4. (Check SSN) Write a program that prompts the user to enter a Social Security number in the format DDD-DD-DDDD, where D is a digit. Your program should check whether the input is valid. Here are sample runs: Enter a SSN: 232-23-5435 JEnter 232-23-5435 is a valid social security number Enter a SSN: 23-23-5435 JEnter 23-23-5435 is an invalid social security number

(Financial: credit card number validation) Credit card numbers follow certain pat- terns. A credit card number must have between 13 and 16 digits. It must start with: 4 for Visa cards 5 for Master cards 37 for American Express cards 6 for Discover cards In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card numbers. The algorithm is useful to determine whether a card number is entered correctly or whether a credit card is scanned correctly by a scanner. Credit card numbers are generated following this validity check, commonly known as the Luhn check or the Mod 10 check, which can be described as follows (for illustra- tion, consider the card number 4388576018402626): 1. Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single-digit number. 4388576018402626 → 2 * 2 = 4 → 2 * 2 = 4 → 4 * 2 = 8 → 1 * 2 = 2 6 * 2 = 12 (1+ 2 = 3) → 5 * 2 = 10 (1+ 0 = 1) → 8 * 2 = 16 (1 + 6 = 7) → 4 * 2 = 8

(Sort Three Integer Numbers) Write a program that prompts the user to input three numbers. The program should then output the numbers in ascending order.

(Count positive and negative numbers and compute the average of numbers) Write a program that reads an unspecified number of integers, determines how many positive and negative values have been read, and computes the total and average of the input values (not counting zeros). Your program ends with the input 0. Display the average as a floating-point number. *********** USE PYTHON ********* Sample Run 1 Sample Output 1: Enter an integer, the input ends if it is 0: 1 Enter an integer, the input ends if it is 0: 2 Enter an integer, the input ends if it is 0: -1 Enter an integer, the input ends if it is 0: 3 Enter an integer, the input ends if it is 0: 0 The number of positives is 3 The number of negatives is 1 The total is 5 The average is 1.25     Sample Run 2 Sample Output 2: Enter an integer, the input ends if it is 0: 0 No numbers are entered except 0

(Phone key pads) The international standard letter/number mapping found on the telephone is shown below: 100 2 abc 3 def 4 ghi 5 ikl 6mno 7pgrs 8 tuv 9wxyz Write a program that prompts the user to enter a letter and displays its correspond- ing number.

(Integer) Bit Operators. Write a program that takes begin and end values and prints out a decimal, binary, octal, hexadecimal chart like below. If any of the characters are printable ASCII characters, then print those, too. If none is, you may omit the ASCII column header.