Lecture 2 Selection and Looping. Program control, if-else, logical expressions, for loop

Transcription

1 Lecture 2 Selection and Looping Program control, if-else, logical expressions, for loop

2 Overview Review Program control if-else statement Relational and logical operators Logical expressions for loop statement

3 How to write a program? 1. Read the problem statement, and identify The input and its range The output The relationship between the input and the output (how to compute the output) [Compreh] 2. Write your thoughts as a sequence of steps. [Algorithm] 3. Convert these steps to Code. [Program] 4. Test your code and compare your program result against a human result. [Testing]

4 Calculate area of a rectangle (Algorithm Program) 1. Get height h h=input( enter height: ); 2. Get width w w=input( enter width: ); 3. Calculate area = h * w area = h * w; 4. Display area disp (area);

5 Program Control How to control the flow of the program? To make the program takes decision To execute some statements and not the other based on the input data or other condition. How to make some statements executed several times.

6 Print the maximum of two numbers (Algorithm and Program) 1. Get number1 as X X=input( enter num1: ); 2. Get number2 as Y Y=input( enter num2: ); 3. If X > Y if X>Y display X disp(x); Otherwise else display Y disp(y);

7 Print the maximum of two numbers % printmax.m X=input( enter num1: ); Y=input( enter num2: ); if X>Y disp(x); else disp(y); (Program and Testing) >>printmax enter num1: 5 enter num2: >>printmax enter num1: 8 enter num2: 3 8 X Y X Y

8 Print two number in ascing order (Algorithm Program) 1. Get number1 as X X=input( enter num1: ); 2. Get number2 as Y Y=input( enter num2: ); 3. If X > Y if X>Y display Y, X disp(y); disp(x); Otherwise else display X, Y disp(x); disp(y);

9 Print two number in ascing order % order2.m X=input( enter num1: ); Y=input( enter num2: ); if X>Y disp(y); disp(x); else disp(x); disp(y); (Program and Testing) >>order2 enter num1: 5 enter num2: >>order2 enter num1: 8 enter num2: X Y

10 If-else statement if condition % do this part if condition is true else %do this part if the condition is false Condition (use relational operators): >, <, >=, <=, ==, ~=

11 Relational Operators Relational operators test the Relationship between two operands (which can be expressions) They are a Truth Statement, and resolve to either True (1) or False (0) Operator Operation == Equal to ~= Not equal to > Greater than >= Greater than or equal to < Less than <= Less than or equal to

12 Logical Operators Relational operators are binary operators so they can have only two operands To have a more complicated condition, we need to use logical operators Logic Operators can combine truth statements together in helpful ways. (and, or, not) Inputs and or not A B A && B A B ~A false false false false true false true false true true true false false true false true true true true false

13 Logical Expressions Examples Write the if condition for the following cases: 1. Check if x > y > z if x>y && y>z 2. Check if x is greater than y or less than or equal to z if x>y x <=z 3. Check if x does not equal y but equal z if x~=y && x==z 4. Check if x is even if rem(x,2)==0 5. Check if y is positive if y>0

14 Repeat a program N times for i=1:1:5 Iteration counter First counter value Step value X=input( enter num1: ); Y=input( enter num2: ); if X>Y disp(x); else disp(y); Last counter value

15 Try it in MATLAB for i=1:1:5 X=input( enter num1: ); Y=input( enter num2: ); if X>Y disp(x); else disp(y); i X Y? How many times these loops will run: for i=1:2:10 for z=10:2:20 for a=10:-2:1 for x=1:10

16 Calculate sum from 1 to N (Program) N=input( enter a number: ); sum=0; for i=1:n sum=sum+i; disp(sum); N i sum ? How to sum only even numbers

17 Calculate factorial of a number N (Program) N=input( enter a number: ); f=1; for i=1:n f=f*i; disp(f); N i f

18 What this program does? N=input( enter a number: ); sum=0; for i=1:n x=input( enter a value: ); sum=sum+x; disp(sum); N i x sum ? How to compute the average of N numbers

19 Sum only positive numbers? N=input( enter a number: ); sum=0; for i=1:n x=input( enter a value: ); if x>0 sum=sum+x; disp(sum); N i x sum

20 Sum and Average positive numbers? N=input( enter a number: ); sum=0; c=0; for i=1:n x=input( enter a value: ); if x>0 sum=sum+x; c=c+1; End Avg=sum/c; C is used to count positive numbers disp(sum); Disp(Avg); N i x sum c

21 Thank You Course Site: Computers for Engineers GENN004