3 Program Flow Control Methods

Course Content#

if Statement, switch Statement#

(C Language Relational Operators)
- 0 → false; 1 → true
- false 👉 0, NULL(null), '\0'
- <= → =<
- ! Negation (Relational Operator)
  - Note the distinction: Bitwise NOT operator ~ (Bitwise Operator)
  - ⭐!! Double Negation: Logical Normalization
    - Unify all true values to 1, false values remain 0
Sequential Structure
- Omitted
Branch Structure

IF Statement#

(Expression)
- Any expression has a logical return value
- For the assignment expression a = 123;
  - The return value is the value of variable a, which is 123
{Code Block;}
- Understand what a statement is?
  - Single Statement: A statement ending with “;”
    - int a = 1, b = 234; // This is a statement, “,” separates into one expression
  - Empty Statement: Only a “;”, has no effect
  - Compound Statement
    - Uses curly braces
    - That is the code block👆
if else Statement
- Can be replaced with a ternary expression (conditional operator)
  - ...? ...: ...;

SWITCH Statement#

Variable a must have a clear unique integer value as a mapping!
- Letters also have such mappings: ASCII code
After entering case, execute subsequent code in order
- Until encountering break, continue, or the end of the structure~
- No need to check if the following cases are satisfied
Default Branch
- When none of the cases are satisfied, it will enter, equivalent to the else of an if else statement

CPU Branch Prediction#

Introduced from LeetCode problem - Palindrome Integer

The captain's highly efficient code is as follows:

The red box implements x<0
- Used to filter integers less than 0, negative numbers are definitely not palindrome integers
This statement appears in the operating system kernel:
!!(x) Logical normalization, the result will only be 1 or 0
- The frequency of occurrence will inform the CPU processing the program

⭐CPU Execution Process Analysis

C Language Program Execution Process: Compiled to generate an executable file → Loaded into memory → CPU executes
Cache
- Level 1 Cache: Fastest speed, but smallest capacity
Comparison of CPU Instruction Execution Methods
- Early Serial (Addressing, Instruction Pre-fetching, Writing Data, Executing, Writing Back to Memory)
- Today's Parallel
- The schematic diagram is as follows:
  - For 5 instructions
    - Serial method requires 5 * 5 = 25 clock cycles
    - Parallel method only requires 5 + 4 = 9 clock cycles
  - When there are many instructions, efficiency increases nearly 5 times
Therefore! CPU prefers sequential structures
- Dislikes branch structures (should use if else statements sparingly)
- Branch prediction issues
- For parallel methods, the CPU does not know what the next step of the branch structure is, which step should be pre-loaded?
  - The CPU actually loads the next step randomly
    - Therefore, if the prediction is wrong, the subsequent work is wasted, and it has to return to reprocess
And __builtin_expect() does exactly that: tells the CPU which branch situation is more likely to occur!!!

Loop Structure#

WHILE Statement
- while () {Code Block;}
  - Check first, then execute
- do {Code Block;} while (Expression); ← Pay attention to the semicolon!
  - Executes at least once
FOR Statement
- for(Initialization; Loop Condition; Post-Execution Operation) {Code Block;}
- Makes the loop very elegant, consisting of three parts
- All three parts can be omitted
  - For example: for(; ; );, which is equivalent to while (1);
  - The above is an infinite loop, doing nothing

In-Class Exercise#

Remember to check for redundant condition checks
When input is invalid (e.g., "q"), it falls into an infinite loop problem
- Solutions and thought processes are detailed below: Thought Point 1
Code
- Redundant Version
- ❗Here scanf's \n is a hidden danger! It needs to be removed; otherwise, it requires reading one more number
  - Because it reads \n only when inputting the next number
  - ⭐Reference Blog: Why does using "%d\n" require reading one more number? — The pitfalls of scanf() function
- Simplified Version
①
②
- Familiarize yourself with the characteristics of switch statement endings
- Code
Code

For the while method, if the first check is true, it is no different from the do...while method

Code

Highlight Notes#

⭐Perform redundant checks on conditional statements (trimming), usually can have more concise code
- For all cases checking if an expression is 0, you can directly use if (!Expression)
  - For if (Expression) single statement, you can use logical AND
    - (Expression) && Single Statement
    - For example, i && printf(" "); to output a space in all iterations except the first
    - The single statement cannot be {Code Block;}
- Replace if...else with a ternary operator
Generally, define the loop variable i in the initialization part: int i = 0
- No need to place it in advance
  - More standardized and concise: define the variable before using it, do not place it too far away
- Scope issues
  - The variable is only used within the loop
Is ++i faster than i++?
- Use ++i whenever possible
  - From the perspective of the function stack: 👇
  - ++i directly pushes the value of i+1 onto the stack
  - While i++, first pushes i onto the stack, then pushes i+1

Code Demonstration#

Branch Structure (6.struct_program.cpp)#

a - b can replace a == b for equality
if else can be replaced with a ternary operator
Output 1: false
- What is the precedence of a++ and &&?
  - ++ has a higher precedence than &&
  - Why doesn't the () outside a++ raise the precedence of ++ over &&?
    - It is prioritized
    - The output false here is unrelated~
      - For the if check, it should check a first, then go to ++
      - The value of the a++ expression is the a without ++
Output 2: a = 1, b = 0
- ⭐Logical AND -- Short-circuit Rule
  - As long as there is a false value in front, it will not proceed further [Smart approach]
(Continuing from a = 1, b = 0)
Output 1: true
Output 2: a = 2, b = 0
- ⭐Logical OR -- Long-circuit Rule
  - As long as there is one true value in front, it will not proceed further
  - To execute the subsequent expression, all preceding expressions must be false
Output method with spaces as separators (no space at the end)
- Method 1: If it is not the first loop, output a preceding space
  - First better obtained: i == 0
  - Can optimize line 43: i && printf(" ");
- Method 2: If it is not the last loop, output a trailing space
rand()
- Need to include <stdlib.h>
- Outputs a random unsigned number
  - rand() % 100 can randomly output numbers from 0 to 99
- ⭐Actually fixed randomness
  - Set the random seed through srand()
    - srand(time(0))
      - time(0)
        
        Get the current time, need to include <time.h>
        
        Accurate to seconds, the number of seconds since 1970.1.1
    - It can be seen that the value is changing, but there is no true randomness in computers
Optimized Version
- The above code has three places that can be simplified! Both if statements can be removed
- ⭐~
  1. Bitwise operations and modulus conversion: % 2 is equivalent to & 1
    - % (2 ^ n) is equivalent to & (2 ^ n - 1)
  2. +1 is equivalent to true value 1
  3. Short-circuit rule of logical AND

Loop Structure (7.cpp)#

Check Palindrome Integer (Decimal)

Remember to perform special checks for negative numbers!
What if you want to check for palindrome integers in binary?
- Just change the circles to 2
  - Computers actually store data in binary at the lowest level
  - Regardless of the base, palindrome can be checked
- Palindrome integer check in base

Calculate Integer Digits (Decimal)

Subtle differences between while loop and do...while loop
- The key is whether the condition holds for the first loop~
  - When inputting a non-zero number, digit and digit2 have no difference
- When inputting the number 0, there is a difference
  - So for the case of 0, check the number of digits
    * Use do...while
    * Or special check

Additional Knowledge Points#

switch Statement
- Variables cannot be declared after case
- Does the end of default also need to add break?
  - Optional, adding break is for uniform style
- Python does not support switch statements
In C language, what will happen if there is no return statement at the end of the main function?
- Starting from C99, it has little effect; the standard requires it to be equivalent to automatically adding return 0;
- Before C99, if control reaches the end of the main function without return, it is undefined behavior.
Floating-point number comparison
- Do not directly use ==, it will be inaccurate; use the difference being less than a certain small value to compare
__builtin_expect() has more than 6 siblings
Bit weight determines the multiple of left and right shifts
- In decimal, left shift by one position, *10
- In binary, left shift by one position, *2

Thought Points#

💡 (Solution) For 5.if.cpp, i.e., in-class exercise 1. Inputting invalid values, such as the letter ‘q’, will lead to an infinite loop
- Is it related to the ASCII code of ‘q’?
  - No relation~
  - Outputting ‘q’ in %d form looks like an address, and the fixed value during a single run should be a value randomly allocated when initializing n
    - If a valid value is input first and then an invalid value, this value is the previous n value
    - Because no value has actually been read in to change n
- Cannot read the value ‘q’
  - The return value of scanf is 0
  - However, it will not move to the next input, similar to the infinite loop event of %[^\n] encountered in the first lecture
- ⭐Of course! You can also use getchar() to swallow this invalid value
  - Is inputting qw invalid?
    - Yes, it needs getchar() to swallow twice
- Revised Code
  - Placing getchar() inside if(!ret) is better, only swallowing characters when reading invalid characters
    - The code above: getchar** will swallow whitespace and invalid characters**
    - The code below: will only swallow invalid characters
      - To demonstrate that the swallowed characters contain no whitespace, only invalid characters, a printf function is added
💡 Similarly, for 5.if.cpp (with loops, needs manual stop), when redirecting standard output using >output, there will be issues with output not being written to the redirected file
- The specific situation has been asked in the Pirate QA, looking forward to discussion: Under what conditions does output redirection write output to a file? (stdout, ./a.out > output, Ctrl+C/Ctrl+D)
- Ctrl+C does not write, Ctrl+D writes

Ctrl+C: The default interrupt key in Linux; when this key is pressed, the system sends an interrupt signal to the running program and shell.
Ctrl+D: The standard input/output EOF in Linux. When this symbol is encountered in standard input/output devices, the program considers it has reached the end of the file, thus ending input or output.

If there is invalid input leading to a large amount of output, Ctrl+C can also write
- Question: Is this because the buffer is insufficient, forcing output to the output file in advance?
  - This is the correct understanding
- Question: Then is the output written to the output file just the overflow output, or all output before Ctrl+C?
  - Overflow output
Code 5.switch.cpp: Why does inputting letters cause an infinite loop?
- Same as 5.if.cpp, see Thought Point 1

Tips#

OJ Problem Solving Method
- Write code in vim
- Use cat to display and copy the code
- Submit
Classic Problem: Given year, month, day, determine if it is reasonable
- Is it necessary to use so many if else?
- You can use space to exchange time: store the number of days in each month in an array
Refer to the textbook Chapter 6 Section 6.4