Programming fundamentals

A named place in memory holding one value. Its data type says what kind of value it is, which decides both what can be stored there and what operations work on it.

Integer: a whole number. Decimal: a number with a fractional part. Char: a single character. String: a sequence of characters. Boolean: exactly True or False.

It adds numbers and joins strings, so 12 + 1 gives 13 but "12" + "1" gives "121". Mixing the two — "12" + 1 — fails, and is one of the commonest bugs.

A local variable is created inside a function, cannot be seen from outside, and is destroyed when the function ends. A global variable is created outside any function and is visible throughout the program.

If a local value is wrong, the cause is inside that function. If a global value is wrong, any part of the program could have changed it — so the bug could be anywhere.

Positions start at 0, and a slice excludes its end position. So text[2:5] gives the characters at 2, 3 and 4 — three characters, which is simply 5 minus 2.

len gives the number of characters. find gives the position where a piece of text starts, or -1 if it is absent. split breaks a string into a list at each occurrence of a separator.

No. upper, lower and replace all return a new string and leave the original untouched. To keep the change you must assign it back: text = text.upper().

Find the position of the @, then take everything before it and everything from one position after it: at = email.find("@"); user = email[:at]; domain = email[at + 1:]. The +1 skips the @ itself.

Whenever the parts can vary in length. code[0:2] works only while the prefix is exactly two characters, whereas splitting at the separator survives a longer or shorter one.

A problem that appears while the program is running, rather than a mistake in the code's grammar. Without handling the program simply stops; with handling it can report the problem and carry on.

try holds only the lines that might fail. except (catch in Java) handles one specific kind of failure. finally runs whether or not anything failed, and is where files get closed.

Unexpected input, usually from people; unavailable resources such as a missing file or unresponsive network; and logic errors, where the code ran on values nobody planned for.

When the problem is predictable and inside your control — an empty list, a zero divisor. Catch the things outside it: files, networks and databases, which can fail between your check and your use.

If a line inside try fails, control jumps straight to except and any later lines in try never run. finally runs either way, so the file is closed rather than left open holding a system resource.

Syntax errors stop the program running at all. Run-time errors stop it partway with a message. Logic errors let it run perfectly and produce the wrong answer — and only the third needs debugging.

Trace table: work through by hand, no tools needed — the exam method. Print statements: show a value at a chosen point. Breakpoints: pause and inspect every variable. Step-by-step execution: run one line at a time and watch values change.

Write down what you expected for one specific input; find the first value that is wrong, halving the search each time; look only at the lines that set that value; then change one thing and test again.

At the boundary: an empty list, a single item, the first and last positions, and values that are exactly equal. If a program is wrong, test the boundary before anything else.

If the change works, you know which change fixed it. If it does not, you have not added a new set of suspects to investigate.