http://koopman.us/bess/chap19_globals.pdf

Scope and extent - 作用域和生存周期

The scope of a variable describes where in a program's text the variable may be used, while the extent (or lifetime) describes when in a program's execution a variable has a (meaningful) value.

Extent, on the other hand, is a runtime (dynamic) aspect of a variable. Each binding of a variable to a value can have its own extent at runtime.

Iteration vs. Recursion

Given relationship between problem/solution and subproblem/partial solution, we can proceed in forward direction, i.e. iteratively, or in backward direction, i.e. recursively.

Summation

$$\begin{eqnarray}
&&sum(\{~\}) &= 0 \\
&&sum(\{a_1,\dots,a_{n+1}\}) &= sum(\{a_1,\dots,a_n\})+a_{n+1}
\end{eqnarray}$$

Or

$$\begin{equation}
sum(\{a_1,\dots,a_n\})=\left\{
\begin{array}{ll}
0 & \text{if } n = 0 \\
sum(\{a_1,\dots,a_{n-1}\})+a_n & \text{if } n > 0
\end{array}
\right.
\end{equation}$$

Coding

???

Array

int a[10];

or

#define SIZE 10

int a [SIZE];

Tyical Operations

1，access：

for loop a[i]

2，left/right shift

for loop，a[i] = a[i + 1]，or a[i] = a[i - 1]。Attention to the order of sequence of operations

Remarks：

1，In case of array as function argument，the array can be accessed in function, because the array name itself is the address of the array.

2，array can not be assigned such as b = a;，INVALID!

3，Do NOT use int a[n];！！！

Is \(a+(b+c)\) same as \((a+b)+c\)?

Not necessary!

while (a)
{
    ...
    ...
    if (b) 
        continue;
    ...
    ...
    ...
}

可以改成

while (a)
{
    ...
    ...
    if (!b) 
   {
    ...
    ...
    ...

    }
}

while (a)
{
    ...
    ...
    if (b) 
        break;
    ...
    ...
    ...
}

建议改为

while (a && !b)
{
    ...
    ...
    if (!b) 
   {
    ...
    ...
    ...

    }
}

for (i=0;i<n;i++)
{
    ...
}

The above for structure can be transformed to a while structure:

i=0;
while (i<n)
{
    ...
    i++;
}

And following while and do-while are equivalent:

do
{
    ...;
}while (b);

{
    ...;
}
while (b)
{
    ...;
}

In case of unknown number of integers to be read, we could use EOF to tell C program the end of input.

In linux system, we can use Control-D to initiate EOF, while in windows system, we use Control-Z to initiate EOF, when input from keyboard.

For example,

#include<stdio.h>

int main()
{
	int num;
        int count = 0;
	int total = 0;
	
	while(scanf("%d", &num) != EOF)
        {
		total = total + num; 
                count++;
	}
	printf("%d integers read，the total = %d\n", count, total);
	return 0;
}

stdin

12
23
34
45
56
^D//indicate a keyboard input Ctrl-D in linux or Ctrl-Z in windows

stdout

5 integers read，the total = 170

当输入输出数据量很大时，通常不适合手工输入。此时可以采用C语言的重定向功能：

把输入数据放入一个文本文件，文件名为：data.in，然后在程序开始处，调用下面的函数。之后正常的输入就会从该文件读取。
freopen("data.in","r",stdin);

如果要把打印结果输出到文件：data.out，可以在程序打印之前，调用下面函数，之后的打印均输出到该文件。
freopen("data.out","w",stdout);

文件的命名不一定如例子所示，只要一致就行。