Thread: 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);
}
}

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.

QuantumPete

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.

QuantumPete

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++?

Originally Posted by whiteflags

>> 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

post updated code

Originally Posted by ಠ_ಠ

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.

QuantumPete

Originally Posted by ಠ_ಠ

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

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