Difference between revisions of "Nested loops"
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| − | + | {{1010Topic|Introduction=Nested loops are just one loop working inside of another loop. The best way to start learning about nested loops is to look at an example using matrices.|Overview=Nested loops allow more complex processing of data. Here we will discuss a simple case of a matrix and how nested loops help to make the code for processing a matrix easier to read.|Chapter_TOC=[[Loops]]}} | |
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==Matrix Example== | ==Matrix Example== | ||
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A simple matrix with two rows and three columns. | A simple matrix with two rows and three columns. | ||
| − | + | 0 1 2 | |
| − | + | 3 4 5 | |
</pre> | </pre> | ||
| − | Now suppose you wanted to print that matrix out in a program by simply using loops. An easy solution would be to use | + | Now suppose you wanted to print that matrix out in a program by simply using loops. An easy solution would be to use a [[For]] loop nested in another [[For]] loop. |
<pre> | <pre> | ||
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int count = 0; | int count = 0; | ||
| − | for(row = 0; row < 2; row++) | + | for( row = 0 ; row < 2 ; row++ ) |
{ | { | ||
| − | for(column = 0; column < 3; column++) | + | for( column = 0 ; column < 3 ; column++ ) |
{ | { | ||
System.out.print(count); | System.out.print(count); | ||
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===Outer Loop=== | ===Outer Loop=== | ||
<pre> | <pre> | ||
| − | for(row = 0; row < 2; row++) | + | for( row = 0 ; row < 2 ; row++ ) |
{ | { | ||
... | ... | ||
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===Inner Loop=== | ===Inner Loop=== | ||
<pre> | <pre> | ||
| − | for(column = 0; column < 3; column++) | + | for( column = 0 ; column < 3 ; column++ ) |
{ | { | ||
System.out.print(count); | System.out.print(count); | ||
| Line 59: | Line 58: | ||
===Putting it All Together=== | ===Putting it All Together=== | ||
| − | * The outer loop will perform whatever is inside | + | * The outer loop will perform whatever is inside two times. |
* The contents of the outer loop is another loop. | * The contents of the outer loop is another loop. | ||
* The inner loop prints out the value of ''count'' in sets of three. | * The inner loop prints out the value of ''count'' in sets of three. | ||
| − | + | * Therefore the inner loop will be executed two times with a total of 6 iterations (3 per execution). | |
| − | + | ||
| − | + | ||
==Other Uses== | ==Other Uses== | ||
| − | + | * When you learn about multi-dimensional arrays, double [[for]] loops will come in handy (see Matrix Example). | |
| + | * You can mix and match different types of loops together to create nested loops. | ||
Example: | Example: | ||
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</pre> | </pre> | ||
| − | + | ==Warnings== | |
| + | Quite often when using for loops we get used to using the variable i as a counter. When you have nested For loops you cannot use the same control variable for both loops. You must have separate control variables for each nested loop so that each loop does not control the other one. | ||
| − | + | The following is a common error: | |
<pre> | <pre> | ||
| − | + | int i; | |
| − | + | int j; | |
| − | + | for( i = 0 ; i < 10 ; i++ ) | |
| − | + | ||
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| − | + | ||
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| − | + | ||
{ | { | ||
| − | + | for( j = 0 ; j < 10 ; i++ ) //notice the i++ should be j++, this is just one spot a mistake may be made | |
| − | + | { | |
| − | + | ..(some code) | |
| − | + | } | |
| + | } | ||
| + | </pre> | ||
| − | + | ==Programming Questions== | |
| − | + | '''Program 1''' | |
| − | + | Create a program that will print out ascii art in the shape of an X. The program will prompt the user for input in the form of a number of at least size 3. For example if the user typed the number 5 your result would look like: | |
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| − | + | <pre> | |
| − | + | * * | |
| − | + | * * | |
| − | + | * | |
| − | + | * * | |
| − | + | * * | |
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</pre> | </pre> | ||
| − | + | [[Nested_Loops_Answers_1|Answer]] | |
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Latest revision as of 23:52, 5 December 2007
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IntroductionNested loops are just one loop working inside of another loop. The best way to start learning about nested loops is to look at an example using matrices.
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{{{Body}}}
Matrix Example
If you have never encountered a matrix before you can think of it as rows and columns of numbers. Or you can think of a matrix as simply a table of rows and columns like so:
A simple matrix with two rows and three columns. 0 1 2 3 4 5
Now suppose you wanted to print that matrix out in a program by simply using loops. An easy solution would be to use a For loop nested in another For loop.
int row, column;
int count = 0;
for( row = 0 ; row < 2 ; row++ )
{
for( column = 0 ; column < 3 ; column++ )
{
System.out.print(count);
count++;
}
System.out.println();
}
The best way to describe what is happening here is to break this code up into the two loops.
Outer Loop
for( row = 0 ; row < 2 ; row++ )
{
...
}
We already know that a for loop will perform the same task until its test condition is met. In this example, the loop will execute the code inside two times meaning that whatever is inside this loop will be performed twice.
Inner Loop
for( column = 0 ; column < 3 ; column++ )
{
System.out.print(count);
count++;
}
System.out.println();
Here is a simple for loop that prints out the value of count, updates count and finishes by adding a new line once the loop's test condition is met. This loop will run three times printing out:
012
Putting it All Together
- The outer loop will perform whatever is inside two times.
- The contents of the outer loop is another loop.
- The inner loop prints out the value of count in sets of three.
- Therefore the inner loop will be executed two times with a total of 6 iterations (3 per execution).
Other Uses
- When you learn about multi-dimensional arrays, double for loops will come in handy (see Matrix Example).
- You can mix and match different types of loops together to create nested loops.
Example:
while(...)
{
for(...)
{
for(...)
{
...
}
}
while(...)
{
...
}
}
Warnings
Quite often when using for loops we get used to using the variable i as a counter. When you have nested For loops you cannot use the same control variable for both loops. You must have separate control variables for each nested loop so that each loop does not control the other one.
The following is a common error:
int i;
int j;
for( i = 0 ; i < 10 ; i++ )
{
for( j = 0 ; j < 10 ; i++ ) //notice the i++ should be j++, this is just one spot a mistake may be made
{
..(some code)
}
}
Programming Questions
Program 1 Create a program that will print out ascii art in the shape of an X. The program will prompt the user for input in the form of a number of at least size 3. For example if the user typed the number 5 your result would look like:
* * * * * * * * *
