Type specifiers (formatting strings): %d, %ld, %f, %c, %s, %lf

📖 Theory

The costume party

You have been trusting %d and %f for three lessons. Time to decode them.

Run this experiment mentally: printf("%d", 78.5);, printing a float in an int's costume. The output is not 78, not 79, but garbage: perhaps -1717986918.

printf does not check who wears what. The type specifier tells it how to read raw bytes, and bytes read in the wrong costume become nonsense. Matching costumes to guests is today's whole art.

📖 Theory

Fill-in-the-blanks with shaped blanks

A printf format string is a sentence with blanks: "Marks: __, Grade: __". But each blank is SHAPED: a %d blank fits an int, a %c blank fits one character, a %f blank fits a decimal. Push a float through an int-shaped blank and it comes out mangled, like forcing a round dabba into a square slot. The blanks are the specifiers; the shapes must match.

📊 At a glance

The specifier family

⌨️ Practical

One marksheet, every specifier

#include <stdio.h>

int main() {
    char name[20] = "Riya";
    int marks = 78;
    float percentage = 78.53;
    char grade = 'A';

    printf("Name : %s\n", name);
    printf("Marks: %d\n", marks);
    printf("Perc : %.2f\n", percentage);
    printf("Grade: %c (ASCII %d)\n", grade, grade);
    return 0;
}

This example runs in Gri-Learn on the web, where you can edit it and see the output.

💭 Think first

One char, two costumes

The last printf prints grade twice: once as %c and once as %d. Predict that line's exact output, using what a char really is.

🧩 Quiz

What does printf("%d", 78.5); actually print?

1. Unpredictable garbage: the bytes are read in the wrong format
2. 78, C removes the decimal part
3. 79, C rounds it
4. 78.5, printf adjusts automatically

🧩 Quiz

You are reading a double with scanf. Which specifier is REQUIRED?

1. %lf
2. %f
3. %d
4. %s

⚠️ Watch out

Where marks leak

Mismatched pairs: %d with floats, %f with ints, garbage both ways. %c vs %d on a char changes what appears (A vs 65), watch which the question wants. In scanf, forgetting that %lf is required for double. And a neat habit worth marks: %.2f prints two decimals (78.53 instead of 78.530000), the display rounds, the stored value stays untouched.

📖 Theory

Unit 1 is complete

Look at what the marks program can now do: declare typed boxes, take input, compute with operators, and print a formatted marksheet, every Unit 1 topic is muscle now, not theory. One small preview hid in today's code: scanf for %s needs no &, because an array's name is already an address; that mystery unlocks in Unit 3 (arrays and pointers). Next: Unit 2, where the program starts making decisions.

✅ Summary

Key takeaways

• Specifiers tell printf/scanf how to read bytes: %d int, %ld long, %f float, %lf double, %c char, %s string.
• Mismatch = garbage: printf converts nothing (%d with 78.5 is undefined).
• scanf is strict (%lf required for double); printf is lenient (%f prints both).
• %c vs %d on the same char: letter vs ASCII number.
• %.2f trims the DISPLAY to two decimals; the stored value is unchanged.
• Memory hook: shaped blanks, the costume must match the guest.