c - How to pass an array of integers to a function as an argument and modify the content? -


i trying modify array of integers through function, array maintains original values. tried accesses values modify it:

this produces error:

*array[i] = *array[i] * *array[i];

this example runs program array modified:

int main() {     int array[] = {1, 2, 3, 4, 4, 5, 6};     int size = sizeof(array)/sizeof(int);      square_array_values(&array, size); }  void square_array_values(int *array, int size) {     int i;     (i = 0; < size; i++) {         printf("array[%d]: %d.\n", i, array[i]);         array[i] = array[i] * array[i];     } }

this late answer party - hope gives ideas how add prints code show going on.

consider output following program. there several ways approach this, 1 possibility. of earlier answers solid , should give ideas.

you're going need build mental model of how memory allocation works in c can predict compiler do. find printing out values of variables , addresses helpful. benefit sketching "memory map" paper , pencil know variables referencing storage.

foo.c output

$ gcc foo.c $ ./a.out hello main(), size=7 array1= 0x7ffc36ef7da0, &(array1)= 0x7ffc36ef7da0   note &(array1[0]) equals both value of array1 , &(array1), above.   also, each subsequent address of array1's elements sizeof(int) = 4 more   previous element, e.g. &(array[n]) = &(array[n-1]) + 4)    ---- before, array1 ----    array1[0]=1            |  0x7ffc36ef7da0    array1[1]=2            |  0x7ffc36ef7da4    array1[2]=3            |  0x7ffc36ef7da8    array1[3]=4            |  0x7ffc36ef7dac    array1[4]=5            |  0x7ffc36ef7db0    array1[5]=6            |  0x7ffc36ef7db4    array1[6]=7            |  0x7ffc36ef7db8 array2=       0x400b0d, &(array2)= 0x7ffc36ef7d98    haven't allocated array2 yet, no contents print out.    in fact, shouldn't printing array2 'points' @ all,    (when doesn't blow on address violation) kind of   emphasize nature of uninitialized values.    ---- before (an uninitialzed array2) ----    array2[0]=29590344     |        0x400b0d    array2[1]=1978349896   |        0x400b11    array2[2]=-998029078   |        0x400b15    array2[3]=1096637192   |        0x400b19    array2[4]=1096630620   |        0x400b1d    array2[5]=-1017167522  |        0x400b21    array2[6]=254699152    |        0x400b25  hello square_array_values(), size=7 array= 0x7ffc36ef7da0, &(array)= 0x7ffc36ef7d58    note value of our array param here same main()'s array1, above.    address of our array param here, &(array) different main()'s &(array1), above. before malloc, result=          (nil), &(result)= 0x7ffc36ef7d70  after malloc, result=      0x2082010, &(result)= 0x7ffc36ef7d70    note none of values above main()'s array2 match have    here square_array_value()'s variable result.    value of main()'s array2 match our value of result after return (see below). so, let's square values...    array[0]=1   | result[0]=1    array[1]=2   | result[1]=4    array[2]=3   | result[2]=9    array[3]=4   | result[3]=16    array[4]=5   | result[4]=25    array[5]=6   | result[5]=36    array[6]=7   | result[6]=49 goodbye square_array_values(), returning result=0x2082010 in main(). array1= 0x7ffc36ef7da0, &(array)= 0x7ffc36ef7da0    note have no differences 'before.array1', above.    of array1's values , addresses same.    ---- after ----    array1[0]=1            |  0x7ffc36ef7da0    array1[1]=2            |  0x7ffc36ef7da4    array1[2]=3            |  0x7ffc36ef7da8    array1[3]=4            |  0x7ffc36ef7dac    array1[4]=5            |  0x7ffc36ef7db0    array1[5]=6            |  0x7ffc36ef7db4    array1[6]=7            |  0x7ffc36ef7db8 array2=      0x2082010, &(array2)= 0x7ffc36ef7d98    array2, however, different 'before.array2', above.    array2 pointing nicely initialized memory - thank you, square_array_values().    ---- after (now array2 initialized) ----    array2[0]=1            |       0x2082010    array2[1]=4            |       0x2082014    array2[2]=9            |       0x2082018    array2[3]=16           |       0x208201c    array2[4]=25           |       0x2082020    array2[5]=36           |       0x2082024    array2[6]=49           |       0x2082028 main(): done. $

foo.c

i stuffed crazy number of printf()s original code, , helper function dump out given array. wouldn't go far in own development, wanted easier "imagine" program's memory locations @ run time.

#include <stdio.h> #include <malloc.h>  int *square_array_values(int *array, int size); /* modified return type */ void dump_array( char *msg, char *label, int *array, int size );  int main() {     /* typo? double 4's ? */     /* original: int array1[] = {1, 2, 3, 4, 4, 5, 6}; */     int array1[] = {1, 2, 3, 4, 5, 6, 7};     int size = sizeof(array1)/sizeof(int);     int *array2; /* can let square_array_values() assign result array */     /* or declare array here , pass in */      printf("hello main(), size=%d\n", size );     printf("array1=%15p, &(array1)=%15p\n", array1, &array1 );     printf("  note &(array1[0]) equals both value of array1 , &(array1), above.\n");     printf("  also, each subsequent address of array1's elements sizeof(int) = %ld more\n", sizeof(int) );     printf("  previous element, e.g. &(array[n]) = &(array[n-1]) + %ld)\n", sizeof(int) );     dump_array( "before, array1", "array1", array1, size );     printf("array2=%15p, &(array2)=%15p\n", array2, &array2 );     printf("   haven't allocated array2 yet, no contents print out.\n");     printf("   in fact, shouldn't printing array2 'points' @ all,\n");     printf("   (when doesn't blow on address violation) kind of\n");     printf( "  emphasize nature of uninitialized values.\n");     dump_array( "before (an uninitialzed array2)", "array2", array2, size );       /* original - try running way too...      *     void square_array_values( &array1, size);      */     array2 = square_array_values(array1, size);     printf("back in main().\n");      printf("array1=%15p, &(array)=%15p\n", array1, &array1 );     printf("   note have no differences 'before.array1', above.\n");     printf("   of array1's values , addresses same.\n");     dump_array( "after", "array1", array1, size );      printf("array2=%15p, &(array2)=%15p\n", array2, &array2 );     printf("   array2, however, different 'before.array2', above.\n");     printf("   array2 pointing nicely initialized memory - thank you, square_array_values().\n");     dump_array( "after (now array2 initialized)", "array2", array2, size );      free( array2 );     printf("main(): done.\n"); }  int *square_array_values(int *array, int size) {     int i;     int *result;     printf("\nhello square_array_values(), size=%d\n", size );     printf("array=%15p, &(array)=%15p\n", array, &array );     printf("   note value of our array param here same main()'s array1, above.\n");     printf("   address of our array param here, &(array) different main()'s &(array1), above.\n");     printf("before malloc, result=%15p, &(result)=%15p\n", result, &result);     result = malloc( sizeof(int) * size );     printf(" after malloc, result=%15p, &(result)=%15p\n", result, &result);     printf("   note none of values above main()'s array2 match have\n");     printf("   here square_array_value()'s variable result.\n");     printf("   value of main()'s array2 match our value of result after return (see below).\n");     printf("so, let's square values...\n");     (i = 0; < size; i++) {         result[i] = array[i] * array[i];         printf("   array[%d]=%d   | result[%d]=%d\n", i, array[i], i, result[i] );     }     printf("goodbye square_array_values(), returning result=%p\n", result);     return result; }  void dump_array( char *msg, char *label, int *array, int size ) {    printf("   ---- %s ----\n", msg );     (int = 0; < size; i++) {         printf("   %s[%d]=%-12d | %15p\n", label, i, array[i], &(array[i]) );     } }

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