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"# Introduction to Python: Loops and Input Functions\n",
"Here, we will explore other looping construct under Python, for example, <br>\n",
"we have **while**, and we also illustrate how to allow user to input arguments into our program.\n",
"\n",
"**Created and updated by:** John C.S. Lui on August 9th, 2020.\n",
"\n",
"**<span style=\"color:red\"> Important note</span>:** *If you want to use and modify this notebook file, please acknowledge the author.*"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## While syntax\n",
"\n",
"## Initialize the condition\n",
"flag == True\n",
"\n",
"## The while loop\n",
"**while** flag: # execute the following statements until the flag becomes False<br>\n",
" # statement 1 <br>\n",
" # statement 2 <br>\n",
" # ........... <br>\n",
" # statement n <br>\n",
"<p><p>\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",
"<p><p>\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. <br>\n",
"For example, we have the following series 1, 1, 2, 3, 5, 8, etc.<br>\n",
"Use the **while** loop to generate a series for up to *upper_limit* numbers."
]
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{
"cell_type": "code",
"execution_count": null,
"metadata": {},
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"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: \") <br>"
]
},
{
"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"
]
},
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"cell_type": "code",
"execution_count": null,
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"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�脌 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"
]
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"cell_type": "code",
"execution_count": null,
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"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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