# TI-Basic Z80 Programming/Conditional Statements

Conditional statements allow a program to take a different path depending on some condition(s). These allow a program to perform a test and then take action based on the result of that test.

## Conditions

Conditions are used to control the path of a program. Conditions perform comparisons between different values through the use of relational operators. These include =, ≠, >, ≥, <, ≤. Relational operators always return a true or false result (known as a boolean). To use them, type the values to compare against on either side of the operator:
To type relational operators, press 2ND [TEST]. You may also want to test two different conditions at the same time and join them using a logical operator. These include and, or, xor, and not. The basic syntax is similar to those of relational operators.
To type logical operators, press 2ND [TEST] LOGIC. For the following list, consider the format A op B, where A and B are conditions, and op is a logical operator. Where op equals...
* and, the entire condition returns true only if A and B are both true.
* or, the entire condition returns true if at least one of A and B is true.
* xor, the entire condition returns true only if one of A or B is true.
The not operator is special because it does not directly compare two conditions. Instead, it negates the condition(s) that are nested inside. For example:
To use conditional statements, they must be placed into a conditional block. Conditional blocks contain the condition to test and the code to execute. The following sections describe the various types of conditional blocks available in TI-Basic.

## If

If (PRGM CTL 1) requires a criteria argument (condition) to be stated which determines whether the following instruction is to be executed. A standard If block can only run one line of code dependent of the condition. For example, if you needed to execute two or more instructions dependent of the condition, you must use an If Then End block. The argument or arguments is a boolean result, meaning that they will be true or false. If an expression is used as the condition, 0 will represent false and non-0 values will represent true.

### Syntax

* Where condition is any statement resulting in a zero or non zero result, or a conditional operator returning true or false
** If condition returns true or non-zero, statement is executed
** If condition returns false or zero, statement is not executed

### Examples

The following example demonstrates a very basic If statement:
    6→X
    If X>5
    Disp "X > FIVE!"
It will display X > FIVE! because the condition X>5 returns true (6 is greater than 5). If X had been 3, condition would return false (3 is not greater than 5), and nothing would display.
The following program will always display Hello World because the condition 1 will always return 1 (which is non-zero) and therefore the condition is always true:
    If 1
    Disp "Hello World"
and likewise the following will never display Goodbye World because it always returns 0.
    If 0
    Disp "Goodbye World"

## If Then End

If Then End (PRGM CTL {1,2,7}) conditional statements are used when more than one statement must be executed if a condition returns true. It is very much like the simple If statement, with the difference that multiple statements are executed instead of one.

### Syntax

The If statement requires a condition, then a Then command must follow on the next line, followed by one or more statements which will execute if the condition returns true. The If block is ended by a End statement which tells the calculator that all following statements are to be executed as normal.
* Where condition is any statement resulting in a true or false result.
** If condition returns true, statement1, statement2... statementn (all statements between Then and End) will execute
** If condition returns false, the statements between Then and End are not executed, and the program continues with the first statement after End
It is also common to manually insert a colon directly after the condition of the If, then type Then. As an example:

### Example

    Prompt X
    If X≥5
    Then
    Disp "You entered",X
    Disp "X ≥ 5"
    End
When the program is run, it will be displayed as:

## If Then Else End

If Elses are used when the programmer needs to make a choice where if the condition returns non-zero, statements are executed, but if the condition returns zero, other statements are executed. Either way, only one set of instructions are executed.

### Syntax

* Where condition is any statement resulting in a zero or non zero result.
** If condition returns nonzero, trueStatements (instructions between Then and Else) will execute, then the instruction after End is executed
** If condition returns zero, falseStatements (instructions between Else and End) are executed, then the instruction after End is executed

### Examples

    If X≥5
    Then
    Disp "Hello World I'm"
    Disp "Big like five"
    Else
    Disp "Goodbye World"
    Disp "I'm small like four"
    End
Would display
If X were greater than or equal to five, but would display the following if X were less than five:

## Else If Workaround

It should be noted, TI-Basic does not support else if statements. However, to workaround this, nest the conditional blocks in the Else portion of the block. For example, in a traditional programming language, the following...
if (condition1) {
} else if (condition2) {
} else if (condition3) {
} else {
would be notated as, in TI-Basic:
    If condition1
    Then
    statements1
    Else
    If condition2
    Then
    statements2
    Else
    If condition3
    Then
    statements3
    Else
    statements4
    End
    End
    End
However, this is hard to read, so for this example, we will add spaces to make it more clear (You cannot add extra white space in practice. You will receive syntax errors.):
    If condition1
    Then
    Else
    End

## You try it!

Try these examples to practice using conditional statements.

### Even or Odd

Write a program that, when given a number, determines if the number is even or odd and prints the result to the screen.
To help you write this program, you can use remainder( (MATH NUM 0). It returns the remainder after diving a dividend by a divisor. The syntax for remainder( is:
Older versions of the TI-84 Plus and the TI-83s do not have a function for calculating the remainder. However, this is still calculatable via an alternate solution:
* Where A is the dividend and B is the divisor.

### BMI

Body mass index (BMI) is a value derived from the mass and height of a person. A person's BMI is calculated with the following formula: ${\frac {{\text{weight}}_{\text{kg}}}{{\text{height}}_{\text{m}}^{2}}}$. The value of a person's BMI can be categorized by the following table:
| BMI | Category |
| < 18.5 | Underweight |
| 18.5-24.9 | Healthy |
| 25-29.9 | Overweight |
| 30+ | Obese |
Write a program that accepts user input for the person's weight in kilograms and their height in meters, and display their BMI and their category.
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