Lecture 09

While Loop

What is the while loop control statement?

The while control statement is a group of code that tells the program to execute some code repeatedly based on a condition

Why is the while loop control statement important?

it allows a program to repeat without having to predetermine how many times the program needs to be repeated

that means: it allows your program to perform repetitive tasks for an unknown number of times
a program without looping is like a train that:
- loops between Honolulu and Waipahu a set number of times
  - some passengers might be left without a train if they've missed the last train
  - sometimes the train might be running with no passengers on board
a program with looping is like a train that:
- loops between Honolulu and Waipahu until all commuters have reached their destinations
  - no passengers will be left stranded
  - the train won't ever run when it's not needed

How to use the while loop control statement?

A group of code that includes:

a condition
a block of code that is executed whenever the condition is true
- after the block of code has finished executing, the condition is re-evaluated and the process is repeated until the condition is false

Syntax:

the keyword is while
the while keyword starts the while loop control statement
the condition follows the while keyword, like while (condition)
the condition is wrapped in parenthesis (condition)
the block of code is wrapped in curly braces, like { /*block of code*/ }
read more at http://en.wikipedia.org/wiki/While_loop

While Loop Example

while_counter.c:

Be careful with the counter:

be careful not to make an infinite loop
- usually you can just hit CTRL+C, if you were working on a real terminal
- since Cloud9 is in a web browser, you would sometimes need to close the terminal tab
- if closing the terminal tab doesn't fix the infinite loop, then exit Cloud9 and re-enter
- be sure to update the code so avoid running another infinite loop again
counter = 1;
while (counter <= 3)
if (counter == 3)
be sure the offset is correct to get the results you expect

Progression of `counter`
loop iteration	value of `counter` at the beginning of the loop	`while (counter <= 3)`	`if (counter == 3)`	value of `counter` at the end of the loop
1	1	`while (1 <= 3)` =`while (true)`	`if (1 == 3)` =`if (false)`	2
2	2	`while (2 <= 3)` =`while (true)`	`if (2 == 3)` =`if (false)`	3
3	3	`while (3 <= 3)` =`while (true)`	`if (3 == 3)` =`if (true)`	4
4	4	`while (4 <= 3)` =`while (false)`	`if` condition never gets evaluated and `if` statements never gets executed	`counter` never gets incremented

Another implementation:

while_counter_offset.c:

counter = 0;
while (counter < 3)
if (counter == 2)

Progression of `counter`
loop iteration	value of `counter` at the beginning of the loop	`while (counter < 3)`	`if (counter == 2)`	value of `counter` at the end of the loop
1	0	`while (0 < 3)` =`while (true)`	`if (0 == 2)` =`if (false)`	1
2	1	`while (1 < 3)` =`while (true)`	`if (1 == 2)` =`if (false)`	2
3	2	`while (2 < 3)` =`while (true)`	`if (2 == 2)` =`if (true)`	3
4	3	`while (3 < 3)` =`while (false)`	`if` statement never gets executed	`counter` never gets incremented

Things to note:

there's a condition to test whether things are true or false
- a condition is an expression that simplifies to true or false
there's one block of code
- the block of code is executed whenever the condition is true
- after the block of code has finished executing, the condition is re-evaluated and the process is repeated until the condition is false
there's no false block of code
- the block of code is skipped when the condition is false
control statements can be nested
nesting control statements too deeply will make the code hard to read
if you need to nest deeply, then consider using functions to abstract away some of the logic; we'll discuss functions and abstraction in a later lecture
while_counter.c is utilizing the while loop control statement
- read more at http://en.wikipedia.org/wiki/While_loop
  - read the introduction
  - read the Equivalent Constructs Section, if you are curious
  - there's not much value in reading the Demonstrating While Loops Section
- read more at http://www.codingunit.com/c-tutorial-for-loop-while-loop-break-and-continue
  - read the While Loop Section
Use Scratch to visualize the loop

Mimicking the while loop

while_mimic.c:

it will be hard to make the code mimic a while loop that iterates 100 times
it will be easier to change the code inside the while loop one time vs changing the mimic code multiple times

While Example Using Flag

while_flag.c:

Things to note:

you can use a float or int instead of an char; use whatever is most descriptive and easy to understand
the flag can be triggered based on user input or some sort of calculation or the loop number (like a counter)
it will be hard to mimic a while loop that terminates based on a flag

Write a while loop to count to 100

Assignment

Homework Problem 9.1, at the end of this problem you should submit to Laulima:

a file named UHusername_09_1.c
example:
- zhaol_09_1.c

write a program that gets a value from the user and then use a while loop to print out the following:

the number to the 1st power
the number to the 2nd power
the number to the 3rd power
the number to the 4th power
the number to the 5th power
...
the number to the 25th power

do not use any built-in C functions like pow
because a number to the 25th power can quickly make C run out of accuracy, testing your program with the 2 test cases below should be sufficient; this is an exercise on the while loop, not C memory management
we will be going over mathematical operators in detail later, but you can read http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B (Section Arithmetic Operators) and see if there's anything you can use for this assignment
for decimal numbers, this class will only be using floats and doubles, but do take note of the precision inaccuracies at the higher exponential calculations

test case 1:

Please enter your number:
[user enters 1.1]
1.100000
1.210000
1.331000
1.464100
1.610510
1.771561
1.948717
2.143589
2.357948
2.593742
2.853117
3.138428
3.452271
3.797498
4.177248
4.594973
5.054470
5.559917
6.115909
6.727500
7.400250
8.140275
8.954302
9.849733
10.834706

test case 2:

Please enter your number:
[user enters 2.2]
2.200000
4.840000
10.648000
23.425600
51.536320
113.379904
249.435789
548.758735
1207.269218
2655.992279
5843.183014
12855.002631
28281.005788
62218.212734
136880.068015
301136.149634
662499.529195
1457498.964228
3206497.721302
7054294.986864
15519448.971101
34142787.736422
75114133.020128
165251092.644283
363552403.817422