lvalue required as left operand of assignment error when using C

Consider the following array:

int arr[5] = {1,2,3,4,5};

We know that name of the array points to address of first element of array i.e. address of 1 here.

So, the following code:

printf("%u",arr);

will give address of 1 in this case. This address of 1 is constant and cannot be changed. So, in a C program, it is not possible to change the value of arr.

The following code will give an error: lvalue required as left operand

#include

int main(){
     int arr[5] = {1,2,3,4,5};  
     arr++;
     return 0;
}

Even if you give a lvalue by modifying the code as below, it will still throw a compilation error.
error: incompatible types when assigning to type 'int[5]' from type 'int *'  

int main(){
     int arr[5] = {1,2,3,4,5};  
     arr = arr +1;
     return 0;
}

Thus, important thing to understand here is, we are missing out on a key concept.
Here, arr is a constant just like a const variable which cannot be changed and any attempt to change that value will result in a compilation error.

It will be more comprehensible if the error "lvalue required as left operand" would have read "arr is a constant and cannot be changed".

Having said that, the following operation is valid.

int main(){
     int arr[5] = {1,2,3,4,5};  
     printf("%d\n", *(arr + 1));
     return 0;
}

This is because, we are fetching the value at index 1 of array arr and not increasing the value of arr.

This is similar to int y = x +5;
We are not increasing the value of x here. We are just adding 5 to value of 5 and assigning it to y.

Fetching elements in multi-dimensional array in C using pointers

As opposed to traditional method of using integer indexes, elements in a multi-dimensional array in C can be fetched using pointers.

Consider the following code using multi-dimensional array.

#include <stdio.h>

int main()
{
 
    int matrix[10][20] = {{1,2,3,4},{5,6,7,8},{9,10,11,12}};
    int i,j;
    for(i = 0; i < 3;i++){
        for(j = 0 ; j< 4 ;j++){
            printf("%d\n",(*(*(matrix +i)) + j));  
        }
    }
 
return 0;
}

Different methods are available to print individual elements from the above array. Three different approaches are described below.

1.  *(a[i] + j) :
    a[i] refers to first element in i th array of matrix. Adding j gives the address of j th element in i th array.
    De-referencing this address gives value at ith row and j th column of matrix array

2. (*(matrix + i))[j]
      *(matrix + i) gives address of first element of i th row of matrix. [j] gives j th column of row i.

3. *(*(matrix + i) + j)
     As above, *(matrix + i) gives address of first element of i th row of matrix. Adding j gives address of j th column of matrix. De-referencing this element gives the value of  i th row and j th column.

Array of Structure

We can use structure to store the value of one particular object but if we want to store the value of 100 such objects we need array of structure.

For eg: If we want to store the value of details of book like author, no of pages and price. We can define such structure as follows:

struct bookinfo{

    char[20] author;

    int pages;

    float price;

};

Now to define three similar structure we can define an array of the above defined structure as follows:

struct bookinfo record[3];

Lets's see this using a C program:

 #include
  struct bookinfo{
           char author[20];
           int pages;
           float price;
   };
 
   int main(){
       int i;
      struct bookinfo record[3];
      char suffix[3][3]={"st","nd","rd"};
      for(i=0;i<3 i="" p="">      {
             printf("Enter the author of first book: ");
             scanf("%s",record[i].author);
             printf("Enter the number of pages in the book: ");
             scanf("%d",&record[i].pages);
             printf("Enter the price of the book: ");
             scanf("%f",&record[i].price);
      }
 
      for(i=0;i<3 i="" p="">      {
 
         printf("\nThe author of %d%s book is: %s \n",i+1,suffix[i],record[i].author);
         printf("The number of pages in %d%s book is: %d \n",i+1,suffix[i],record[i].pages);
         printf("The price of %d%s book is: %f \n",i+1,suffix[i],record[i].price);
      }
      return 0;
  }

OUTPUT:

Enter the author of first book: fasdf
Enter the number of pages in the book: 32
Enter the price of the book: 34
Enter the author of first book: phy
Enter the number of pages in the book: 45
Enter the price of the book: 56.5
Enter the author of first book: math
Enter the number of pages in the book: 54
Enter the price of the book: 45.7

The author of 1st book is: fasdf
The number of pages in 1st book is: 32
The price of 1st book is: 34.000000

The author of 2nd book is: phy
The number of pages in 2nd book is: 45
The price of 2nd book is: 56.500000

The author of 3rd book is: math
The number of pages in 3rd book is: 54
The price of 3rd book is: 45.700001