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Explain the concept of uninitialized array access in C language

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Release： 2023-09-01 20:53:05

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Explain the concept of uninitialized array access in C language

Question

In C language, if an uninitialized array is used, will the program be executed?

Solution

If we use any uninitialized array, the compiler will not generate any compilation and execution errors.
If the array is not initialized, you may get unpredictable results.
Therefore, it is better that we always initialize array elements with default values.

Sample program

The following is a C program to access an uninitialized array -

Live demonstration

#include <stdio.h>
int main(void){
   int a[4];
   int b[4] = {1};
   int c[4] = {1,2,3,4};
   int i; //for loop counter
   //printing all alements of all arrays
   printf("</p><p>Array a:</p><p>");
   for( i=0; i<4; i++ )
      printf("arr[%d]: %d</p><p>",i,a[i]);
   printf("</p><p>Array b:</p><p>");
   for( i=0; i<4; i++)
      printf("arr[%d]: %d</p><p>",i,b[i]);
   printf("</p><p>Array c:</p><p>");
   for( i=0; i<4; i++ )
      printf("arr[%d]: %d</p><p>",i, c[i]);
   return 0;
}

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Output

When the above program is executed, the following results are produced -

Array a:
arr[0]: 4195872
arr[1]: 0
arr[2]: 4195408
arr[3]: 0

Array b:
arr[0]: 1
arr[1]: 0
arr[2]: 0
arr[3]: 0

Array c:
arr[0]: 1
arr[1]: 2
arr[2]: 3
arr[3]: 4

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NOTE

If we do not initialize the array, by default, it will print garbage values and never show the error.

Consider another C program to access an uninitialized array -

Example

Live demonstration

#include <stdio.h>
int main(void){
   int A[4];
   int B[4] ;
   int C[4] = {1,2};
   int i; //for loop counter
   //printing all alements of all arrays
   printf("</p><p>Array a:</p><p>");
   for( i=0; i<4; i++ )
      printf("arr[%d]: %d</p><p>",i,A[i]);
   printf("</p><p>Array b:</p><p>");
   for( i=0; i<4; i++)
      printf("arr[%d]: %d</p><p>",i,B[i]);
   printf("</p><p>Array c:</p><p>");
   for( i=0; i<4; i++ )
      printf("arr[%d]: %d</p><p>",i, C[i]);
   return 0;
}

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Output

When executing the above program, the following results will be produced -

Array a:
arr[0]: 4195856
arr[1]: 0
arr[2]: 4195408
arr[3]: 0

Array b:
arr[0]: -915120393
arr[1]: 32767
arr[2]: 0
arr[3]: 0

Array c:
arr[0]: 1
arr[1]: 2
arr[2]: 0
arr[3]: 0

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