Strings & String Functions

#include <stdio.h>
#include <string.h>

int main(void) {
    /* Declaration styles */
    char hobby[] = "rowing";        /* size = 7 (6 chars + '\0') */
    char buf[64] = "";              /* empty string: buf[0] = '\0' */
    char greeting[64];              /* uninitialized — must write before reading */

    /* strlen: count characters before '\0' */
    printf("hobby    = \"%s\"\n", hobby);
    printf("strlen   = %zu\n",   strlen(hobby));   /* 6 */
    printf("sizeof   = %zu\n",   sizeof(hobby));   /* 7 (includes \0) */

    /* strcpy: copy a string into a buffer */
    strcpy(greeting, "Hello, ");   /* buf must fit "Hello, \0" = 8 bytes */
    printf("After strcpy: \"%s\"\n", greeting);

    /* strcat: append one string to another */
    strcat(greeting, hobby);       /* greeting = "Hello, rowing" */
    strcat(greeting, "!");         /* greeting = "Hello, rowing!" */
    printf("After strcat: \"%s\"\n", greeting);
    printf("Final strlen = %zu\n", strlen(greeting));

    /* sprintf: build a string with formatting */
    int score = 95;
    sprintf(buf, "Grade: %d%%", score);   /* %% prints literal % */
    printf("%s\n", buf);

    return 0;
}

#include <stdio.h>
#include <string.h>

#define BUFSIZE 16

int main(void) {
    char dst[BUFSIZE];
    const char *src = "tiger";       /* 5 chars */
    const char *long_src = "superlongstring";  /* 15 chars — fits exactly */

    /* strncpy: copy at most n bytes */
    strncpy(dst, src, BUFSIZE);
    dst[BUFSIZE - 1] = '\0';         /* ALWAYS force-terminate! */
    printf("strncpy: \"%s\"\n", dst);

    /* What if src is too long? strncpy truncates but may omit '\0' */
    strncpy(dst, long_src, BUFSIZE);
    dst[BUFSIZE - 1] = '\0';         /* critical: ensures null terminator */
    printf("truncated: \"%s\"\n", dst);

    /* strncat: safe concatenation — always adds '\0' */
    char result[32] = "Hello";
    strncat(result, ", World", sizeof(result) - strlen(result) - 1);
    printf("strncat: \"%s\"\n", result);

    /* strcmp: string equality — NEVER use == on strings! */
    const char *a = "apple";
    const char *b = "banana";
    const char *c = "apple";

    printf("strcmp(apple, banana) = %d  (negative = a comes first)\n",
           strcmp(a, b));
    printf("strcmp(apple, apple)  = %d  (zero = equal)\n",
           strcmp(a, c));
    printf("strcmp(banana, apple) = %d  (positive = b comes after)\n",
           strcmp(b, a));

    /* strncmp: compare only first n chars — great for command parsing */
    const char *cmd = "GET /index.html";
    if (strncmp(cmd, "GET", 3) == 0) {
        printf("HTTP method is GET\n");
    }

    return 0;
}

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

int main(void) {
    /* atoi: UNSAFE — silently returns 0 on failure, no error info */
    int bad = atoi("abc");         /* returns 0 — you cannot tell it failed */
    int also_bad = atoi("0");      /* also returns 0 — indistinguishable! */
    printf("atoi(\"abc\") = %d  (silent failure!)\n", bad);

    /* strtol: SAFE — preferred in COMP2017 */
    /* Signature: long strtol(const char *s, char **endptr, int base); */
    char *endptr;
    const char *s1 = "42abc";

    long val = strtol(s1, &endptr, 10);
    /* val = 42, endptr points to "abc" (the first non-digit) */
    printf("strtol(\"%s\", ..., 10): val=%ld, rest=\"%s\"\n", s1, val, endptr);

    /* Detect complete conversion: *endptr should be '\0' */
    if (*endptr != '\0') {
        printf("  Warning: partial conversion, trailing: \"%s\"\n", endptr);
    }

    /* Detect no digits at all: endptr == original string */
    const char *s2 = "xyz";
    long val2 = strtol(s2, &endptr, 10);
    if (endptr == s2) {
        printf("strtol(\"%s\"): no digits found!\n", s2);
    }

    /* Detect overflow using errno */
    errno = 0;                          /* reset before the call */
    long big = strtol("99999999999999999999", &endptr, 10);
    if (errno != 0) {
        printf("Overflow detected via errno!\n");
    }

    /* Base 0: auto-detect hex (0x) and octal (0) prefixes */
    printf("0x1F  = %ld\n", strtol("0x1F",  NULL, 0));  /* 31 */
    printf("017   = %ld\n", strtol("017",   NULL, 0));  /* 15 (octal) */
    printf("255   = %ld\n", strtol("255",   NULL, 0));  /* 255 (decimal) */

    return 0;
}

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

int main(void) {
    /* atof: UNSAFE — silently returns 0.0 on failure, no error info */
    double bad = atof("xyz");          /* returns 0.0 — indistinguishable from atof("0") */
    printf("atof(\"xyz\") = %f  (silent failure!)\n", bad);

    /* strtod: SAFE — preferred alternative */
    /* Signature: double strtod(const char *str, char **endptr); */
    char *end;
    const char *s1 = "3.14abc";

    double val = strtod(s1, &end);
    /* val = 3.14, end points to "abc" (first unconverted char) */
    printf("strtod(\"%s\", ...): val=%f, rest=\"%s\"\n", s1, val, end);

    /* Detect complete conversion: *end should be '\0' */
    if (*end != '\0') {
        printf("  Warning: partial conversion, trailing: \"%s\"\n", end);
    }

    /* Detect no digits at all: end == original string pointer */
    const char *s2 = "not-a-number";
    double val2 = strtod(s2, &end);
    if (end == s2) {
        printf("strtod(\"%s\"): no valid number found!\n", s2);
    }
    (void)val2;  /* suppress unused-variable warning */

    /* Using strtod for command-line argument validation */
    const char *arg = "2.718";
    errno = 0;
    double e = strtod(arg, &end);
    if (end == arg) {
        fprintf(stderr, "Not a number\n");
    } else if (errno != 0) {
        fprintf(stderr, "Overflow/underflow\n");
    } else {
        printf("Parsed: %f\n", e);
    }

    return 0;
}

#include <stdio.h>
#include <string.h>

int main(void) {
    /* sscanf parses a string the same way scanf parses stdin */
    char line[] = "Alice 21 88.5";
    char name[20];
    int  age;
    float gpa;

    int n = sscanf(line, "%19s %d %f", name, &age, &gpa);
    /* %19s: read at most 19 chars into name (leaves room for \0) */

    if (n == 3) {
        printf("Name: %s\n", name);
        printf("Age:  %d\n", age);
        printf("GPA:  %.1f\n", gpa);
    } else {
        printf("Parse error: only got %d fields\n", n);
    }

    /* sprintf round-trip: format then parse */
    char buf[64];
    sprintf(buf, "x=%d y=%d", 10, 20);
    int x, y;
    sscanf(buf, "x=%d y=%d", &x, &y);
    printf("Parsed: x=%d, y=%d\n", x, y);

    return 0;
}