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A TUTORIAL ON POINTERS AND ARRAYS IN C - Part 1 |
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Written by Hemanshu
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Saturday, 29 December 2007 |
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Page 7 of 9
Now, let' consider some of the things the above examples have shown us. First off, consider the fact that *ptr++ is to be interpreted as returning the value pointed to by ptr and then incrementing the pointer value. This has to do with the precedence of the operators. Were we to write (*ptr)++ we would increment, not the pointer, but that which the pointer points to! i.e. if used on the first character of the above example string the ' would be incremented to a ' . You can write some simple example code to illustrate this.
Recall again that a string is nothing more than an array of characters, with the last character being a '\0'. What we have done above is deal with copying an array. It happens to be an array of characters but the technique could be applied to an array of integers, doubles, etc. In those cases, however, we would not be dealing with strings and hence the end of the array would not be marked with a special value like the nul character. We could implement a version that relied on a special value to identify the end. For example, we could copy an array of positive integers by marking the end with a negative integer.
On the other hand, it is more usual that when we write a function to copy an array of items other than strings we pass the function the number of items to be copied as well as the address of the array, e.g. something like the following prototype might indicate:
void int_copy(int *ptrA, int *ptrB, int nbr);
where nbr is the number of integers to be copied. You might want to play with this idea and create an array of integers and see if you can write the function int_copy() and make it work.
This permits using functions to manipulate large arrays. For example, if we have an array of 5000 integers that we want to manipulate with a function, we need only pass to that function the address of the array (and any auxiliary information such as nbr above, depending on what we are doing). The array itself does not get passed, i.e. the whole
array is not copied and put on the stack before calling the function, only its address is sent.
This is different from passing, say an integer, to a function. When we pass an integer we make a copy of the integer, i.e. get its value and put it on the stack. Within the function any manipulation of the value passed can in no way effect the original integer. But, with arrays and pointers we can pass the address of the variable and hence manipulate the
values of the original variables.
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Last Updated ( Saturday, 29 December 2007 )
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C programming 