MYP Design Variables, Data Types and Operators Notes

A Variable Is a Labelled Box the Program Can Change

1. A variable is a name your program attaches to a value, so the code can use the name while the value changes underneath it.
2. Assignment is how a value gets in, written in Python with a single equals sign as in temperature = 21.
3. Assignment works right to left, so count = count + 1 reads the old count, adds one, and stores the answer back under the same name.
4. One equals sign assigns and two equals signs compare, and swapping them is behind a huge share of first programs that quietly do nothing.
5. Being able to change is the whole point, since a score that cannot change is a score of zero forever.

Four Data Types Cover Nearly Everything You Store

1. An integer is a whole number with no decimal point, such as 7, 0 or -3, and it is what you count with.
2. A float is a number with a decimal point, such as 21.5, which is the sort of value a temperature sensor hands you.
3. A string is text inside quotes, such as "Bag packed?", and a string of digits like "42" is still text.
4. A boolean holds only True or False, and every comparison your program makes hands one back.
5. The type decides what an operator does, so 5 + 3 gives 8 while "5" + "3" gives "53".
6. Choosing the right type early saves conversion later, so store a door state as a boolean rather than as the words "open" and "closed".

Casting Converts a Value From One Type to Another

1. Casting means deliberately converting a value from one type into another.
2. Python uses int(), float() and str(), so int("42") gives the number 42 and str(42) gives the text "42".
3. Anything a user types arrives as a string, so you need age = int(input("Age? ")) before you can compare it to 13.
4. Casting a float to an integer chops the decimal off instead of rounding, so int(21.9) gives 21 and not 22.
5. Casting fails loudly when it cannot work, since int("twelve") stops the program with a ValueError.

Arithmetic Operators Do the Sums

1. The four ordinary ones are + for add, - for subtract, * for multiply and / for divide.
2. Python has two extras worth knowing for design work: // gives whole number division and % gives the remainder.
3. The % operator is how you make something happen every fifth time, because count % 5 == 0 is true on 5, 10 and 15.
4. Brackets beat the usual order, so (a + b) / 2 averages two numbers while a + b / 2 does something else entirely.
5. Dividing by zero stops the program, so check the divisor first whenever it came from a sensor reading.

Comparison and Logical Operators Ask the Questions

1. The comparison operators are ==, !=, <, >, <= and >=, and every one of them hands back a boolean.
2. The double == asks "are these equal" while a single = says "make this equal", which is why a condition never uses a single equals sign.
3. Logical operators join conditions together, where and needs both sides true, or needs at least one, and not flips the answer over.
4. So if moisture < 400 and hours_since_water > 6 waters only soil that is both dry and has been left alone for a while.
5. Comparing floats with == is unreliable, because 0.1 + 0.2 does not land exactly on 0.3, so compare with > or < and a small margin instead.

Variable Names Are Free Documentation

1. A name like temp_c tells the next reader the unit as well as the meaning, while t tells them nothing at all.
2. Names cannot start with a digit or contain spaces, so join words with an underscore as in soil_moisture.
3. Most Python code uses lower case with underscores, and staying consistent across one program matters more than which style you chose.
4. Single letters are worth avoiding except for a loop counter, where i is understood by every programmer who reads it.
5. Rename as soon as a meaning drifts, because a variable called count that now holds seconds will mislead you at 11pm the night before a deadline.
6. Well named code needs far fewer comments, since the line already says what it is doing.