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Latest revision as of 10:00, 13 June 2014
Title of script: Iterations
Author: Anuradha Amrutkar
Keywords: for loop, while loop
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| Slide | Welcome to the spoken tutorial on iterative calculations using Scilab. |
| Slide | I am using scilab version 5.2 in Mac operating system , but these calculations should work in other versions and also in Scilab that runs in linux and windows. |
| Narration | I will use the code available in the file iteration.sce |
| Narration | I have opened this file using Scilab editor, which I plan to use only as an editor. |
| Demonstration | Let us create a vector using the colon operator.
i = 1:5 creates a vector from 1 to 5, in increments of 1. |
| Demonstration | In this command,
i = 1:2:5, we see that the middle argument of 2 indicates the increment. |
| Demonstration | 1 is the first argument where the vector starts. i cannot go beyond 5. It can be equal to 5, however. |
| Demonstration | Note that if the ending argument changes to 6 the result remains the same.
i = 1:2:6 |
| Narration | It is not difficult to explain this behaviour. |
| Narration | Can you think for a moment why this happens? |
| Demonstration | We will now demonstrate the use of the for statement to perform iterative calculations.
for i = 1:2:7
disp(i)
end
This code prints out i, as we go through the loop. The display is due to the command disp - the passed argument is displayed. Recall that the for loop is used for integer values. In this case, four integer values, namely, 1, 3, 5 and 7 are displayed. The number of times the iterations take place is known as priori in for loops. |
| Demonstration | In the rest of this tutorial, we will stick to the default increment of 1. Let us begin with the loop that displays i equal to 1 to 5.
for i = 1:5
disp(i)
end
|
| Demonstration | We will modify this code by introducing the break statement.
for i = 1:5
disp(i)
if (i==2),
break
end
end
|
| Demonstration | Note that i is displayed only up to 2. |
| Narration | The iteration is not carried out till the last value of i, namely, 5. |
| Narration | When i is equal to 2, the if block is executed for the first time. |
| Narration | The break command, however, terminates the loop. |
| Narration | If we want to get out of a loop when some intermediate condition is satisfied, we can use the break statement. |
| Demonstration | Note that "i is equal to 2" statement uses the "equal to" sign twice. |
| Narration | This is the standard way to compare the equality in programming languages. |
| Narration | The result of this comparison statement is a boolean: true or false. |
| Demonstration | We will introduce the continue statement.
for i = 1:5
if (i<=3) then
continue
else
disp(i)
end
end
|
| Demonstration | This results in i getting displayed only for 4 and 5. |
| Narration | For i less than or equal to 3, as given by the i<=3 statement, nothing happens. |
| Narration | The continue statement makes the program skip the rest of the loop. |
| Narration | Unlike the break statement, however, it does not exit the loop. |
| Narration | The parameter i is incremented and all the calculations of the loop are executed for this new i. |
| Demonstration | We take a small break and show how to get help for operators of the type <=. Let us type
help <= |
| Narration | This opens the scilab help browser. |
| Demonstartion | We see that the help is available under the option less. So now we type
help less |
| Narration | We see the required help instructions here. |
| Narration | The for statement in Scilab is more powerful than in programming languages. |
| Demonstartion | For example, let us perform a for loop over a vector:
v = [1 5 3];
for x = v
disp(x)
end
|
| Narration | This script displays all values of v. |
| Narration | Until now we have been displaying only the variables. |
| Narration | We can indeed display the result of a calculation as well. |
| Demonstration | The following code displays the square of the numbers.
v = [1 5 3];
for x = v
disp(x^2)
end
|
| Narration | We have spent quite a bit of time explaining the for loop. |
| Narration | Let us now move on to the while loops. |
| Narration | The while statement allows us to perform a loop when a boolean expression is true |
| Narration | At the beginning of the loop, if the expression is true, the statements in the body of the while loop are executed. |
| Narration | If the program is written well, the expression becomes false and the loop is ended. |
| Demonstration | Now let us see an example for the while loop:
i = 0;
while(i<=5)
i = i+1;
disp(i)
end
|
| Demonstration | The values of i, from 1 to 6 are displayed. |
| Narration | Break and continue statements inside the while loop work exactly as they did in the for loop, as we demonstrate using break: |
| Demonstration | i = 0;
while(i<=5)
i = i+1;
disp(i)
if(i==3) then
break
end
end
|
| Narration | We can see that the moment i becomes equal to 3, the program exits the loop, thanks to the break statement. |
| Narration | You can also try the example for continue statement in while loop. |
| Slide | This brings us to the end of this spoken tutorial on iterative calculations using Scilab. |
| Slide | Spoken Tutorials are part of the Talk to a Teacher project, supported by the National Mission on Education through ICT. |
| Slide | More information on the same is available at http://spoken-tutorial.org/NMEICT-Intro. |
