4. Structured Algorithms


Students Learns About:

Structured algorithms

  • control structures which form the basic building blocks of all algorithms:

  • sequence

  • selection (binary, multiway)

  • repetition (pre-test, post-test), including for … next loops

  • use of subroutines


  • methods for representing algorithms:

  • pseudocode

  • flowcharts incorporating standard control structures

  • software structure

  • use of a clear uncluttered mainline and subroutines

  • use of a modular approach

  • use of stubs to represent incomplete modules

  • use of standard algorithms, including:

– load an array and print its contents

– add the contents of an array of numbers

  • checking the algorithm for errors

  • benefits of using structured algorithms

  • ease of development

  • ease of understanding

  • ease of modification

Students Learn To:

  • identify control structures in an algorithm



  • interpret and create algorithms represented in both pseudocode and flowcharts that use standard control structures






  • detect logic errors in an algorithm by performing a desk check


  • gather solutions from a number of sources and modify them to form an appropriate solution to a specified problem


  • represent code from different sources as an algorithm to assist in understanding its purpose and to assess its relevance in a proposed solution


  • incorporate a stub for modules for which the detail has not yet been developed




Introduction to Structured Algorithms

An algorithm is a method of solving a problem. The algorithm describes the processing steps necessary to transform the inputs into the outputs.

Standard methods for representing algorithms. 

There are many standard methods for representing algorithms. 
  • Pseudocode is an English-like algorithm description language. 
    • Keywords are used in pairs to indicate the start and finish of each control structure. 
    • These keywords are written in capitals to make them stand out. 
    • Between each pair of keywords, statements are indented to further emphasise each control structure.
  • Flowcharts use rectangles for processes, parallelograms for input and output and diamonds for decisions. 
    • These components are connected with flow lines. 
    • Although it is possible to draw flow lines in any direction there are standard constructs for each of the standard control structures used for structured algorithms. 
    • When using flowcharts we must be careful to use only recognised control structures. 
    • It is possible to draw flowcharts that cannot be implemented in programming code.

Control Structures

Structured algorithms are built using control structures. 
  • This process is known as structured programming. 
  • Control structures determine the direction or order in which statements within an algorithm are executed. 
  • Control is the influence that directs the flow of execution. 
  • Control structures are standard constructs used when creating structured algorithms.
The theory behind structured programming is that all problems can be solved using just three different control structures; 
  • sequence
    • processes executed in a certain order
  • selection
    • Binary Selection - "either/or" - 2 alternate paths 
    • Multiway Selection - multiple choices, more than 2 alternate paths
  • iteration (or repetition).
    • Iteration is the repetition of a sequence of steps. 
    • A termination condition is used either at the start or at the end of the sequence to stop the repetition. 
    • Iteration is often called looping as control passes from the last statement in the sequence back to the first forming a loop. 
    • The processes within the loop structure are known as the body of the loop.
    • There are two main types of iteration; 
      • pre-test where the termination condition is at the start of the loop and 
      • post-test where the termination condition follows the body of the loop. 
      • A third structure known as a counting loop or for...next loop is a special case of the pre-test languages.










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