Computational Thinking - Counting

By

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Dept. of CSE

PVPSIT, Kanuru.

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_{PRASAD V. POTLURI SIDDHARTHA INSTITUTE OF TECHNOLOGY}

• Problem
• Given a set of n students’ examination marks (in the range 0 to 100) make a count of the number of students that passed the examination. A pass is awarded for all marks of 50 and above.
• Algorithm development
• Counting mechanisms are very frequently used in computer algorithms.
• Generally a count must be made of the number of items in a set which possess some particular property or which satisfy some particular condition or conditions.

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Dept. of CSE

2026

Algorithm 2.2

COUNTING

PVPSIT (Autonomous)

Problem Solving Techniques

• Suppose that we are given the set of marks

55, 42, 77, 63, 29, 57, 89

• To make a count of the passes for this set we can start at the left, examine the first mark (i.e. 55), see if it is >=50, and remember that one student has passed so far.
• The second mark is then examined and no adjustment is made to the count.
• The process continues in a similar manner until all marks have been tested.

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• In more detail we have:

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Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• The two steps in this process can be represented by

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• Obtain the similar expression

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• Viewing the mechanism in this way makes it clear that the existence of the variable previous_count in its own right is unnecessary.
• The essential steps in our pass-counting algorithm can therefore be summarized as:

While less than n marks have been examined do

(a) read next mark,

(b) if current mark satisfies pass requirement then add one to count.

• Before any marks have been examined the count must have the value zero.

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• Algorithm description

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• Pseudo-code / logic
• begin

read n

set passmark as 50

count := 0

i := 0

while i < n do

begin

i:= i+1

read m

if m >= passmark then count := count+1

end

writeout “no. of passes=”, count

• end

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• Notes on design:
• Initially, and each time through the loop, the variable count represents the number of passes so far encountered.
• On termination (i=n) count represents the total number of passes in the set.
• Applications
• All forms of counting.

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

• Supplementary Problems
• 2.2.2 Design an algorithm that reads a list of numbers and makes a count of the number of negatives and the number of non-negative members in the set.

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques

2.2.2 Design an algorithm that reads a list of numbers and makes a count of the number of negatives and the number of non-negative members in the set.

• Algorithm description
• Prompt and read the number of numbers to be processed.
• Initialize negative and non-negative counts to zero.
• While there are still numbers to be processed repeatedly do

(a) read next number.

(b) if it is < 0 then add one to negative count

else add one to non-negative count.

4. Write out negative and non-negative counts.

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Dept. of CSE

2026

Solution

PVPSIT (Autonomous)

Problem Solving Techniques

• Pseudo-code:
• begin

read (n)

negative_count := 0

non-negative_count := 0

total_count := 0

while total_count < n do

begin

read (number)

total_count := total_count+1

if number < 0 then

negative_count := negative_count+1

else

non-negative_count := non-negative_count+1

end

write out negative_count and non-negative_count

• end

Dept. of CSE

2026

PVPSIT (Autonomous)

Problem Solving Techniques