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Revision• To understand the term Nesting

– To be able to write programs that contain nested loops.– To be able to dry run programs containing nested loops.

• To introduce the concepts of Iteration– To understand how to use the while loop construct– To understand how to use the for loop construct– To understand how to use the do … while loop construct

• To understand a different kind of selection statement: switch

Nested if … elseint a = 15;int b = 10;if(10 <= a) { if(b <= 10) { a = a * b; } else { a = a + b; }} else { a = a - b;}

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Very poor layout.

You should indent every time you have : -then clauseelse clauseif inside another if (which is really then/else clause)

Put } at the start of the line.

Put { at the end of a line

Nested if … else Value of a ?int a = 15;int b = 10;if(10 <= a) { if(b <= 10) { a = a * b; } else { a = a + b; } // end inner if} else { a = a - b;} // end outer if

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150

Better layout?

You should indent every time you have : -then clauseelse clauseif inside another if (which is really then/else clause)

Put } at the start of the line.

Put { at the end of a line

What is the value of a ?int a = 15;int b = 10;if(25 > a) { if(10 > b) { a = a * b; } else { a = a + b; } // end inner if} else { a = a - b;} // end if

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Selection - revision• Have a look at this youtube video on selection

http://www.youtube.com/watch?v=42Azc4Yjj68

• In your own time watch this one

https://www.youtube.com/watch?v=CZT00BnNscY

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Additional Information• A BIG problem you may encounter when using

nesting of if … else code is the matching of “else” with the appropriate “if”.

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String calculateGrade(int marks) {

String result = "Error";

if(marks >= 70) result = "A"; else

if(marks >= 60)

result = "B";

else

if(marks >= 50)

result = "C"; else

if(marks >= 40)

result = "D";

result = "Fail"; else

return result;

}

If you want to spend hours debugging your code don’t follow indenting and bracketing standards.

Why won't this program compile?

The return statement may not necessarily be reached.

The else on line 11 should be on line 10. You can see that there is no “if” just before line 11 statement.

Conform to the Coding Conventions

String calculateGrade(int marks) { String result = "Error"; if(marks >= 70){ result = "A"; } else { if(marks >= 60){ result = "B"; } else { if(marks >= 50){ result = "C"; } else { if(marks >= 40){ result = "D"; } else { result = "Fail"; } } } } return result; }

Brackets can help reduce errors.

Brackets and indents can help debugging.

When will message 2 be printed?

if(x>18 && x<30){

if (q > 200) {

System.out.println("message 1");

} else {

System.out.println("message 2");

}

} if(x>18 && x<30){

if (q > 200) {

System.out.println("message 1");

}

} else {

System.out.println("message 2");

}

if(x>18 && x<30)

if (q > 200)

System.out.println("message 1");

else

System.out.println("message 2");

else is matched with most recently defined if. Here we have the same behaviour as the bracket free version

Different behaviour can be achieved with different positioning of braces

Indents should make reading easier

The “Dangling else” Problem

Answer: when x>18 and x<30 and q>200

Do loop. What is the value of s ?

int t = 12;int s = 2;do { t = t - 2; s = s + 3;} while(t > 8); // end do loop

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t s t>812 210 5 True8 8 False

While Loop. What is the value of x ?int y = 10;int x = 9;while(y > 7) { y = y - 1; x = x + 3;} // end while

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y x y>710 9 True9 12 True8 15 True7 18 False

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For loop. What is the value of s ?int s = 15;int r;for(r=4; r>=0; r=r-2) { s = s + 5;} // end for

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s r r>=015 4 True20 2 True25 0 True30 -2 False

For loop. What is the value of h ?int h = 14;int i, j, k;for(i=1; i<3; i=i+1) { for(j=2; j<3; j=j+1) { for (k=2; k<=3; k=k+1) { h = h + 1; } // end inner for } // end middle for} // end outer for

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int h = 14;int i, j, k;for(i=1; i<3; i=i+1) { for(j=2; j<3; j=j+1) { for (k=2; k<=3; k=k+1) { h = h + 1; } // end inner for } // end middle for} // end outer for

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h i j k i<3 j<3 k<=314 1 T 2 T 2 T15 3 T16 4 F 3 F 2 T 2 T 2 T17 3 T18 4 F 3 F 3 F

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Value of h ?134565654345656543

123456789

Switch. What is the value of a ?int a = 12;int b = 2;switch(b) { case 1: a = a - b; case 2: a = b * b; case 3: a = a + b; default: a = a - 8;}

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-2

a b 12 2 4 6-2

Switch. What is the value of b ?int a = 3;int b = 17;switch(a) { case 3: b = a + b; break; case 6: b = a * b; break; case 9: b = a * b * 2; break; default: b = a * b * 3; break;}

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Switch with data type charchar a = 'B';int b = 7;Int c = 10;switch(a) { case 'A': b = c + b; break; case 'B': b = c * b; break; case 'C': b = c * b * 2; break; default: b = c * b * 3; break;}

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Can we deal with upper or lower case?char a = 'a';int b = 7;Int c = 10;switch(a) { case 'A': case 'a': b = c + b; break; case 'B': case 'b': b = c * b; break; case 'c': case 'C': b = c * b * 2; break; default: b = c * b * 3; break;}

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Any Requests for Thursday?

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