1. Advanced Program Design

2. Review  Step 1: Problem analysis and specification –Specification description of the problem’s inputs and output –Analysis generalize specification to solve given problem and related problems of same kind Program Development and Problem Solving

3. Review  Step 2: Data organization and algorithm design  Algorithm – a procedure to process input and produce the required output  Data organization – representation of input, output, and intermediate values –assignment of variable names to values Basic description of an algorithm 1.get input 2.compute output for given input values 3.return output

4. Assignment statements  Assigns the value of an expression to a variable:  All variables that appear on the right side of an assignment statement must have previously defined values  The value resulting from the evaluation of the expression is assigned to the variable on the left side of the assignment statement =

5. Examples  Variables with previously assigned values can appear on both sides of the assignment statement pi=3.14159; x=15; y=30; z=x+y; a=80; b=95; average=(a+b)/2; sum=0; sum=sum+1; the equals sign should be interpreted as “is assigned the value of ” or “is replaced by” 

6. Review - Control Structures  Sequential execution –instructions follow one another in a logical progression  Selective execution –provides a choice depending upon whether a logical expression is true or false  Repetitive execution –the same sequence of instructions is to be repeated a number of times

7. Selective execution: if, if-else  if –executes instructions only when the logical expression is true –used to select “special cases”  if-else –executes one set of instructions when the logical expression is true and different instructions when the expression is false –used to select between two cases –example from last class: minimum of two numbers

8. Selective execution: if else-if else  if else-if else –distinguish between three or more cases –example: convert numerical grade to A-F  If a logical expression is true, the remainder of the statements are bypassed –good design - check likeliest cases first if(grade  90) letter_grade = A; else if(grade  80) letter_grade = B; else if(grade  70) letter_grade = C; else if(grade  60) letter_grade = D; else letter_grade = F;

9. Repetitive execution: for-loops  Repetition controlled by a counter –instructions executed once for each value of a variable in a specified range  Example:  If a = 3, b = 7, and x = 10 initially, what is the value of x at the end of the loop? for k = a to b x=x+k; k is the counter variable a,b,x must have assigned values  

10. Repetitive execution: while-loops  Repetition controlled by a logical expression –instructions executed while the logical expression is true –some variable in the logical expression must change with each repetition otherwise, we loop forever! for k = a to b x=x+k; j=a; while(j<b) { x=x+k; j=j+1; } a for loop can also be written as a while loop

11. For-loop or while-loop?  When to use a for-loop: –always for counting! –you know how many times to execute the loop –counter variable is not changed in the loop –counter variable is not used outside the loop  When to use a while-loop: –number of repetitions needed is unknown –drawback: infinite loops be extremely careful when you design while loops!

12. Detecting infinite loops  Problem: compute sum of positive integers  n  Assume n is an input value. Are the following while-loops correct or incorrect? Why? sum=0; while(n>0) sum=sum+n; sum=0; while(n>0) { sum=sum+n; n=n+1; } k=1; sum=0; while(sum<n) { sum=sum+k; k=k+1; }

13. Exercises  Design an algorithm compute sum of all positive integers  n –for example, if n = 5 then your algorithm should return 15 (because 1+2+3+4+5=15)  Design an algorithm that computes x n for an integer x and an integer n  0 –if n = 0, then x 0 = 1 otherwise, x n = x x x x x x n times