Merge sort in ascending order

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

stdio.h supplies terminal input/output. stdlib.h declares conversion and allocation functions. string.h supplies string and memory utilities. ctype.h supplies isspace, and errno.h supplies conversion error reporting. Headers declare facilities; including them does not execute an operation.

/* Read one entire token; reject malformed, oversized and out-of-range input. */
int readToken(char text[], size_t capacity) {
    int ch; size_t used = 0; int fits = 1;

readToken receives a writable buffer and its capacity. ch is int so it can represent EOF as well as input characters. used counts stored characters, and fits starts as a success flag. One slot must remain for the null terminator.

    while ((ch = getchar()) != EOF && isspace((unsigned char)ch)) { }
    if (ch == EOF) return 0;

Read until a non-whitespace character appears. The unsigned-char cast supplies an allowed value to isspace. Return 0 if EOF arrives before a token begins.

    do {
        if (ch == 0) fits = 0;
        if (used + 1 < capacity) text[used++] = (char)ch;
        else fits = 0;

The do body handles the character already read. An embedded zero byte marks the token invalid. Store a character only while there is room for it and a null terminator; otherwise clear fits. Continue reading even after the buffer fills.

        ch = getchar();
    } while (ch != EOF && !isspace((unsigned char)ch));
    text[used] = '\0';
    return fits;
}

Read through the next whitespace or EOF, add a null character after the stored prefix, then return fits. A too-long token returns 0 rather than pretending that its stored prefix is the complete token.

int readInt(int *value) {
    char token[64], *end;
    if (!readToken(token, sizeof token)) return 0;

value is an output address. A 64-slot local buffer receives the token; a failed token read returns 0 before attempting conversion.

    errno = 0; long result = strtol(token, &end, 10);
    if (errno || end == token || *end || result < -1000000 || result > 1000000) return 0;

Reset errno before strtol. &end receives the stopping address. Reject conversion errors, no converted digits, leftover characters, or values outside -1,000,000 to 1,000,000. Thus 12x is not accepted as the integer 12.

    *value = (int)result; return 1;
}
int invalid(void) { puts("Invalid or incomplete input."); return 1; }

Store a validated integer through *value and return 1 from readInt. invalid is a separate helper that prints the error and returns 1 as main’s failure status; the same number has different roles in these two function contracts.

#define MAX_N 100
int readArray(int a[], int *n, int nonnegative) {
    if (!readInt(n) || *n < 1 || *n > MAX_N) return 0;

MAX_N is 100. readArray receives an array, the caller’s count address, and a flag requesting nonnegative values. Require a count from 1 to 100; return 0 before using an invalid count.

    for (int i = 0; i < *n; ++i)
        if (!readInt(&a[i]) || (nonnegative && a[i] < 0)) return 0;
    return 1;
}

readInt checks each complete integer token. If nonnegative is nonzero, negative elements are also rejected. Return 1 only after every requested element has been accepted.

void printArray(const int a[], int n) {
    printf("Sorted:"); for (int i = 0; i < n; ++i) printf(" %d",a[i]); putchar('\n');
}

printArray reads the first n elements without modifying them. It prints the Sorted label, a space before each value, and a final newline.

void merge(int a[], int temp[], int low, int mid, int high) {
    int i = low, j = mid + 1, k = low;

i scans low..mid, j scans mid+1..high, and k identifies the next output position in temp. The helper assumes both input subranges have already been sorted.

    while (i <= mid && j <= high) temp[k++] = a[i] <= a[j] ? a[i++] : a[j++];

While both halves have elements, write one into temp and advance its source index. The <= choice takes an equal value from the left half first, preserving stable order for ties.

    while (i <= mid) temp[k++] = a[i++];
    while (j <= high) temp[k++] = a[j++];
    for (i = low; i <= high; ++i) a[i] = temp[i];
}

At most one half still contains elements. Copy its remainder, then copy temp[low..high] back into a. The brace ends merge.

void mergeSort(int a[], int temp[], int low, int high) {
    if (low >= high) return;
    int mid = low + (high - low) / 2;

A range with low >= high needs no sorting. Otherwise calculate a midpoint using the range difference, avoiding direct addition of the two bounds.

    mergeSort(a,temp,low,mid); mergeSort(a,temp,mid + 1,high);
    merge(a,temp,low,mid,high);
}

The recursive calls finish the left and right halves, then merge combines them. Each caller resumes only after its child calls return.

int main(void) {
    int a[MAX_N], temp[MAX_N], n;
    if (!readArray(a,&n,0)) return invalid();
    mergeSort(a,temp,0,n - 1); printArray(a,n); return 0;
}

main reserves an input and temporary array. Passing 0 to readArray allows negative elements. Sort the used indexes 0..n-1, print the result and return success.