Algorithms and Pseudo-code

• We have already encountered assembly code. So I am going to skip the first part of chapter 6.
• We have also encountered an algorithm A set of instructions that solve a problem in a finite amount of time. (I don't care what the book says, use this definition.)
• Pseudocode
  • An informal language used to express algorithms.
  • The book's definition makes me think this is something formal, which is is NOT.
  • From Wikipedia:

    In computer science, pseudocode is a plain language description of the steps in an algorithm or another system. Pseudocode often uses structural conventions of a normal programming language, but is intended for human reading rather than machine reading. It typically omits details that are essential for machine understanding of the algorithm, such as variable declarations and language-specific code. The programming language is augmented with natural language description details, where convenient, or with compact mathematical notation. The purpose of using pseudocode is that it is easier for people to understand than conventional programming language code, and that it is an efficient and environment-independent description of the key principles of an algorithm. It is commonly used in textbooks and scientific publications to document algorithms and in planning of software and other algorithms.

  • There is no formal syntax or semantics
  • The syntax of a language is a set of rules describing how to create a well formed statement in that language.
  • The semantics of a language is the assigned meaning of a statement in that language.
  • We will discuss both of these in more detail later.
  • The book gives some formal rules for pseudocode.
    • I think that is wrong.
    • We will be less formal.
• For now let's solve a problem to understand all three.
• I want to estimate how much I will spend on a trip for gas.
  • First I need to understand the problem.
  • Next I need to come up with an algorithm
  • Finally I will implement this as a program.
• Understanding the problem.
  • What things about this problem do I know?
  • Answer
    • The average miles per gallon of my car
    • The miles I will travel on my trip
    • The cost of gas
  • Can I solve this problem for an example trip?
    • For example if I travel for 100 miles, with a car that gets 25MGP and gas costs $3.00.
    • Answer

    •  
      
                      1 gal     $3.00
          100 miles * -------  * -----  = 
                      25 mile    1 gal
      
                      1 gal     $3.00
          100 miles * -------  * -----  = 
                      25 mile    1 gal
      
          100 
          --- * $3 = $12
           25 
       

  • This is a crucial first step, If I can not solve the problem, then I can not write a program to solve it.
  • Next, can I write down the steps to solving the problem?
    • Try 1:

      • priceOfTrip = milesTraveled / mpg * costOfGas 

    • This is not really sufficient.
      • In most cases, we need to indicate where the values we are computing with come from.
      • And we probably need to indicate what we will do with these values.
      • The above solution is the heart of the algorithm but it is really not enough.
    • Try 2

      • #include < iostream>
        
        using namespace std;
        
        int main() {
           float milesTraveled{0},
                 mpg{0},
                 priceOfTrip{0},
                 costOfGas{0};
        
           cout << "Enter the miles traveled: ";
           cin >> milesTraveled;
        
           cout << "Enter the miles per gallon for your car: ";
           cin >> mpg;
        
           cout << "Enter the costOfGas per gallon for gas: ";
           cin >> costOfGas;
        
           priceOfTrip = milesTraveled / mpg * costOfGas;
        
           cout << "The cost of your trip will be $" << priceOfTrip << endl;
        
           return 0;
        }

      • This is bad
        • It is the final code, more or less.
        • And it is worried about the syntax of the program.
    • Try 3

      • prompt the user for miles traveled
        get milesTraveled
        
        prompt the user for mpg
        get mpg
        
        prompt the user for costOfGas
        get costOfGas
        
        priceOfTrip = milesTraveled / mpg * costOfGas
        
        print "the price of the trip is $" priceOfTrip

      • This is much better.
        • It gives all of the steps.
        • In the right order.
  • Notice that our algorithm uses variables.
    • These are places to write down information.
      • In the final program they will be put in memory.
      • Notice that they have reasonable names.
        • Say what they are used for.
        • ie they have meaning, you can tell what they are used for.
      • Notice that we do not "declare" these variables.
        • Again, you do that in programming, but generally not in an algorithm.
  • Notice that our algorithm has simple English statements telling us what to do.
  • The last step of this process is to test our algorithm.

    input:
       get miles: 100
       get mpg:  25
       get cost of gas: 3
    
    Output:
       The trip will cost $12.00
    
    Work Area:
    milesTraveled     mpg     costOfGas    priceOfTrip
    -------------     ---     ---------    -----------
        100            25         3           12            100/25 = 4
                                                            4 * 3 = 12
             

  • This step is also essential.
    • You want to verify your algorithm to make sure you have a valid solution.
    • It would be a real problem to spend a bunch of time writing code only to find you had the wrong solution.