1 Statements and Expressions

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1 ME 5241 Computer Aided Engineering Tom Chase Fall 2000 PSEUDO-CODE Pseudo-code is used to refine the logic of functions to its simplest possible form before writing the logic in any specific programming language. Pseudo-code encourages you to simplify logic by restricting you to only three action constructs: if-then, if-then-, and for. Functions developed using pseudo-code are generally simpler, more reliable, easier to read, and easier to maintain than functions developed without it. Therefore, investing in the extra step of writing a preliminary draft of a function in pseudo-code is likely to reduce your overall development time. The pseudo-code that we will use in this class 1 is defined below. Our pseudo-code is loosely based on the C or C++ programming language. We will introduce its usage with simple examples. Since our pseudo-code has only three action constructs, learning it requires little study. First, formats for statements and expressions used in the three basic action constructs are defined. Then, two of the three basic action constructs, if-then and if-then-, are presented. The convention for delineating functions in pseudo-code is described next. The third action construct, for, is then defined, with clarification of how to set up for statements to do iterative computations. This document ends by noting a tempting go to construct from the early days of FORTRAN that should never be used today. 1 Statements and Expressions 1.1 Simple Statements x = 3.0 call mat_mult y = sin( deg * PI / ) write( ymin, cupcrv( ymin ), ymax, cupcrv( ymax ) ) Note 1: Calling arguments of functions need not be listed in pseudo-code if an input-output chart for the function is attached. Note 2: Read and write statements should simply indicate variables that are read into or printed from the program; formats are not required in the pseudo-code. 1 No specific format for pseudo-code has been adopted as a standard. The literature presents a wide variety of rules and definitions ranging from very lenient to very constrained. The pseudo-code used in this class is relatively constrained. 1

2 1.2 Compound Statements Compound statements are needed when more than one operation must be performed as one condition of an if statement or one iteration in a for statement. The compound statement consists of multiple simple and/or compound statements delimited by opening and closing brackets (, ): xmxob = x - xob ymyob = y - yob c1 = xmxob * cos( gamma ) + ymyob * sin( gamma ) 1.3 Expression An expression is a statement whose result is a logical type. In C and C++, a logical is typically represented as a simple integer taking the value of 1 ( TRUE ) or 0 ( FALSE ). FORTRAN supports an intrinsic LOGICAL parameter type. i >= j x < 0.0 iok (Note that iok is assumed to be a logical variable in the last example.) 2 Conditional Statements 2.1 if-then if expression then statement if iok then if ( ( cupcrv( ymin) * cupcrv( ymax ) ) > 0.0 ) then write( ymin, cupcrv( ymin ), ymax, cupcrv( ymax ) ) iok = 0 Note that the statement to be performed if iok is TRUE is a simple statement, which happens to be another if statement. However, the statement performed if the second expression is TRUE is a compound statement. 2

3 Example C translation: if ( iok ) if ( cupcrv( ymin ) * cupcrv( ymax ) > 0.0 ) printf( "%lf %lf %lf %lf\n", ymin, cupcrv( ymin ), ymax, cupcrv( ymax ) ); iok = 0; Example FORTRAN translation: if ( iok ) then if ( cupcrv( ymin ) * cupcrv( ymax ).GT.0.0 ) then write( 6, 100 ) ymin, cupcrv( ymin ), $ ymax, cupcrv( ymax ) iok =.FALSE. endif endif 2.2 if-then- if expression then statement statement if iok then call results write( error message ) call lnkdes Example C translation: if iok results( ans ); printf( "Warning: Error!\n" ); lnkdes(); 3

4 Example FORTRAN translation: if ( iok ) then call results( ans ) write( 6, 200 ) call lnkdes() endif 3 Program and Function Delineation The executable statements of a function are delineated by opening and closing brackets (similar to a compound statement): function func1 (simple or compound statement) function message write( "this is a message" ) Example C translation: void message printf( " this is a message\n" ); Example FORTRAN translation: subroutine message() write( 6, 20 ) 20 format( this is a message ) end Note: Calling arguments for functions are not shown in the pseudo-code, as they are presumed to be defined on an associated input-output chart for the function. 4

5 4 Repetitive Statments 4.1 for for (integer parameter) = (integer or integer parameter) to/downto (integer or integer parameter) do statement for row = 1 to nrow do for col = 1 to ncol do write( matrix( nrow, ncol ) ) Example C translation: for ( row = 0; row < nrow; row++ ) for ( col = 0; col < ncol; col++ ) printf( "%10.5lf", matrix[ row ][ col ] ); printf( "\n" ); Example FORTRAN translation: do row = 1, nrow write( 6, 30 ) ( matrix( row, col ), col = 1, ncol ) enddo 4.2 Using for Statements to Avoid Infinite Loops While and repeat statements are not included as recommended pseudo-code constructs, since their usage may lead to infinite loops. Instead, I recommend that you construct all loops using for statements. The only justifiable exception to this rule is event-driven loops, which will not be necessary in the projects assigned in this class. 4.3 Implementing Iterative Computations Iterative computations should be implemented by writing a function containing nothing but the iterative computation in a for loop. The upper limit for the loop should be set as a maximum sensible number of iterations. Convergence is checked within the loop. If convergence is achieved before completion of the loop, the loop and function are terminated simultaneously with a return statement within the loop. If the loop completes without converging, an informative error message is printed. Please note that writing an iterative loop using a while or repeat statement is not allowed in this class. We ask that you use the for loop construct instead because it yields more robust code. Specifically, a user can accidently specify unrealistic convergence criteria 5

6 that will send a while or repeat loop into an infinite loop. If the iteration is done using a for loop instead, the loop is guaranteed to stop in a finite number of attempts in the case that convergence doesn t occur. As an example, consider a function written to ensure that an angle, measured in degrees, is between 0 and 360 deg. The input-output chart for the function is: arbitrary input angle (angin) angchk output angle in range 0 to 360 deg (angout) The pseudo-code to implement this function might appear as follows: function angchk angout = angin for i = 1 to 5 do if ( angout >= ) then angout = angout if ( angout < 0.0 ) then angout = angout return write( "error: input angle out of conversion range" ) Note that this function will successfully convert any angle between deg to deg to an equivalent angle between 0 and 360 deg. If the input angle is outside this range, the function prints an informative error message and terminates. The for loop prevents iterating more than 5 times in attempting to convert a nonsensical angle to the desired range. 5 A Note on GO TO Statements The following GO TO construct is not permitted in this class: 100 continue angle = angle if ( angle.ge ) go to 100 This construct is disallowed because: 1. Such constructs readily lead to infinite loops. 2. The construct is not obvious as a loop; i.e., it is difficult to read. 6