Pointers and arrays

the concept of pointers and arrays in C or C++ go together. there is a very strong relationship between these two concepts and in this lesson we will be discussing this relationship when we declare an array let's say we declare an Integer array A of size 5 then we create 5 integer variables named A0 A1 A2 A3 ad A4 and now these five integers will be stored in the memory as a block of five consecutive integers something like this what we are showing here in the right if A0 or the first element in the array stored at address let's say address 200 and in a typical compile modern day compiler integer is stored in four bytes so A1 will be four bytes ahead of A0 at 204 and A2 will be at 208 A3 will be at 212 A4 will be at 216 the overall size of array will be 20 bytes and these 20 bytes will be one consecutive block we are only showing the section of the memory in which A is stored sometimes we also show the memory horizontally something like this from left to right we increase the address but these two representations are just for the sake of understanding let's use the horizontal representation of the memory this time I will show this memory a little more extending towards the right so that I can accommodate a couple of more variables let's say apart from this integer Array A I have an integer variable X and its value is 5 and let's say X is located at address 300 now let's say I have a pointer to integer P and in P I store address of X if we print P then the value and P would be 300 so this statement will print 300 and if we dereference P and try to bring the address stored in, try to print the value stored in this location P in the value would be Five this is fine but we also know that we can do an operation something like increment and decrement a pointer variable by a constant so we can do something like P equal the P Plus one and this will take us to the address of the next integer and because integer is four bytes so now P would be 304 so if I want to print p now then the output should be 304 but if we try to dereference P now and try to print * p then we do not know the value at this address so we cannot say what will be printed it's like we know that Mr ex lives at address 300 but we do not know who is his neighbor who lives at address 304 but for this integer Array A lets us say I'm writing this same integer array in the right here which is located at address 200 if I declare a pointer to integer P and store the address of the first element by putting an ampersand operator in front of A0 then printing P would give us in this scenario the output 200 and printing *P would give us but before that lets say we have these values in the array lets fill up some values in the array so *P would be 2 and if i want to print P plus one then the address would be 204 and if I dereference P+1 and try to bring this value then print this value it will be Four and similarly if we wanted the third element in the array we could do a P plus 2 here so using pointer arithmetic make sense in the case of pointers because in the case arrays because we know what is in the adjacent location there is one more property of array if we just use the name of the Array A then A gives us the pointer to the first element in the array so we can write a statement like P is equal to A in fact we do not even need to take this address in another pointer variable if we simply print A then this gives us nothing but the address of the first element in the array and if we want to dereference this and try to print *A then this will give us the value so if we want to perform something like we want to print A plus one then this will give us the address 204 and *(A+1) will give us the address of the second element in the array value of the second element in the array. For an elemen in the array at index i we can retrieve the address of this particular element in the memory using either ampersand A[i] or simply A plus i and these two will give us the address of A[ i] and the value of A[ i] can be retrieved using either we simply use A[ i] or we can also use * A plus i and *(A+1) will also give us the value now this is an important concept you can write ampersand A[i] or A plus i for each other and they mean the same and we can write A[i] or *(A+1) for each other and they mean the same tha address of the first element in the array can also be called the Base address and A simply using the variable name A gives us the base address of the array let us now see some code examples and try to solidify our concepts further in my program lets say we have and integer array A now as we said if we simply print A then this should give us the address of the first element in the array and we can also get the address of the first element in the array by using ampersand written in front A[0] if I simply print A[0] it will print the first element in the array and we can also print the first element in the array by using * operator in front of the variable name A let us now run this and see what happens and i also need to put an endline afte r each of these print statements okay so the output is that the first two lines are the same they are giving us the address of the first element in array. and the second lines are giving us the value in fact if we run a loop like this from 0 to 4 then they can print address of the element at index i as &A[i] or (A+i) and we can print the value of ith element as A[i] or *(A+i) now if we see the output here this is a fresh run a new run so old address allocations will change but if we see the address printed in two lines for A[0] is same the value same again for A1 address is same four bytes ahead of the previous address and the value is as expected and we can go on like this now one more thing even though just using the variable name A returns us pointer to the base address or the address of the first element and we can equate the variable name A again some pointer variable like this you cannot do something like A plus plus increment thel value of A this will give us compilation error we can do something like P++ once we assign A to some pointer variable, other pointer variable but incrementing A itself would be in valid so this was how arrays are stored in memory and how that addresses can be manipulated and how they can access the values using pointers in the next lessons in the coming lessons we will talk about character arrays and even talk about passing arrays as function arguments.so thanks fo r watching

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Length: 8min 42sec (522 seconds)

Published: Wed Feb 20 2013