Question

I’m trying to modify a previous program I wrote using pure pointer notation. It’s a program that generates a random string of 40 uppercase letter, takes input of up to 20 uppercase letter, and input of a character. The program replaces reoccurring characters in the generated string with the character entered. Right now, I’m trying to pass the parameters of the randomly generated string so I can access them in the second function and can’t figure out how to do it.

Thank you for the help.

#include <stdio.h>
#include <stdlib.h>

/* Function prototypes */
void fillS1(char * x);

void fillS2(char * x, char * y, char z);

void strFilter(char * a, char * b, char c);

int main(int argc, const char * argv[])
{

char s1[41];
char s2[21];
char x = 0;

char * pointerToS1;
char * pointerToS2;

pointerToS1 = s1;
pointerToS2 = s2;

fillS2(pointerToS2, pointerToS1, x);

return 0;
}

/* Function to generate a random string of 40 uppercase letters */
void fillS1(char * randomlyGeneratedPointer)
{
char randomlyGeneratedArray[41];

randomlyGeneratedPointer = randomlyGeneratedArray;

int i;

for (i = 0; i < 40; i++) {
    *(randomlyGeneratedPointer + i) = 'A' + rand() % 26;
}


}

/* Function to get user input of characters */
void fillS2(char * userStringPointer, char * randomStringPointer, char     replacementCharacter)
{

char userstring[21];
char randomString[41];
char copyString[42];
//char * pointerToCopyString = copyString;

userStringPointer = userstring;



int i = 0;
int n = 0;
int lowercaseCheck = 0;

char loopContinue = 0;




fillS1(randomStringPointer);  //here is the function call for the randomly generated       string.



printf("This is the random string: %s", randomStringPointer);

do {

    /* For loop to copy the first randomly generated string */
    for(i = 0; i < 42; i++)
        *(randomStringPointer + i) = copyString[i];

    randomStringPointer = copyString;

    i = 0;
    lowercaseCheck = 0;

    /* While loop to to get user input */
    printf("Please enter at least 2 capital letters and a maximum of 20.\n");
    while  ((((*(userStringPointer + i)) = getchar()) != '\n'))  {

        /* Counter to determine how many characters were entered */
        i++;

    }

    /* Adding 1 to add to add null character */
    i++;

    *(userStringPointer + i) = '\0';

    //printf("This is the user's string %s", userStringPointer);

    /* Capital letter check */
    for (n = 0; n < 20; n++) {
        if (((*(userStringPointer + n)) >= 'a') && (*(userStringPointer + n) <= 'z')) {

            lowercaseCheck++;
        }
    }

    if (--i < 3) {
        printf("You need at least two letters\n");
    }

    else if (i > 21){
        printf("You cannot have more than twenty letters\n");
    }

    else if (lowercaseCheck == 0) {

        puts(userStringPointer);

        printf("Enter a character to replace occuring letters.\n");
        scanf("%c", &replacementCharacter);
        getchar();

        //printf("this is the copy string before strFilter: %s", randomStringPointer);
        //printf("This is the replacement character %c", replacementCharacter);

        strFilter(randomStringPointer, userStringPointer, replacementCharacter);

    }

    else

        printf("You  must have 2 capital letters.\n");


    printf("Would you like to enter another string (y/n)?\n");
    loopContinue = getchar();
    getchar();


} while (loopContinue != 'n' && loopContinue != 'N');

}

/* Function to replace letters with the character chosen by the user */
void strFilter(char * replacementCopyStringPointer, char * replacementUserStringPointer,         char c)
{

int i = 0;
int n = 0;

while (n < 20) {

    for (i = 0; i < 40; i++) {
        if ((*(replacementCopyStringPointer + i)) == *(replacementUserStringPointer + n)){
            *(replacementCopyStringPointer + i) = c;
        }
    }
    i = 0;
    n++;
}

puts(replacementCopyStringPointer);
}

Answer 1

randomlyGeneratedArray array in fillS1 would get destroyed once fillS1 function returns.
You should allocate the memory from heap for randomlyGeneratedArray.

randomlyGeneratedArray = (char *)malloc(sizeof(char)*41)

Also do the same for userStringPointer in fillS2.
That should solve the problem.

For difference between stack and heap read this question What and where are the stack and heap?