A problem with a pointer to a string

This is a discussion on A problem with a pointer to a string within the C Programming forums, part of the General Programming Boards category; Hi. i was bored so i wrote a very simple encryption program using a global variable 'text' (the text to ...

  1. #1
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    A problem with a pointer to a string

    Hi. i was bored so i wrote a very simple encryption program using a global variable 'text' (the text to be encrypted) which i pass into two functions, encrypt and decrypt. This works fine, but as i've been told before that global variables are bad (i'm unsure why) i want to use a pointer to text instead of the global variable. However i don't have a very good understanding of pointers at all, so both my attempts to implement the code below with pointers failed miserably. Any help regarding this would be greatly appreciated.

    Thanks

    My code:

    Code:
    #include <stdio.h>
    #include <string.h>
    
    void encrypt(char[]);
    void decrypt(char[]);
    
    char text[100];
        
    int main(void)
    {     
       char decision[10];
        
       printf("Enter text to be encrypted:");
       fgets(text, sizeof(text), stdin);
       text[strlen(text)-1] = '\0';
        
       encrypt(text);
       printf("%s\n", text);
    	
       printf("Do you want to decrypt the string?(yes/no)\n");
       fgets(decision, sizeof(decision), stdin);
       decision[strlen(decision)-1] = '\0';
    	
       if (strcmp(decision, "yes") == 0)
       {
          decrypt(text);
          printf("%s\n", text);
       }
       return 0;
    }
    
    void encrypt(char txt[])
    {
        int i;
        
        for (i = 0; i < strlen(txt); i++)
        {
            txt[i]++;
        }
    }
    
    void decrypt(char txt[])
    {
        int i;
        
        for (i = 0; i < strlen(txt); i++)
        {
            txt[i]--;
        }
    }

  2. #2
    Just Lurking Dave_Sinkula's Avatar
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    My test:
    Enter text to be encrypted:hello world
    ifmmp!xpsme
    Do you want to decrypt the string?(yes/no)
    yes
    hello world
    What's the question? If you just want text local to main, do so.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  3. #3
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    my understanding was that if the text variable was not global, but local, i would be able to pass it into my functions, but i would not be able to change the variable from the function itself, unless i used a pointer to the variable. sorry if it was unclear but my question was simply how would i go about implementing the code i had already written (above), with a pointer to the variable 'text' (where text would be inside main()) instead of using 'text' as a global variable. Hope this makes sence.

    Thanks

  4. #4
    Just Lurking Dave_Sinkula's Avatar
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    I may me a little off at the moment, but this does exactly the same as when text is global.
    Code:
    int main(void)
    {     
       char text[100];
       char decision[10];
        
       printf("Enter text to be encrypted:");
       fgets(text, sizeof(text), stdin);
       text[strlen(text)-1] = '\0';
        
       encrypt(text);
       printf("%s\n", text);
    	
       printf("Do you want to decrypt the string?(yes/no)\n");
       fgets(decision, sizeof(decision), stdin);
       decision[strlen(decision)-1] = '\0';
    	
       if (strcmp(decision, "yes") == 0)
       {
          decrypt(text);
          printf("%s\n", text);
       }
       return 0;
    }
    unless i used a pointer to the variable
    These are passing pointers.
    Code:
    void encrypt(char[]);
    void decrypt(char[]);
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

  5. #5
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    Thanks i did not realise this.
    Last edited by edd1986; 03-20-2005 at 04:46 PM.

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