Introduction to Game Design, Prototyping, and Development (2015)

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The Iteration Clause Doesn’t Have to Increment

Though the iteration clause is commonly an increment statement like i++, it doesn’t have to be. Any operation can be used in the iteration clause.

Decrement

One of the most common alternate iteration clauses is counting down rather than counting up. This is accomplished by using a decrement operator in a for loop.

6 void Start() {
7 for ( int i=5; i>2; i-- ) {
8 print( "Loop: "+i );
9 }
10 }

And the output to the Console pane would be as follows:

Loop: 5
Loop: 4
Loop: 3

foreach Loops

A foreach loop is kind of like an automatic for loop that can be used on anything that is enumerable. In C#, most collections of data are enumerable, including the Lists and arrays covered in the next chapter as well as strings (which are a collection of chars). Try this example in Unity.

1 using UnityEngine;
2 using System.Collections;
3
4 public class Loops : MonoBehaviour {
5
6 void Start() {
7 string str = "Hello";
8 foreach( char chr in str ) {
9 print( chr );
10 }
11 }
12
13 }

The Console output will print a single char from the string str on each iteration, resulting in:

H
e
l
l
o

The foreach loop guarantees that it will iterate over all the elements of the enumerable object. In this case, it iterates over each character in the string "Hello". foreach loops are covered further in the next chapter as part of the discussion of Lists and arrays.

Jump Statements within Loops

A jump statement is any statement that causes code execution to jump to another location in the code. One example that has already been covered is the break statement at the end of each case in a switch statement.

break

break statements can also be used to prematurely break out of any kind of loop structure. To see an example, change your Start() function to read as follows:

6 void Start() {
7 for ( int i=0; i<10; i++ ) {
8 print( i );
9 if ( i==3 ) {
10 break;
11 }
12 }
13 }

Note that in the this code listing, lines 1-5 and the final line containing only a closing brace } (which was formerly line 13) have been omitted because they are identical to the lines in previous code listings. They should still be there in your script in MonoDevelop, and you should just replace the foreach loop from lines 7-10 of the preceding foreach code listing with lines 7-12 of this code listing.

Run this in Unity, and you will get this output:

0
1
2
3

The break statement exits the for loop prematurely. A break can also be used in while, do...while, and foreach statements.

7 int i = 3;
8 do { 3
9 print( i ); 2
10 i--;
11 if ( i==1 ) break; // 2
12 } while ( i > 0 );

7 foreach (char c in "Hello") {
8 if (c == 'l') { H
9 break; e
10 }
11 print( c );
12 }

The numbers in the following list refer to lines in the preceding code that are marked with // 1 and // 2 to the right of the line (these have been bolded in the preceding code for emphasis).

1. This line shows the single-line version of an if statement. If there is only one line, the braces are not necessary.

2. This code only outputs 3 and 2 because on the second iteration of the loop, the i-- decrement reduces i to 1, and then the condition clause for the if statement on line 11 is true and breaks out of the loop.

Take the time to look at each preceding code example and make sure you understand why each is generating the output shown. If any look confusing, type the code into Unity and then run through it with the debugger. (The debugger is explained in detail in Chapter 24, “Debugging.”)

continue

continue is used to force the program to skip the remainder of the current iteration and continue to the next.

In the preceding code, any time that i%90 != 0 (that is, i/90 has a remainder other than 0), the continue will cause the for loop to move on to the next iteration, skipping the print( i ); line. The continue statement can also be used in while, do...while, and foreachloops.