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Lecture 2 |
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Introduction To Structured Programming |
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What is Structured Programming? |
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Sequence |
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Selection |
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Iteration |
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Program instructions are carried out in strict
order, in other words they are carried out in Sequence. |
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All programs will run in sequence. |
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Real Life Example: A Recipe |
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Selection occurs when there are two or more
possible routes through your code. |
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Route selection is based on a condition. |
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The route taken through the program instructions
depends on the result of the condition. |
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Example:
If the water is cold, boil it. |
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Iteration occurs when instructions in a computer
program are repeated. |
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Iteration is useful as it means that repetitive
tasks can be coded without having to enter the same instructions several
times into the program. |
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Iteration is often called LOOPING |
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Imagine the same task has to be carried out 10
times. |
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Rather than entering the instructions into the
program 10 times (which would be repetitive and time-consuming)… |
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… we write the instructions out once, with a
further instruction noting that they should be executed 10 times rather
than only the once. |
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There are two main types of “loops”. |
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Deterministic Loops |
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Here, as in our earlier example, the number of
iterations execute is known in advance. |
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e.g.
Loop 10 times |
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Conditional Loop |
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The number of iterations is based on a
condition. |
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e.g. Loop until the total is greater than 100 |
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Program ProgramName; |
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Uses RequiredUnits; |
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Var |
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Variables declared here |
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Begin |
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Main program code here |
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End. |
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Name your program |
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Name must be alpha-numeric. |
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Name must be unique. |
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No punctuation characters allowed. |
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Must be followed by a semi-colon |
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E.g. |
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Program CreditCheck; |
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Not all Turbo Pascal commands are immediately
available. To access some, it is
necessary to call the Turbo Pascal Unit that stores their instructions. |
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This is done using the USES clause. |
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We will nearly always want to call the CRT Unit,
which includes standard routines for formatting the computer screen. |
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E.g. |
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Uses Crt; |
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This section is used to declare variables for
our program. (More on this later) |
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Virtually all programs will require variables,
though if no variables are required this section can be omitted. |
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The main program code, where your instructions
reside, must start with the Begin keyword and finish with the End. keyword. |
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Begins and ends are also used within the program
to delineate program blocks. |
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Note the full stop at the end of the End. |
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This indicates that it is the last End in the program. |
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Only this last End. can have this full stop. |
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All program instructions between a Begin and an End
must be terminated by a semi-colon (;). |
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Program Hello1; |
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Begin |
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writeln(‘Hello World’); |
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End. |
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Note the Indentation of the writeln command. |
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This is standard practice to improve the
readability of the program. |
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Program Hello2; |
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Uses Crt; |
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Begin |
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clrscr; |
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writeln(‘Hello World’); |
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readln; |
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End. |
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Notes