For anything but the most simple programs, decisions will have to be made by your code. There are numerous ways of doing this, and all are similar to other BASICs and generally have equivalents across most languages. For all decisions, standard Blitz operators can be used for the comparisons, or direct numeric evaluation can be carried out without a comparison. In these cases, any numeric value other than 0 counts as true, while 0 counts as false.
The most straightforward decision-making command, If simply carries out a comparison and executes some code if the result of the comparison is true. The format is:
If <expression> Then <command(s) to execute>
When <expression> results in anything other than 0/False, the <command(s) to execute> will be executed. If the result is 0/False, the program flow skips onto the next line. For example:
If x > 5 Then NPrint "Score is greater than 5!"
If more than a short piece of code is required to be executed by the If statement, it can be expanded by leaving out the Then keyword and instead finishing the block of code with End If. For example:
If x > 5 NPrint "Score is greater than 5!" x = 0 Gosub gamewon End If
In this example, all three lines following the If statement are executed only if x is greater than 5. It is good practice in cases like this to indent your code as shown to make it easier to see what code is affected by the comparison.
Else can be added to an If statement in order to provide code to be executed if the expression is not true. For example:
If x > 5 NPrint "Score is greater than 5!" x = 0 Gosub gamewon Else NPrint "Score isn't high enough!" End If
In this example, if x is greater than 5, the message "Score is greater than 5!" will be displayed, x will be set to 0 and the code will jump to the gamewon subroutine. If x is not greater than 5 (i.e., any value less than or equal to 5), the message "Score isn't high enough!" will be printed instead. Only one of the two code segments will be executed, even if the value of the comparison is changed during the execution of the conditional code. This is because the comparison has already been calculated and is not re-checked when the Else statement is reached.
Else can also be used in the single line version of the If statement:
If x > 5 Then NPrint "Score over 5!" Else NPrint "Score not over 5!"
For situations where choosing between multiple code segments might be required instead of just two as in the If/Then/Else blocks, the Select and Case statements can be used. These statements take a variable and execute a particular code segment if the variable contains a particular value. Any number of different code segments are possible where each segment is executed only if the variable contains the value associated with that segment. The Select statement specifies the variable to compare, a Case statement starts each block with the value to be checked for, and End Select finishes the Select block and returns the program to normal flow. For example:
Select level Case 0 NPrint "Level 0" Gosub resetgame Case 1 NPrint "Level 1" Case 5 NPrint "Final Level!" Gosub lastlevel End Select
In this example, the "Level 0" message is printed and the resetgame subroutine is executed if level is 0, the "Level 1" message is printed if level is 1, and the "Final Level!" message is printed and the lastlevel subroutine is executed if level is 5. Note that no other block is executed once one Case statement matches the variable's contents. Once the selected block is completed, program flow continues after the End Select statement.
Select / Case / End Select is very similar to the Switch statement in C, however no break; statement is needed at the end of each Case block, instead, the next Case, Default or End Select statement marks the end of the current block.
Note also that in the example above, if level doesn't contain 0, 1 or 5, none of the blocks are executed at all, and the program jumps directly to the statement following the End Select statement. An optional Default statement can be used at the end of the Select block to provide a special block of code to execute if none of the Case statements have been satisfied. For example:
Select level Case 0 NPrint "Level 0" Gosub resetgame Case 1 NPrint "Level 1" Case 5 NPrint "Final Level!" Gosub lastlevel Default NPrint "Random Other Level!" End Select
This example works similarly to the first example above, except that the message "Random Other Level!" is printed whenever the level variable contains a value other than 0, 1 or 5.