\n", "* All **while** loop has an initial condition as true.\n", "* The **while** statement includes a condition to test.\n", "* All statements in the loop will be executed as long as the condition is true.\n", "* When the condition changes to False (within the statement loop), the loop stops\n", "* Codes that are defined after the **while** loop will be executed\n", " \n", "
\n",
"Let's illustrate it with some examples."
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# Computing the sum from 1 to upper_limit\n",
"\n",
"upper_limit = 10 # Define the upper limit\n",
"\n",
"sum = 0\n",
"num = 1\n",
"\n",
"while num <= upper_limit :\n",
" sum = sum + num\n",
" print(\"The sum from 1 to\", num, \"is\", sum)\n",
" num = num + 1\n",
"\n",
"print(\"\\n**** The sum from 1 to\", upper_limit, \"is\", sum)\n",
" "
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Example\n",
"A Fibonacci series is a series of numbers in which each number is the sum of the two preceding numbers.
\n",
"For example, we have the following series 1, 1, 2, 3, 5, 8, etc.
\n",
"Use the **while** loop to generate a series for up to *upper_limit* numbers."
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"upper_limit = 30 # set upper limit for the Fibonacci series\n",
"previous_num = 0\n",
"current_num = 1\n",
"next_num = 0\n",
"index = 1\n",
"\n",
"print(\"The Fibonacci series: \" + str(1) +\",\", end=\" \") # instead of newline, use \" \" !!!!!\n",
"while index <= upper_limit:\n",
" next_num = previous_num + current_num # generate next number to print\n",
" if index < upper_limit: # if not last number, generate \",\" and \" \"\n",
" print (str(next_num) +\",\", end=\" \")\n",
" else: # if last number, generate \".\" and a newline\n",
" print (str(next_num) + \".\")\n",
" previous_num = current_num # change previous and current number\n",
" current_num = next_num\n",
" index = index + 1 # modifying the condition of while loop\n"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Exercise\n",
"Use the **while** loop to generate the first 10 numbers of 2^n, where n is an integer with an upper bound."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Allowing user input\n",
"We have learnt how to generate output, but how do we accept input by the users of the program? \n",
"It is by the *input()* function. Let's illustrate."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## General syntax for input\n",
"\n",
"variable = input ('Message you want to show: \")
"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Example"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# create a list of teachers in the first floor\n",
"teachers = ['john c. s. lui', 'patrick p. c. lee', 'james cheng', 'wei meng']\n",
"\n",
"# Ask the user to input a name\n",
"name = input('Enter a name of the teacher:')\n",
"\n",
"# Now check whether the name is in the list\n",
"\n",
"if name in teachers:\n",
" print(\"Yes, \", name.title(), \"is in the list\")\n",
"else:\n",
" print(\"Sorry, \", name.title(), \"is NOT in the list\")"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Note on *input()* and *raw_input()* in Python 2.7\n",
"\n",
"In Python 3, you use *input()* to get a **string** back. In Python 2.7,\n",
"the counter-part is *raw_input()*\n"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Exercise\n",
"\n",
"Write a program with the following:\n",
"- Create a list of your favorite sports\n",
"- Print out your favorite sports \n",
"- Prompt user to enter another sport\n",
"- If the sport is not in the list, append it to the list, else print out an error message\n",
"- display the content of the current list"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Examples"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# Keep adding new names into the list\n",
"teachers = [] # create an empty list\n",
"\n",
"name = '' # initialize the name\n",
"\n",
"while name.upper() != 'QUIT':\n",
" name = input ('Input the name of a professor whose office is on the 1st floor: ')\n",
" if name.upper() != 'QUIT':\n",
" teachers.append(name)\n",
" print(\"Right now the list is:\", teachers)\n",
"\n",
"print(\"Done!\")"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# Give the user some context for \"menu input\".\n",
"print(\"\\nWelcome to the nature center. What would you like to do?\")\n",
"\n",
"# Set an initial value for choice other than the value for 'quit'.\n",
"choice = ''\n",
"\n",
"# Start a loop that runs until the user enters the value for 'quit'.\n",
"while choice != 'q':\n",
" # Give all the choices in a series of print statements.\n",
" print(\"\\n[1] Enter 1 to take a course in Python.\")\n",
" print(\"[2] Enter 2 to take a course in C++.\")\n",
" print(\"[3] Enter 3 to take a course in Java.\")\n",
" print(\"[q] Enter q to quit.\")\n",
" \n",
" # Ask for the user's choice.\n",
" choice = input(\"\\nWhich course you like to take ? \")\n",
" \n",
" # Respond to the user's choice.\n",
" if choice == '1':\n",
" print(\"\\nPython is great. Have fun!\\n\")\n",
" elif choice == '2':\n",
" print(\"\\nC++ is also a good language. It has good performance!\\n\")\n",
" elif choice == '3':\n",
" print(\"\\nYou want to take Java? Are you sure?\\n\")\n",
" elif choice == 'q':\n",
" print(\"\\nOK, I know you have enough programming languages\\n\")\n",
" else:\n",
" print(\"\\nI don't understand your choice, please try again.\\n\")\n",
" \n",
"# Print a message that we are all finished.\n",
"print(\"Thanks again, Chiao.\")"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# Example of using while loop to go through a list\n",
"\n",
"# Start with a list of teachers, and an empty list of smart teachers.\n",
"teachers = ['john c. s. lui', 'patrick p. c. lee', 'james cheng', 'wei meng']\n",
"smart_teachers = []\n",
"\n",
"# Let's go through the teachers\n",
"while len(teachers) != 1: # process all but the first item in the teacher list\n",
" \n",
" # Get the last teachers from the list, put he/she in the smart_teaher list\n",
" current_teacher = teachers.pop()\n",
" print(\"We have %s from the list!\" % current_teacher.title())\n",
" \n",
" # Move it to the smart_teachers list\n",
" smart_teachers.append(current_teacher)\n",
" \n",
"# Prove that we have finished confirming all users.\n",
"print(\"\\nSmart teachers are: \")\n",
"for people in smart_teachers:\n",
" print('- ' + people.title())\n",
" \n",
"print(\"\\nDumb teachers are:\")\n",
"for people in teachers:\n",
" print('- ' + people.title())"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Final examples"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# A program with while \n",
"a, b = 0, 1 # parallel assignment\n",
"while b < 10:\n",
" print('a = ', a, '; b = ', b)\n",
" a, b = b, a+b\n",
"print ('end')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Nested while"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"x = 0\n",
"while x < 3: # outer loop\n",
" y = 0\n",
" while y < 3: # inner loop\n",
" print(\"x =\", x, \";\", \"y =\", y)\n",
" y = y+1\n",
" x = x+1"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Else Clause with Python While Loop\n",
"In Python, we can add an optional `else` clause after the end of `while` loop.\n",
"\n",
"The code inside the `else` clause would **always** run but after the while loop finishes execution. The **one situation** when it won’t run is if the loop exits after a `break` statement.\n",
"\n",
"Using the else clause would make sense when you wish to execute a set of instructions after the while loop ends, i.e., without using a break statement.\n"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"def while_else_demo():\n",
"\n",
" count = 0\n",
" while count < 5 :\n",
" num = int(input(\"Enter number between 0-100?\"))\n",
" if (num < 0) or (num > 100):\n",
" print(\"Aborted while: You've entered an invalid number.\")\n",
" break\n",
" count += 1\n",
" else:\n",
" print(\"While loop ended gracefully.\")\n",
"\n",
"while_else_demo()"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Exercise\n",
"\n",
"Let's see whether you really understand the *while* structure in Python"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# What is the output for the following snippet?\n",
"\n",
"x = 0\n",
"\n",
"while False :\n",
" x = x + 1\n",
" print('So good to see John C.S. Lui.')"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# What is the output for the following snippet?\n",
"\n",
"x=2\n",
"while x<=10 :\n",
" print(x)\n",
" x = x + 1"
]
},
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"# What is the output for the following snippet?\n",
"\n",
"#custom debug print function\n",
"def dbgprint(x):\n",
" if debug == True:\n",
" print(x)\n",
"\n",
"#ask user to enter the range values\n",
"debug = False\n",
"r1 = int(input(\"Enter the starting range value?\"))\n",
"r2 = int(input(\"Enter the ending range value?\"))\n",
" \n",
"dbgprint(\"Range: %d - %d\" % (r1, r2))\n",
"\n",
"num = r1 + 1\n",
"count = 0\n",
"\n",
"while num < r2:\n",
" dbgprint(\"num: %d\" % (num))\n",
" res = num % 2\n",
" dbgprint(\"res: %d\" % (res))\n",
" if (num % 2) > 0:\n",
" count += 1\n",
" num += 1\n",
"\n",
"print(\"Odd count: %d\" % (count))"
]
},
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"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": []
}
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