.3 Break Statement**
section，script中可以独立运行的部分，通过%%开始



e.g. edit BreakExamples %% Examples of skipping remainder of iterations

%% Example 1 % Skipping is accomplished in the while condition. ii = 1; while ii < length(readings) && readings(ii) <= 100 readings(ii) = 0; ii = ii + 1; end



**6.4 Logical Indexing**



holmes = logical([1 -2 0 0 9.12 -2]) holmes = 1 1 0 0 1 1 如果c = [1 0 1]是逻辑值而不是数字，另a = 1:3，那么a(c)则会输出a中和c对应的逻辑为真的数字，即第一个和第三个 如何提取v里的非负数： keepers = v >= 0 w = v(keepers) 或者：直接跳过keepers，w = v(v>=0) 可以同时附加多个条件，并排筛选 如何将v里的负数改为0： v(v<0) = 0 在等式左右都用logical indexing: v(v<0) = v(v<0)+100 如果logical indexing用在一个矩阵上且在等式右侧，输出结果会把矩阵变成列向量 矩阵里的数字顺序是从上往下，从左往右 设计矩阵的logical index在等式两侧，也不会改变矩阵形状： A(A > 0.1) = sqrt(A(A > 0.1)) A = 0.7333 -1.3077 -1.3499 -0.2050 0.8194 1.3542 -0.4336 1.7421 -0.1241 -1.2075 -2.2588 0.5853 0.8517 1.2205 0.8469 0.9285 1.8917 -0.0631 1.1870 1.2768 0.5646 1.6642 0.8454 1.1905 0.6992 取A和B矩阵里的较大值放在A里： A((A>B)) = A(A>B) -B(A>B) 用好Logical Indexing可以省掉很多loops！ 

6.5 Preallocation

How to time the function:
Tick - to start timing
Tock - to stop timing
e.g.
>> tic; sum(1:1e3); toc
edit noprealloc:
function noprealloc
N = 5000;
for ii = 1:N
for jj = 1:N
A(ii,jj) = ii*jj;
end
end
tic, noprealloc, toc - 73''
edit prealloc:
function noprealloc
N = 5000;
A = zeros(N,N);
for ii = 1:N
for jj = 1:N
A(ii,jj) = ii*jj;
end
end
tic, prealloc, toc - 0.6''

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