Loop patterns, sums and star patterns · C

A loop on its own just repeats. Most real loops build up an answer one step at a time: a total, a count, a product. This lesson collects the patterns you'll meet in class and in exams; run each one and change the numbers.

Iteration: one trip round the loop

Each pass through a loop's body is an iteration. Tracing the loop below: i is 1, 2, then 3, and when it reaches 4 the condition i <= 3 is false, so the loop stops.

#include <stdio.h>

int main() {
    for (int i = 1; i <= 3; i++) {
        printf("Iteration number %d\n", i);
    }
    printf("Loop finished\n");
    return 0;
}

Adding up: the sum of 1 to 10

An accumulator holds the running total: start it at 0 before the loop, add to it inside, use it after:

#include <stdio.h>

int main() {
    int sum = 0;                        /* 1. start the total at 0 */
    for (int i = 1; i <= 10; i++) {
        sum = sum + i;                  /* 2. add this number (also: sum += i) */
        printf("after adding %d, the total is %d\n", i, sum);
    }
    printf("Sum of 1 to 10 = %d\n", sum);   /* 3. use it after the loop */
    return 0;
}

i	sum before	sum after
1	0	1
2	1	3
3	3	6
...	...	...
10	45	55

Counting down: 10 to 1 in reverse

Start high, continue while i >= 1, and subtract with i--. Adding while counting down gives the same 55:

#include <stdio.h>

int main() {
    int sum = 0;
    for (int i = 10; i >= 1; i--) {
        printf("%d ", i);
        sum += i;
    }
    printf("\nSum of 10 down to 1 = %d\n", sum);

    int n = 5;
    while (n >= 1) {           /* the same countdown with while */
        printf("%d... ", n);
        n--;
    }
    printf("lift off!\n");
    return 0;
}

Conditional sums, counting and factorials

#include <stdio.h>

int main() {
    int evens = 0, odds = 0, sevens = 0;
    for (int i = 1; i <= 10; i++) {
        if (i % 2 == 0) evens += i;
        else odds += i;
    }
    for (int i = 1; i <= 100; i++) {
        if (i % 7 == 0) sevens++;      /* a counter adds 1 each time */
    }
    int fact = 1;                      /* a product starts at 1, not 0 */
    for (int i = 1; i <= 10; i++) {
        fact *= i;
    }
    printf("Even sum: %d, odd sum: %d\n", evens, odds);
    printf("%d numbers from 1 to 100 divide by 7\n", sevens);
    printf("10! = %d\n", fact);
    return 0;
}

Times tables and array totals

#include <stdio.h>

int main() {
    int n = 7;
    for (int i = 1; i <= 10; i++) {
        printf("%d x %d = %d\n", n, i, n * i);
    }

    int marks[5] = {72, 85, 90, 64, 88};
    int total = 0, highest = marks[0];
    for (int i = 0; i < 5; i++) {
        total += marks[i];
        if (marks[i] > highest) highest = marks[i];
    }
    printf("Total: %d, highest: %d\n", total, highest);
    printf("Average: %.1f\n", total / 5.0);
    return 0;
}

total / 5 would be integer division (79); dividing by 5.0 keeps the decimals.

Digit sum and reversing a number

n % 10 is the last digit; n / 10 drops it:

#include <stdio.h>

int main() {
    int n = 4096, digitSum = 0, reversed = 0;
    while (n > 0) {
        int digit = n % 10;
        digitSum += digit;
        reversed = reversed * 10 + digit;
        n = n / 10;
    }
    printf("Digit sum: %d\n", digitSum);
    printf("Reversed: %d\n", reversed);
    return 0;
}

Patterns with nested loops

The outer loop picks the row; the inner loop prints that row. Row row of a triangle has row stars; a pyramid row has n - row spaces then 2 * row - 1 stars:

#include <stdio.h>

int main() {
    int n = 5;
    for (int row = 1; row <= n; row++) {
        for (int col = 1; col <= row; col++) {
            printf("* ");
        }
        printf("\n");
    }
    for (int row = 1; row <= n; row++) {
        for (int s = 1; s <= n - row; s++) printf(" ");
        for (int k = 1; k <= 2 * row - 1; k++) printf("*");
        printf("\n");
    }
    int num = 1;                        /* Floyd's triangle */
    for (int row = 1; row <= 4; row++) {
        for (int col = 1; col <= row; col++) {
            printf("%d ", num);
            num++;
        }
        printf("\n");
    }
    return 0;
}

Your turn: sum of 1 to n

This example reads n with scanf; its input is 100:

#include <stdio.h>

int main() {
    int n, sum = 0;
    printf("Add up 1 to which number? ");
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    printf("\n1 + 2 + ... + %d = %d\n", n, sum);
    printf("Formula check: %d\n", n * (n + 1) / 2);
    return 0;
}