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Title of script: Iterations

Author: Anuradha Amrutkar

Keywords: for loop, while loop

Visual Cue
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

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
Demonstration We will modify this code by introducing the break statement.
 for i = 1:5
    if (i==2),
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
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
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
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;
    i = i+1;
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;
    i = i+1;
    if(i==3) then
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

Contributors and Content Editors

Chandrika, PoojaMoolya