我正在关注iTunes大学：哈佛CS50讲座，以学习编程的基础知识。我目前正在尝试为文本密码创建程序。这是整个程序的代码。
#include<stdio.h>
#include<cs50.h>
#include<stdlib.h>
#include<string.h>
int cyphmain (void);
int decyphmain (void);
int maincalc (int k);
int dmaincalc (int k);
int uppercalc (int u, int k);
int lowercalc (int u, int k);
int duppercalc (int u, int k);
int dlowercalc (int u, int k);
int
main(void) //Main Function begins by asking the user if he would like to Cypher or Decypher his text
{
char h;
printf("Do You Want Cypher(c) or Decypher(d)?:");
scanf("%c",&h);
if (h==99) //Checks ascii of "c"(99) and runs the cypher main function
cyphmain();
else if (h==100) //Checks ascii of "d"(100) and runs the decypher main function
decyphmain();
else
return 0;
}
int cyphmain (void) //Cypher Main Function
{
int k;
printf("What is the Cypher Number:"); //Gets the rot number
scanf("%d",&k);
if (k>25) //Ensures that the User only choses a number from 1-25
{ while (k>25) //Continues to ask user for a number until they input a value <=25
{
printf("Sorry Please Choose A Number from 1-25 only:");
int l=GetInt();
k=l;
}
maincalc(k); //as soon as a valid input is received maincalc function is run
}
else //in this case a valid input has been recieved and maincalc runs
maincalc(k);
return 0;
}
int decyphmain (void) //Decypher Main Function
{
int k;
printf("What is the Cypher Number:"); //Gets the rot number
scanf("%d",&k);
if (k>25) //Ensures that the User only choses a number from 1-25
{ while (k>25) //Continues to ask user for a number until they input a value <=25
{
printf("Sorry Please Choose A Number from 1-25 only:");
int l=GetInt();
k=l;
}
dmaincalc(k); //as soon as a valid input is received maincalc funtion is run
}
else //in this case a valid input has been recieved and maincalc runs
dmaincalc(k);
return 0;
}
int maincalc(int k) //The Calculation Function for Cyphering
{
char *s;
printf("Please enter the phrase that you would like to code:");
scanf("%s",&s);
int i=0;
int n=strlen(s);
while(i<n)
{
if(s[i]>=65&&s[i]<=90) //For Uppercase letters
{
int u=s[i];
int c=uppercalc(u,k); //Hands off the character to a function to cypher Upper Case Letters
printf("%c",c);
}
else if(s[i]>=97&&s[i]<=122) //For Lowercase letters
{
int u=s[i];
int c=lowercalc(u,k); //Hands off the character to a function to cypher Lower Case Letters
printf("%c",c);
}
else
printf("%c",s[i]); //For non letters
i++;
}
printf("\n");
return 0;
}
int uppercalc(int u, int k) //Algorithm used to cypher Upper Case letters
{
if(u+k<=90)
{
int c=u+k;
return (c);
}
else
{
if (((u+k)/26)==3)
{
int c=((u+k)%26)+52;
return (c);
}
else
{
int c=((u+k)%26)+78;
return (c);
}
}
}
int lowercalc(int u, int k) //Algorithms used to Cypher lower case letters
{
if(u+k<=122)
{
int c=u+k;
return (c);
}
else
{
if (((u+k)/26)==4)
{
int c=((u+k)%26)+78;
return (c);
}
else
{
int c=((u+k)%26)+104;
return (c);
}
}
}
int dmaincalc(int k) //The Calculation Function for Decyphering
{
char *s;
printf("Please enter the phrase that you would like to decode:");
scanf("%s",&s);
int i=0;
int n=strlen(s);
while(i<n)
{
if(s[i]>=65&&s[i]<=90) //For Uppercase letters
{
int u=s[i];
int c=duppercalc(u,k);
printf("%c",c);
}
else if(s[i]>=97&&s[i]<=122) //For Lowercase letters
{
int u=s[i];
int c=dlowercalc(u,k);
printf("%c",c);
}
else
printf("%c",s[i]); //For non letters
i++;
}
printf("\n");
return 0;
}
int duppercalc(int u, int k) //Algorithm to decypher Upper Case letters
{
if(u-k>=65)
{
int c=u-k;
return (c);
}
else
{
if (((u-k)/26)==2)
{
int c=((u-k)%26)+78;
return (c);
}
else
{
int c=((u-k)%26)+52;
return (c);
}
}
}
int dlowercalc(int u, int k) //Algorithms to decypher lower case letters
{
if(u-k>=97)
{
int c=u-k;
return (c);
}
else
{
if (((u-k)/26)==3)
{
int c=((u-k)%26)+104;
return (c);
}
else
{
int c=((u-k)%26)+78;
return (c);
}
}
}
本程序首先通过插入char“c”或“d”询问用户是否想要Cypher或Decypher，然后运行cyphmain（如果是cyphering）或dcecyphmain（如果是decphering）函数。这很好用
程序然后询问用户腐烂编号，然后要求用户输入短语。这也很好。
然而，在输入（de）cypher的单词后，程序崩溃并出现分段错误，这让我相信错误在于maincalc / dmaincalc函数。 （请注意，每个函数基本上有两个副本用于加密，另一个用于decyphering，它们完全相同，除了文本中的一些更改以及对文本进行加密或解密的实际数学时）。
这是故障的一个例子
Do You Want Cypher(c) or Decypher(d)?:d
What is the Cypher Number:2
Please enter the phrase that you would like to decode: Hello
Segmentation fault (core dumped)
答案 0 :(得分：0)
您的错误在这里：
char *s;
printf("Please enter the phrase that you would like to code:");
scanf("%s",&s);
这不会为字符串分配空间。您可以使用malloc来分配空间，也可以将字符串放在堆栈中。
char *s = malloc(sizeof(char) * 50); /* option 1*/
char s[50]; /* option 2 */
对于选项1，请注意sizeof(char)始终为1，我只是为了完整性而将其放在那里。另外，请不要忘记在完成字符串后必须调用free(s)。