finding substring

This is a discussion on finding substring within the C Programming forums, part of the General Programming Boards category; Hello everyone; Whats the problem in following code for searching a substring in a string. The code is here- Code: ...

  1. #1
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    Question finding substring

    Hello everyone;

    Whats the problem in following code for searching a substring in a string. The code is here-

    Code:
    #include<stdio.h>
    #include<stdlib.h>
    void sarch( char *, int , char *, int );
    void main()
    {
    int i;
    char *str="thegreathero";
    char *pattern="hero";
    int m,n;
    search(char *str,int m, char* pattern, int n);
    printf("\n \n");
    
    }
    
    void search( char *x, int m, char *y, int n)
    {
    int i,j;
    for(j=0;j<(n-m);++j)
    {
    for(i=0; i<m && x[i]=y[i+j]; ++i);
    if(i>=m){
    printf("\n match found at \n \n->[%d]\n->[%s]\n",j,y+j);
    }
    }
    Last edited by rits; 03-11-2009 at 11:03 AM.

  2. #2
    Technical Lead QuantumPete's Avatar
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    There are a number of problems, though it would be helpful if you posted what kind of errors you got and where. Compiler/Linker/Run time?
    In any case, you don't need to specify the types of the arguments when you call the search function inside main. Because you have already declared the function at the top, the compiler knows what kind of arguments you are passing to the function. Also, you never initialise m or n, so what you pass into search could be anything from 0 to the maximum value an int can hold, it's entirely random.

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  3. #3
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    Yes, silly mistake inialization of m and n are missing. But one more thing i am getting compilation time error in the for loop in search function (for(i=0; i<m && x[i]=y[i+j]; ++i);
    saying l value required. Please help me out....

  4. #4
    Technical Lead QuantumPete's Avatar
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    Before I do that, indent your code properly and post the latest version of it. That way we're all reading from the same page.

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  5. #5
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    Here it is-

    Code:
    #include<stdio.h>
    #include<stdlib.h>
    void search( char *, int , char *, int );
    void main()
    {
         int i;
         char *str="thegreathero";
         char *pattern="hero";
         int m=4,n=12;
         search(str, m,  pattern,  n);
         printf("\n \n");
    
    }
    
    void search( char *x, int m, char *y, int n)
    {
         int i,j;
         for(j=0;j<(n-m);++j)
        {
           for(i=0; i<m && x[i]=y[i+j]; ++i);
            if(i>=m){
            printf("\n match found at \n \n->[%d]\n->[%s]\n",j,y+j);
          }
       }
    }

  6. #6
    Banned ಠ_ಠ's Avatar
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    Code:
    for(i=0; i<m && x[i]=y[i+j]; ++i);
    this will store the value of y[i + j] into x[i] every time the loop runs

    If you want it to check if x[i] has the same value of y[i+j] you need to use == like this
    Code:
    for(i=0; i<m && x[i]==y[i+j]; ++i);
    also you terminated the for loop with a semicolon and you didn't use opening/closing curly braces (I don't think either of those are valid)

    Code:
          for(i=0; i<m && x[i]=y[i+j]; ++i);
            if(i>=m){
            printf("\n match found at \n \n->[%d]\n->[%s]\n",j,y+j);
          }
    I think this is what the correct version would look like

    Code:
               for(i=0; i<m && x[i]==y[i+j]; ++i) {
                 if(i>=m) {
                   printf("\n match found at \n \n->[%d]\n->[%s]\n",j,y+j);
                 }
               }

    is there any particular reason you are using ++i instead of i++?
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  7. #7
    Registered User whiteflags's Avatar
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    >> is there any particular reason you are using ++i instead of i++?
    You say that as if it would be wrong, it isn't.

  8. #8
    Banned ಠ_ಠ's Avatar
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    Quote Originally Posted by whiteflags View Post
    >> is there any particular reason you are using ++i instead of i++?
    You say that as if it would be wrong, it isn't.
    yeah, I just never see anyone else do that
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    Yes i made the changes its compiling sucessfully but damn thing is not showing any result.. is the mistake in passing m and n i have inialize these with my string and substring length.

  10. #10
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    post updated code
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  11. #11
    Technical Lead QuantumPete's Avatar
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    Quote Originally Posted by ಠ_ಠ View Post
    yeah, I just never see anyone else do that
    It's ever so slightly faster than i++, because the register only has to be loaded once. In practice and on today's computers it makes very little difference.
    Also, you don't *need* braces for a for loop, as long as the looped block is only one line (or in this case an if statement with one line). However, it's good practice to *always* use braces, in case you want to add debug print statements later on.

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  12. #12
    CSharpener vart's Avatar
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    Quote Originally Posted by ಠ_ಠ View Post
    yeah, I just never see anyone else do that
    If you read more C++ code you should see it more often. For simple types (used in C) it makes no difference.

    In C++ - for types having complex copy-constructor - It could affect performance. So C++ programmers just have a habbit to prefer prefix over postfix increment decrement in places where there is no logical difference to using one over another
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  13. #13
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    Here is the latest code-
    Code:
    #include<stdio.h>
    #include<stdlib.h>
    void search( char *, int , char *, int );
    void main()
    {
        int i;
        char *str="thegreathero";
        char *pattern="hero";
        int m=4,n=12;
        search(str, m,  pattern,  n);
        printf("\n \n");
      
      }
      
    void search( char *x, int m, char *y, int n)
    {
        int i,j;
        for(j=0;j<(n-m);++j)
        {
          for(i=0; i<m && x[i]==y[i+j]; i++){
          if(i>=m){
                          printf("\n match found at \n \n->[%d]\n->[%s]\n",j,y+j);
                       }
                }
         }
    }
    

  14. #14
    Frequently Quite Prolix dwks's Avatar
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    Code:
          for(i=0; i<m && x[i]==y[i+j]; i++){
          if(i>=m){
    Do you think it will ever be the case that i >= m when part of your for loop condition is i < m?

    Also: cpwiki.sf.net/Void_main
    dwk

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  15. #15
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    Similar thread here that you may wanna check it out.

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