matlab译文第4章.ppt

4. If it is possible to implement a calculation either with a for loop or using vectors, always implement the calculation with vectors, your program will be much faster.
The code in a while loop is repeated an indefinite number of times until some user-specified condition is satisfied.
Example 4.1----statistical analysis ex4.4
3.always preallocate all arrays used in a loop before executing the loop, this practice greatly increases the execution speed of the loop. The steps of extending an array: 1.Create a new array 2.Copy the contents of the old array to the new longer array 3.Add the new value to the array 4.Delete the old array
Spot the flaw! 4.2 the for loop Executes a block of statements a specified number of times. for index=expr
commands end Index is the loop variable. The loop is executed once for each column in expr.
for ii=[5 9 7] commands
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for ii=[1 2 3; 4 5 6] commands
end
Quiz4.1 P162 1,2,3,4,5,6,7
Example4.3----calculating the day of year P145
Solution: for loop switch construct
Quiz 4.1 P162 8,9
Nesting loops P156 Note: inner for loop executes completely before the index variable of the outer for loop is incremented. (seldom use, too slow)
Example 4.5-----Comparing Loops and Vectorization Three ways P152
Using the MATLAB functions tic and toc
The break and continue statements
Break----terminates the execution of a loop and passes control to the next statement after the end of the loop. Continue----terminates the current pass through the loop and returns control to the top of the loop.
3.After the statements in the body of the loop have been executed, the program assigns the next column of the expression to the loop variable index, and the program executes the statements within the body of the loop again.
4.Step 3 is repeated over and over as long as there are additional columns in the control expression.
for ii=1:10 commands
end
for ii=1:2:10 commands
end
Good programming practice: 1.always indent the body of a for loop by 2 or more spaces to improve the readability of the code. 2.never modify the value of a loop index with the body of the loop.
The for loop construct functions as follows.
1.At the beginning of the loop, MATLAB generates the control expression.
2.The first time through the loop, the program assigns the first column of the expression to the loop variable index, and the program executes the statements within the body of the loop.
loops
Function: execute a sequence of statements more than once Type: while loops , for loops 4.1 the while loops General form: while expression
commands end