Algorithms, Flowcharts & Pseudocode

Most programming mistakes happen before any code is typed: the plan was wrong or vague. Professional developers plan solutions first. This lesson gives you three tools for planning.

#Computational thinking

#Algorithms

An algorithm is a finite, ordered list of unambiguous steps that solves a problem. A recipe is an algorithm for cooking.

Algorithm: find the largest of three numbers

1. Start.
2. Read A, B, C.
3. If A ≥ B and A ≥ C, then LARGEST = A.
4. Otherwise, if B ≥ C, then LARGEST = B.
5. Otherwise, LARGEST = C.
6. Print LARGEST.
7. Stop.

A good algorithm is correct for every input (what if two numbers are equal?), finite, unambiguous and efficient.

#Flowcharts

A flowchart draws an algorithm as connected shapes:

Loops appear as an arrow going back to an earlier step:

#Pseudocode

Pseudocode writes an algorithm in structured, code-like English — precise like code, but free of any one language's syntax.

ALGORITHM SumOfDigits
    INPUT n
    sum ← 0
    WHILE n > 0 DO
        digit ← n MOD 10
        sum ← sum + digit
        n ← n DIV 10
    END WHILE
    OUTPUT sum
END

Common conventions: ← for assignment, IF … THEN … ELSE … END IF, FOR i ← 1 TO n DO … END FOR, WHILE … DO … END WHILE, INPUT and OUTPUT, and indentation to show blocks.

#From plan to program

The pseudocode translates almost line by line into any language:

#include <stdio.h>

int main(void) {
    int n, sum = 0;
    printf("Enter a number: ");
    scanf("%d", &n);
    while (n > 0) {
        int digit = n % 10;
        sum = sum + digit;
        n = n / 10;
    }
    printf("Sum of digits = %d\n", sum);
    return 0;
}

Enter a number: 40213
Sum of digits = 10

n = int(input("Enter a number: "))
total = 0
while n > 0:
    total += n % 10
    n //= 10
print("Sum of digits =", total)

Enter a number: 40213
Sum of digits = 10

#Dry runs

Before running code, trace it by hand with a table of variables — the single best debugging habit: