The computer scientist Niklaus Wirth stated that:
Programs = Algorithms + Data
The algorithm is part of the blueprint or plan for the computer program; an algorithm is:
"An effective procedure for solving a problem in a finite number of steps."
It is effective, which means that an answer is found and it finishes, that is it has a finite
number of steps. A well-designed algorithm will always provide an answer, and it may not be the
answer you want but there will be an answer. It may be that the answer is that there is no answer.
A well-designed algorithm is also guaranteed to terminate.
The key features of an algorithm are:
Sequence (also known as Process),
Decision (also known as Selection)
Repetition (also known as Iteration or Looping)
In 1964 the mathematicians Corrado Bohm and Guiseppe Jacopini demonstrated that any
algorithm can be stated using sequence, decision and repetition. The work of Bohm and Jacopini
was of great importance since it eventually led to the disciplines of structured program design
that are much used today.
Sequence means that each step or process in the algorithm is executed in the specified order.
In an algorithm each process must be in the correct place, otherwise the algorithm will most
probably fail.
The Decision constructsIf ... then, If ... then ... else ...
In algorithms the outcome of a decision is either true or false, and there is no in between.
The outcome of the decision is based on some condition that can only result in a true or false
value[1].
The decision takes the form: if proposition then process
A proposition in this sense is a statement, which can only be true or false. It is either true
that today is Wednesday or false that today is Wednesday. It can't be both true and false. If the
proposition is true then the process, which follows the then, is executed.
The decision can also be stated as:
if proposition
then process1
else process2
This is the if ... then ... else ... form of the decision. This means that if the proposition is true

then execute process1 else or otherwise execute process2.
The first form of the decision if proposition then process has a null else, that is, there is no
else.
The Repetition constructsRepeat and While
Repetition takes two forms, the Repeat loop and the While loop.
The repeat loop is used to iterate or repeat a process or sequence of processes until some
condition becomes true. It has the general form:
Repeat
Process1
Process2
ProcessN
Until proposition
The repeat loop does some processing before testing the state of the proposition.
The while loop is used to iterate or repeat a process or sequence of processes while some
condition becomes true. It has the general form:
While proposition
Process1
Process2
ProcessN
The while loop tests the state of the proposition first.
There are four different ways of stating algorithms: Step-Form, Pseudocode, Flowchart, and
Nassi-Schneiderman.
The first two are written forms. The written form is just normal language.[2]A problem with
human language is that it can seem to be imprecise. In terms of meaning, what I write may not be
the same as what you read.[3]Pseudocode is also human language but tends toward more precision
by using a limited vocabulary.
The last two are graphically-oriented, that is they use symbols and language to represent
sequence, decision and repetition.
Flow charts are a graphical method of designing programs and once
the rules are learned they are very easy to draw. A well-drawn flow chart
is also very easy to read.
The major symbols are the DECISION (also known as selection)
and the SEQUENCE (or process) symbols. The START and STOP symbols are called the
terminals. The SUBPROCESS symbol is a variation on the sequence symbol.

1, imprecise  [,impri'sais]
adj. 不精确的；不严密的；不确切的