String Pattern Matching in C - c

Question

I was trying this pattern matching method in C but whenever I give all the input, the vscode terminal waits for a while and just stops the program without any warnings/message. Can anyone point to what is wrong here?
#include <stdio.h>
#include <string.h>
int main()
{
char STR[100], PAT[100], REP[100], ANS[100];
int i, m, j, k, flag, slP, slR, len;
i = m = k = j = flag = len = 0;
printf("\nMain String: ");
gets(STR);
printf("\nPattern String: ");
gets(PAT);
slP = strlen(PAT);
printf("\nReplace String: ");
gets(REP);
slR = strlen(REP);
while (STR[i] != '\0')
{
if (STR[i] = PAT[j])
{
len = 0;
for (k = 0; k < slP; k++)
{
if (STR[k] = PAT[k])
len++;
}
if (len == slP)
{
flag = 1;
for (k = 0; k < slR; k++, m++)
ANS[m] = REP[k];
}
}
else
{
ANS[m] = STR[i];
m++;
i++;
}
}
if (flag == 0)
{
printf("\nPattern not found!");
}
else
{
ANS[m] = '\0';
printf("\nResultant String: %s\n", ANS);
}
return 0;
}

Answer 1

There are multiple problems in the code:
using gets() is risky, this function was removed from the C Standard because it cannot be used safely.
if (STR[i] = PAT[j]) copied the pattern to the string. You should use:
if (STR[i] == PAT[j])
similarly, if (STR[k] = PAT[k]) is incorrect. You should compare PAT[k] and STR[i + k]:
if (STR[i + k] == PAT[k])
you should test for buffer overflow for the output string as replacing a short string by a larger one may produce a string that will not fit in ANS
you do not increment i properly.
Here is a modified version:
#include <stdio.h>
int getstr(const char *prompt, char *dest, int size) {
int c, len = 0;
printf("%s", prompt);
while ((c = getchar()) != EOF && c != '\n') {
if (len + 1 < size)
dest[len++] = c;
}
if (size > 0)
dest[len] = '\0';
printf("\n");
if (c == EOF && len == 0)
return -1;
else
return len;
}
int main() {
char STR[100], PAT[100], REP[100], ANS[100];
int i, m, k, flag;
if (getstr("Main String: ", STR, sizeof STR) < 0)
return 1;
if (getstr("Pattern String: ", PAT, sizeof PAT) < 0)
return 1;
if (getstr("Replace String: ", REP, sizeof REP) < 0)
return 1;
i = m = flag = 0;
while (STR[i] != '\0') {
if (STR[i] == PAT[0]) { // initial match
// compare the rest of the pattern
for (k = 1; PAT[k] != '\0' && PAT[k] == STR[i + k]; k++)
continue;
if (PAT[k] == '\0') { // complete match
flag = 1;
// copy the replacement string
for (k = 0; REP[k] != '\0'; k++) {
if (m + 1 < sizeof ANS)
ANS[m++] = REP[k];
}
i += k; // skip the matching characters
continue;
}
}
// otherwise copy a single character
if (m + 1 < sizeof ANS)
ANS[m++] = STR[i];
i++;
}
ANS[m] = '\0';
if (flag == 0) {
printf("Pattern not found!\n");
} else {
printf("Resultant String: %s\n", ANS);
}
return 0;
}