“歡迎回到C語言的世界”

這是我時隔好幾個月再次開啟VS Code後，腦中出現的低沉你而又智慧的陌生聲音。這個聲音帶有驅動和鼓勵，又帶有一絲不滿與勉勵。這個聲音推動人繼續敲擊鍵盤，用指尖點選字母方塊，編織出自己奇妙的王國。

話不多說，繼續在這裡記錄吧。此篇權當是迴歸篇，簡要複習一下基礎，找回手感。

字串和格式化輸入輸出

eg.1:

//演示與使用者互動
#include<stdio.h>
#include<string.h> //提供strlen()函式
#include<windows.h>
#define DENSITY 62.4 //人體密度
int main(void)
{
  float weight,volume;
  int size,letters;
  char name[40];
  printf("Hi,what's your first name?\n");
  scanf("%s",&name);
  printf("%s,what's your weight in pounds?\n",name);
  scanf("%f",&weight);
  size = sizeof(name);
  letters = strlen(name);
  volume = weight/DENSITY;
  printf("Well,%S,your volume is %2.2f cubic feet.\n",name,volume);
  printf("Also,your first name has %d letters,\n",letters);
  printf("and we have %d bytes to store it.\n",size);
  system("pause");
  return 0;
}
 

eg.2:

//一些不匹配的整形轉換
#include<stdio.h>
#include<windows.h>
#define PAGES 336
#define WORDS 65618
int main(void)
{
  short num = PAGES;
  short mnum = -PAGES;
  printf("num as short and unsigned short: %hd %hu\n",num,num);
  printf("-num as short and unsigned short:%hd %hu\n",num,num);
  printf("num as int and char:%d %c\n",num,num);
  printf("WORDS as int,short,and char:%d %hd %c\n",WORDS,WORDS,WORDS);
  system("pause");
  return 0;
}
//short 型別與 int 型別的區別：
1）位元組數不同
2）資料範圍不同
 

eg.3:

//不匹配的浮點型別轉換
#include<stdio.h>
int main(void)
{
  float n1 = 3.0;
  double n2 = 3.0;
  long n3 = 2000000000;
  long n4 = 1234567890;
  printf("%.le %.le %.le %.le\n",n1,n2,n3,n4);
  printf("%ld %ld\n",n3,n4);
  printf("%ld %ld %ld %ld\n",n1,n2,n3,n4);
  getchar();
  return 0;
}
//關於引數傳遞與型別轉換

eg.4：

//input.c---何時使用&
#include<stdio.h>
#include<windows.h>
int main(void)
{
  int age;
  float assets;
  char pet[30];
  printf("Enter your age,assets,and favorite pet:\n");
  scanf("%d %f",&age,&assets);//scanf()讀取基本變數型別的值，在變數名前加上一個&
  scanf("%s",pet);//scanf()把字串讀入字元陣列中，不要使用&
  printf("%d $%2.2f %s\n",age,assets,pet);
  system("pause");
  return 0;
}
​
 

eg.5:

//varwid.c---使用變寬輸出欄位
#include<stdio.h>
#include<windows.h>
int main(void)
​
  unsigned width,precision;
  int number = 256;
  double weight = 242.5;
  printf("Enter a field wigth:\n");
  scanf("%d",&width);
  printf("The number is :%*d.\n",width,number);
  printf("Now enter a width and a precision:\n");
  scanf("%d %d",&width,&precision);
  printf("weight = %*.*f\n",width,precision,weight);
  printf("Done!\n");
  system("pause");
  return 0;
}
/*printf()中可以用*修飾符代替字元寬度
而scanf()中，*則表示跳過相應輸入項*/

運算子、表示式和語句

eg.1:

//shoes.c---計算多個不同鞋碼對應的腳長
#include<stdio.h>
#define ADJUST 7.31
int main(void)
{
  const double SCALE = 0.333;
  double shoe,foot;
  printf("Shoe size(men's) foot length\n");
  shoe = 3.0;
  while(shoe<18.5)
   {
    foot = SCALE*shoe+ADJUST;
    printf("%10.lf %15.2f inches\n",shoe,foot);
    shoe = shoe+1.0;
   }
  printf("If the shoe fits,wear it.\n");
  getchar();
  return 0;
}

eg.2:

//sizeof.c---sizeof()的使用
#include<stdio.h>
int main(void)
{
  int n = 0;
  size_t intsize;
  intsize = sizeof(int);
  printf("n = %d,n has %zd bytes;all ints have %zd bytes.\n",n,sizeof n,intsize);
  getchar();
  return 0;
}
/*sizeof返回size_t型別的值
typedef機制允許為現有型別建立別名
'typedef double real'
這樣，real就是double的別名*/

eg.3：

//pound.c---定義一個帶引數的函式
#include<stdio.h>
void pound(int n);//n 為int型別變數
int main(void)
{
  int times = 5;
  char ch = '!';
  float f = 6.0;
  pound(times);
  pound(ch);
  pound(f);
  getchar();
  return 0;
}
​
void pound(int n)
{
  while(n-->0)
    printf("#");
  printf("\n");
}

迴圈控制語句

eg.1:

//suming.c--根據使用者鍵入的整數求和
#include<stdio.h>
int main(void)
{
  long num;
  long sum =0L;//long 型別後要加L
  int status; 
  printf("please enter an integer to be summed");
  printf("(q to quit):");
  status = scanf("%ld",&num);
  while(status == 1)
   {
    sum += num;
    printf("Please enter next integer(q to quit):");
    status = scanf("%ld",&num);
   }
  printf("Those integer sum to %ld.\n",sum);
  getchar();
  return 0;
}
//注意，scanf()返回值賦給status，如果scanf讀取一個整數，則返回1；若讀取的不是數字，則返回0

eg.2:

//cmpflt.c---浮點數比較
#include<stdio.h>
#include<math.h>
int main(void)
{
  const double ANSWER = 3.14159;
  double response;
  printf("What is the value of pi?\n");
  scanf("%lf",&response);
  while(fabs(response-ANSWER)>0.01)//fabs()取精度更高的double、float型別的絕對值，在math標頭檔案裡
   {
    printf("Try again!\n");
    scanf("%lf",&response);
   }
  printf("Close enough!\n");
  getchar();
  return 0;
}

eg.3:

//truth.c---哪些值是為真
#include<stdio.h>
int main(void)
{
  int n=3;
  while(n)
    printf("%2d is true.\n",n--);
  printf("%2d is false.\n",n);
  n = -3;
  while(n)
   {
    printf("%2d is true.\n",n++);
   }
  printf("%2d is false.\n",n);
  getchar();
  return 0;
}

Ps:關於for語句：

for語句使用三個表示式控制迴圈過程

for(initialize;test;update)
    statement

initialize表示式在執行for語句之前執行一次，然後對test表示式求值，接著對update求值。

eg.4:

//entry.c---入口條件迴圈
#include<stdio.h>
int main(void)
{
  const int secret_code =13;
  int code_entered;
  printf("To enter the triskaidekaphobia therapy club,\n");
  printf("please enter the secret code number:");
  scanf("%d",&code_entered);
  while(code_entered != secret_code)
   {
    printf("To enter the ttc,\n");
    printf("please enter the secret code number:");
    scanf("%d",&code_entered);
   }
  printf("Congratulations!You a cured!\n");
  return 0;
}

eg.5:

//power.c---計算數的整數冪
#include<stdio.h>
double power(double n,int p);
int main(void)
{
  double x,xpow;
  int exp;
  printf("Enter a number and the positive integer power\n");
  printf("to which the number will be raised.Enter q to quit.");
  while(scanf("%lf %d",&x,&exp)==2)
   {
    xpow = power(x,exp);
    printf("%.3g to the power %d is %.5g\n",x,exp,xpow);
    printf("enter next pair of numbers or q to quit.\n");
   }
  printf("hope you enjoyed this power trip--byebye!\n");
  getchar();
  return 0;
}
double power(double n,int p)
{
  double pow=1;
  int i;
  for(i = 1;i<=p;i++)
    pow*=n;
  return pow;
}

分支和跳轉控制語句

eg.1:

//colddays---統計寒冷天氣百分比
#include<stdio.h>
#include<windows.h>
int main(void)
{
  const int FREEZING = 0;
  float temperature;
  int cold_days = 0;
  int all_days = 0;
  printf("Enter the list of daily low temperatures.\n");
  printf("Use Celsius,and enter q to quit.\n");
  while(scanf("%f",&temperature)==1)
   {
    all_days++;
    if(temperature<FREEZING)
     {
      cold_days++;
     }
   }
  if(all_days!=0)
    printf("%d days total:%.lf%% were below freezing.\n",all_days,100.0 * (float) cold_days/all_days);
  if(all_days == 0)
    printf("No data entered!\nplz try again!");
  system("pause");
  return 0;
}

eg.2:

//divisors.c---使用巢狀if語句顯示一個數的約數
#include<stdio.h>
#include<stdbool.h>
#include<windows.h>
int main(void)
{
  unsigned long num;//待測試的數
  unsigned long div;//可能的約束
  bool isPrime;
  printf("Please enter an integer for analysis;");
  printf("Enter q to quit.\n");
  while(scanf("%lu",&num)==1)
   {
    for(div = 2,isPrime = true;(div*div)<=num;div++)
     {
      if(num%div==0)
       {
        if((div*div)!=num)
        printf("%lu is divsible by %lu and %lu.\n",num,div,num/div);
        else
        printf("%lu is divisible by %lu and %lu.\n",num,div);
        isPrime = false;//該數不是素數
       }
     }
    if(isPrime)
      printf("%lu is is prime.\n",num);
    printf("Please enter another integer for analysis;");
    printf("Enter q to quit.\n");
   }
  printf("byebye!\n");
  system("pause");
  return 0;
}

此外關於C語言中分支與跳轉還有許多方法，例如if語句，switch語句，break、continue、goto語句等跳轉語句，用法都類似，貫徹迴圈重複執行任務的思想。

函式

//lethead1.c
#include<stdio.h>
#define NAME "GALATHINK,INC."
#define ADDRESS "101 MEGABUCK PLAZA"
#define PLACE "MEGApolis,CA 94904"
#define WIDTH 40
void starbar(void);
int main(void)
{
  starbar();
  printf("%s\n",NAME);
  printf("%s\n",ADDRESS);
  printf("%s\n",PLACE);
  starbar();
  return 0;
}
void starbar(void)//簡單地用這個函式做個邊框
{
  int count;
  for(count = 1;count<=WIDTH;count++)
    putchar('*');
  putchar('\n');
}