CE317      Computer Methods in Civil Engineering

Lecture 2*: Computer Implementation



Steps of Computer Implementation

1-     Algorithm: the sequence of logical steps required to perform a numerical procedure for solving a certain problem


2-    Flow Chart: a graphical representation of an algorithm. It consists of a series of blocks and arrows. Each block represent a certain task or operation. The arrows represent the sequence in which the operations are performed.


3-  Code: a set of step-by-step commands that instructs the computer to carry out the required calculations according to the designed algorithm or flow chart. The code can be written in various computer languages such as Fortran, BASIC, Visual BASIC, C, etc. There are also software “packages” that have more powerful built-in functions to perform different types of numerical, symbolic and graphical analyses such as MATLAB, Mathematica, Excel, etc. 

To illustrates the above 3 steps, let consider the example of obtaining the roots of the quadratic equation Ax2+Bx+C=0:


The algorithm and flow chart for this problem can be written as:


Step 1:     Begin the calculation.


Step 2:    Input the values of the

               coefficients A, B and C


Step 3:    Compute DISC=B2-4AC            


Step 4:    If DISC < 0, print:


Otherwise, compute:




Step 5:    Print the values of the roots

The above flow chart can be implemented in the following Fortran code:


















If Mathematica is used the above code reduces to a single command:







If a=1, b=4 and c=1, we can evaluate the roots using  %/. :






If we want a numerical value, we use the function N:





Approximation & Errors


Mathematical Definition of Errors:


If the true solution is known, then the accuracy of the numerical solution can be measured using the true percent relative error that can be defined as




The exact solution is not known for many problems and therefore, the above formula becomes useless. Since the numerical solution is obtained using an iterative procedure, we can measure the accuracy of the numerical solution using



where εa is the approximate percent relative error.


Types of Errors:


1-     Round-off errors:

They are due to the computer imperfection in dealing with numbers. For example: the quantities π, 71/2, e, etc. cannot be represented exactly by the a limited number of digits provided by a certain computer. 


2-    Truncation errors:

They are due to approximating the exact formulation of the problem by an approximate (numerical) formulation. These errors are related to Taylor series as will be explained later.


3-    Blunders: They are due to human errors and machine imperfections.


4-    Formulation errors


5-    Errors due to assumptions and data uncertainty



We will discuss the second type of errors in more details because they are associated with the process of approximation performed in most of the numerical methods considered in this course.


The Taylor Series

Taylor’s theorem: Let the function f(x) and its n derivatives be continuous on an interval [xi ,  xi+1], then the function evaluated at xi+1 can be estimated from its value at xi through the following series:


Zero order Taylor series:



1st order Taylor series:   


2nd order Taylor series:


nth order Taylor series:

The accuracy of the above representation depends on the following factors:

1-     Type of f(x): The series is exact if nth order series is used to represent an nth order polynomial. The case is not true if f(x) involves functions that do not have vanishing (n+1)th derivatives. Examples are: Sin(x), ex, Ln(x).


2-    The size of the interval [xi , xi+1]: More accurate for shorter intervals.


3-    The number of terms in the series: More accurate for more terms (not always true)


Use of Taylor series to approximate functions



Estimate the polynomial f(x)=-0.1 x4 -0.15 x3 -0.5 x2 -0.25 x +1.2 at x=1 from its value and derivatives at x=0, using zero, 1st, 2nd, 3rd and 4th order Taylor series. Compare the relative errors in the five approximations.




* Important: This handout is only a summery of the lecture. The student  take detailed notes during the class and refer to the textbook for more examples and discussion.