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#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
void encrypt(FILE * f, int key) {
int c;
while ((c = fgetc(f)) != EOF) {
if (isalpha(c)) {
c = tolower(c);
c -= 'a';
c += key;
c %= 26;
c += 'a';
}
printf("%c", c);
}
}
int main(int argc, char ** argv) {
if (argc != 3) {
fprintf(stderr,"Usage: encrypt key inputFileName\n");
return EXIT_FAILURE;
}
int key = atoi(argv[1]);
if (key == 0) {
fprintf(stderr,"Invalid key (%s): must be a non-zero integer\n", argv[1]);
return EXIT_FAILURE;
}
FILE * f = fopen(argv[2], "r");
if (f == NULL) {
perror("Could not open file");
return EXIT_FAILURE;
}
encrypt(f,key);
if (fclose(f) != 0) {
perror("Failed to close the input file!");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
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