string match function

[jmprince01]

in C, write a function [char *strmatch(char *str, char *s)]
which will find the first occurance of the string s in the string str and will return a pointer to the first match if such a match occurs. if there is no match, then strmatch() should return NULL

Here is what i came up with. am i close?

char *strmatch(char *str, char *s)
while (char *str != '\0'){

if (*str == *s){
return (*s);
*s++;
}
}
return (NULL);

[Banfa]

No

Correcting your code so it would at least compile

char *strmatch(char *str, char *s){

  while (char *str != '\0'){

    if (*str == *s){
      return (s);  // Note can't return *s that has type char
      *s++;
    }
  }

  return (NULL);
}

Lets take an example

strmatch("Hello ", "Help");

Help does not occur in Hello so NULL should be returned

At the while *str == 'H' so the loop continues
At the if *str == 'H', *s == 'H' so *str == *s and we enter the if block
In the if block return s pointer to "Help"

Function fails

Try another example

strmatch("Red dog eats meat", "dog");

This should return a pointer to "dog eats meat".

At the while *str == 'R' so the loop continues
At the if *str == 'R', *s == 'd' so *str!== *s and we don't enter the if block

There are no other statements so the while loop iterates and we start again

At the while *str == 'R' so the loop continues
At the if *str == 'R', *s == 'd' so *str!== *s and we don't enter the if block

this is exactly the same as the previous itteration because str and s are not changed, if fact the function has entered an infinite loop.

The behaviour of this function is

If the first characters match return a pointer to the string to search for else enter an infinite loop.


When writing a small simple function like this then trying a few test examples can easily prove or disprove the function, especially if you have a symbolic debugger to let you step through the code. Even if you don't have a symbolic debugger use of printf statements will help you work out what is going on.

[jmprince01]

you're the greatest. thank you.