A. while loop
1. • Need a loop variable
– Initialized outside loop
– Changes within loop
– Test for termination depends on variable
• Useful to think about a decrementing function
– Maps set of program variables into an integer
– When loop is entered, value is non-negative
– When value is <= 0, loop terminates, and
– Value is decreased every time through loop
eg: abs(x) – ans**3 (smaller the solution, nearer we will done.)
2. different between branching program and looping program:
• Branching structures (conditionals) let us jump to different pieces of code based on a test
– Programs are constant time
• Looping structures (e.g., while) let us repeat pieces of code until a condition is satisfied
– Programs now take time that depends on values of variables, as well as length of program
3. some command
a.
b. If the command break is executed within a loop, it halts evaluation of the loop at that point and passes control to the next expression.
c. stop infinite loop : control C
B. for loop: for <identifier> in <sequence>:
<code block>
1. some logics:
• Identifier bound to first value in sequence
• Code block executed
• Identifier bound to next value
• Code block executed
• Continues until sequence exhausted or a break statement is executed
• To generate a sequence of integers, use
– range(n) = [0, 1, 2, 3, …, n-1]
– range(m,n) = [m, m+1, …, n-1]
PS: spaces in strings as characters too!
2. range function:
more info: https://docs.python.org/3/library/functions.html#range
3. in range, 'num' variable will assign new value. (In python you can assign new values to variables. In this case the for loop will iterate in the list [0, 1, 2, 3, 4], each time the for loop iterates from the list it will assign num a new value.)
4. 5%2=1 5/2=2 5.0/2=2.5
5. if no indent, continue execute following if
6. Approximation Methods
• Suppose we now want to find the square root of any non-negative number?
• Can’t guarantee exact answer, but just look for something close enough
• Start with exhaustive enumeration
– Take small steps to generate guesses in order
– Check to see if close enough
•general method:
-Exhaustive enumeration
– Bisection search
– Newton-Raphson (for root finding)
7. Print on the same line: use comma
# 习题中出现的问题：
a. 多次重复的部分可以命名成变量
b. 尽量避免在循环判断当中有重复。
