for The loop
reading: 2.3
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So far, when we wanted to perform a task multiple times, we have written redundant code: System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("I am so smart"); System.out.println("S-M-R-T"); System.out.println("I mean S-M-A-R-T"); Java has a statement called a for loop statement that instructs the computer to perform a task many times. for (int i = 1; i <= 5; i++) { // repeat 5 times System.out.println("I am so smart"); } System.out.println("S-M-R-T");
Repetition with for loops
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for loop syntax
for loop: A Java statement that executes a group of statements repeatedly until a given test fails.
General syntax:
header for (<initialization> ; <test> ; <update>) { <statement>; <statement>; body ... <statement>; }
Example:
for (int i = 1; i <= 10; i++) { System.out.println("His name is Robert Paulson"); }
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for loop over range of ints
We'll write for loops over integers in a given range.
The loop declares a loop counter variable that is used in the test, update, and body of the loop.
for (int <name> = 1; <name> <= <value>; <name>++)
Example:
for (int i = 1; i <= 6; i++) { System.out.println(i + " squared is " + (i * i)); }
Possible interpretation: "For each int i from 1 through 6, ..." Output:
1 2 3 4 5
squared squared squared squared squared
is is is is is
1 4 9 16 25
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for loop flow diagram
Behavior of the for loop:
Start out by performing the <initialization> once. Repeatedly execute the <statement(s)> followed by the <update> as long as the <test> is still a true statement.
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Loop walkthrough
Let's walk through the following for loop:
for (int i = 1; i <= 3; i++) { System.out.println(i + " squared is " + (i * i)); }
Output 1 squared is 1 2 squared is 4 3 squared is 9
i
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Another example for loop
The body of a for loop can contain multiple lines.
Example:
System.out.println("+----+"); for (int i = 1; i <= 3; i++) { System.out.println("\\ /"); System.out.println("/ \\"); } System.out.println("+----+");
Output:
+----+ \ / / \ \ / / \ \ / / \
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Some for loop variations
The initial and final values for the loop counter variable can be arbitrary numbers or expressions:
Example: for (int i = -3; i <= 2; i++) { System.out.println(i); } Output: -3 -2 -1 0 1 2 Example: for (int i = 1 + 3 * 4; i <= 5248 % 100; i++) { System.out.println(i + " squared is " + (i * i)); }
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Downwardcounting for loop
The update can also be a -- or other operator, to make the loop count down instead of up.
This also requires changing the test to say >= instead of <= .
System.out.print("T-minus "); for (int i = 10; i >= 1; i--) { System.out.print(i + ", "); } System.out.println("blastoff!"); Output: T-minus 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, blastoff!
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Singleline for loop
When a for loop only has one statement in its body, the { } braces may be omitted. for (int i = 1; i <= 6; i++) System.out.println(i + " squared is " + (i * i)); However, this can lead to mistakes where a line appears to be inside a loop, but is not: for (int i = 1; i <= 3; i++) System.out.println("This is printed 3 times"); System.out.println("So is this... or is it?"); Output: This is printed 3 times
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for loop questions
Write a loop that produces the following output. On day #1 of Christmas, my true love sent to me On day #2 of Christmas, my true love sent to me On day #3 of Christmas, my true love sent to me On day #4 of Christmas, my true love sent to me On day #5 of Christmas, my true love sent to me ... On day #12 of Christmas, my true love sent to me Write a loop that produces the following output.
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Mapping loops to numbers
Suppose that we have the following loop: for (int count = 1; count <= 5; count++) { ... }
What statement could we write in the body of the loop that would make the loop print the following output?
3 6 9 12 15
Answer: for (int count = 1; count <= 5; count++) { System.out.print(3 * count + " "); }
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Mapping loops to numbers 2
Now consider another loop of the same style: for (int count = 1; count <= 5; count++) { ... }
What statement could we write in the body of the loop that would make the loop print the following output?
4 7 10 13 16
Answer: for (int count = 1; count <= 5; count++) { System.out.print(3 * count + 1 + " "); }
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Loop number tables
What statement could we write in the body of the loop that would make the loop print the following output? 2 7 12 17 22 To find the pattern, it can help to make a table of the count and the number to print.
Each time count goes up by 1, the number should go up by 5. But count * 5 is too great by 3, so we must subtract 3.
count 1 2 3 4 5 number to print 2 7 12 17 22 count * 5 5 10 15 20 25 count * 5 3 2 7 12 17 22
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Loop table question
What statement could we write in the body of the loop that would make the loop print the following output? 17 13 9 5 1 Let's create the loop table.
Each time count goes up 1, the number should ... But this multiple is off by a margin of ...
number to print 17 13 9 5 1 count * -4 4 8 12 16 20 count * -4 + 21 17 13 9 5 1
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count 1 2 3 4 5
Degenerate loops
Some loops execute 0 times, because of the nature of their test and update. // a degenerate loop for (int i = 10; i < 5; i++) { System.out.println("How many times do I print?"); }
Some loops execute endlessly (or far too many times), because the loop test never fails. A loop that never terminates is called an infinite loop. for (int i = 10; i >= 1; i++) { System.out.println("Runaway Java program!!!");
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Nested loops
nested loop: Loops placed inside one another.
The inner loop's counter variable must have a different name.
for (int i = 1; i <= 3; i++) { System.out.println("i = " + i); for (int j = 1; j <= 2; j++) { System.out.println(" j = " + j); } } Output: i = 1 j = j = i = 2 j = j = i = 3 j = j = 1 2 1 2 1 2
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More nested loops
In this example, all of the statements in the outer loop's body are executed 5 times.
The inner loop prints 10 numbers each of those 5 times, for a total of 50 numbers printed.
for (int i = 1; i <= 5; i++) { for (int j = 1; j <= 10; j++) { System.out.print((i * j) + " "); } System.out.println(); // to end the line } Output: 1 2 3 4 5 6 7 8 9 10 2 4 6 8 10 12 14 16 18 20 3 6 9 12 15 18 21 24 27 30 4 8 12 16 20 24 28 32 36 40
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Nested for loop exercise
What is the output of the following nested for loops? for (int i = 1; i <= 6; i++) { for (int j = 1; j <= 10; j++) { System.out.print("*"); } System.out.println(); }
Output: ********** ********** ********** ********** **********
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Nested for loop exercise
What is the output of the following nested for loops? for (int i = 1; i 6; <= i++) { for (int j = 1; j <= i; j++) { System.out.print("*"); } System.out.println(); }
Output: * ** *** **** *****
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Nested for loop exercise
What is the output of the following nested for loops? for (int i = 1; i <= 6; i++) { for (int j = 1; j <= i; j++) { System.out.print(i); } System.out.println(); }
Output: 1 22 333 4444 55555
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Nested for loop exercise
What nested for loops produce the following output? 1, 2, 3, 1, 2, 3, 1 1 1 2 2 2
Answer: for (int y = 1; y <= 2; y++) { for (int x = 1; x <= 3; x++) { System.out.println(x + ", " + y); } }
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Nested for loop exercise
What nested for loops produce the following output?
inner loop (repeated characters on each line)
....1 ...2 p (loops 5 times because there are 5 lines) ..3 .4 5
This is an example of a nested loop problem where we build multiple complex lines of output:
outer "vertical" loop for each of the lines inner "horizontal" loop(s) for the patterns within each line
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Nested for loop exercise
First we write the outer loop, which always goes from 1 to the number of lines desired: for (int line = 1; line <= 5; line++) { ... }
We notice that each line has the following pattern:
some number of dots (0 dots on the last line) a number
....1 ...2 ..3 .4 5
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Nested for loop exercise
Next we make a table to represent any necessary patterns on that line: ....1 ...2 ..3 .4 5
line # of dots 1 2 3 4 5 4 3 2 1 0 value displayed 1 2 3 4 5
Answer: for (int line = 1; line <= 5; line++) { for (int j = 1; j <= (-1 * line + 5); j++) { System.out.print("."); }
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Nested for loop exercise
A for loop can have more than one loop nested in it. What is the output of the following nested for loops?
for (int i = 1; i <= 5; i++) { for (int j = 1; j <= (5 - i); j++) { System.out.print(" "); } for (int k = 1; k <= i; k++) { System.out.print(i); } System.out.println(); }
Answer:
1 22 333 4444 55555
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Nested for loop exercise
Modify the previous code to produce this output: ....1 ...2. ..3.. .4... 5....
line 1 2 3 4 5 # of dots 4 3 2 1 0 value displayed 1 2 3 4 5 # of dots 0 1 2 3 4
Answer: for (int line = 1; line <= 5; line++) { for (int j = 1; j <= (-1 * line + 5); j++) { System.out.print("."); } System.out.print(line); for (int j = 1; j <= (line - 1); j++) { System.out.print("."); 27
Common nested loop bugs
It is easy to accidentally type the wrong loop variable.
What is the output of the following nested loops?
for (int i = 1; i <= 10; i++) { for (int j = 1; i <= 5; j++) { System.out.print(j); } System.out.println(); }
What is the output of the following nested loops?
for (int i = 1; i <= 10; i++) { for (int j = 1; j <= 5; i++) { System.out.print(j); } System.out.println(); }
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How to comment: for loops
Place a comment on complex loops explaining what they do conceptually, not the mechanics of the syntax.
Bad:
// This loop repeats 10 times, with i from 1 to 10. for (int i = 1; i <= 10; i++) { for (int j = 1; j <= 5; j++) { // loop goes 5 times System.out.print(j); // print the j } System.out.println(); }
Better:
// Prints 12345 ten times on ten separate lines. for (int i = 1; i <= 10; i++) { for (int j = 1; j <= 5; j++) { System.out.print(j); } System.out.println(); // end the line of output }
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Drawing complex figures
reading: 2.4 2.5
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Drawing complex figures
Write a program that produces the following output.
Use nested for loops to capture the repetition.
#================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================#
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Drawing complex figures
When the task is nontrivial, write down steps on paper before writing code:
1. A pseudocode description of the algorithm (written in English) 2. A table of each line's contents, to help see the pattern in the input
#================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================# 32
Pseudocode
pseudocode: A wr...