Colormap and colorbar in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this colormap and colorbar code?

Consider the following MATLAB code snippet:

imagesc(peaks(10));
colormap(jet);
colorbar;

What will the colorbar represent?

MATLAB

imagesc(peaks(10));
colormap(jet);
colorbar;

A A colorbar showing a gradient from blue to red representing the data values

B A colorbar showing a gradient from green to yellow representing the data values

C A colorbar with discrete colors only, no gradient

D No colorbar will be displayed

💡 Hint

Think about what the 'jet' colormap looks like and what colorbar does.

The 'jet' colormap in MATLAB creates a smooth gradient from blue to red. The colorbar shows this gradient representing the range of data values in the image.

❓ Predict Output

intermediate

What is the effect of changing colormap to 'hot'?

Given this MATLAB code:

imagesc(magic(5));
colormap(hot);
colorbar;

What color range will the colorbar display?

MATLAB

imagesc(magic(5));
colormap(hot);
colorbar;

A A gradient from purple to pink to white

B A gradient from blue to green to yellow

C A gradient from black through red and yellow to white

D A grayscale gradient from black to white

💡 Hint

Recall the colors in the 'hot' colormap.

The 'hot' colormap starts with black, then red, then yellow, and finally white for the highest values.

❓ Predict Output

advanced

What error occurs with this colormap command?

What error will this MATLAB code produce?

imagesc(rand(5));
colormap('unknownmap');
colorbar;

MATLAB

imagesc(rand(5));
colormap('unknownmap');
colorbar;

A Error: Undefined function or variable 'unknownmap'.

B No error, displays default colormap

C Warning: Colormap ignored, colorbar not shown

D Error: Invalid syntax in colormap function

💡 Hint

Check if 'unknownmap' is a valid colormap name.

MATLAB throws an error because 'unknownmap' is not a recognized colormap name or variable.

❓ Predict Output

advanced

What is the number of colors in this colormap?

Consider this MATLAB code:

imagesc(peaks(20));
colormap(parula(10));
colorbar;

How many distinct colors does the colormap have?

MATLAB

imagesc(peaks(20));
colormap(parula(10));
colorbar;

A 20

B 256

C 64

D 10

💡 Hint

Look at the argument passed to parula.

The number inside parula() specifies how many colors the colormap contains. Here it is 10.

🧠 Conceptual

expert

How does colorbar scale relate to data range?

In MATLAB, if you use:

imagesc(data);
colormap(jet);
colorbar;

and then change the color limits with:

caxis([10 20]);

What effect does this have on the colorbar and image colors?

MATLAB

imagesc(data);
colormap(jet);
colorbar;
caxis([10 20]);

A The colorbar scale remains unchanged; image colors do not change

B The colorbar scale adjusts to show colors only for data values between 10 and 20; values outside are clipped

C The colorbar disappears because caxis limits are set manually

D The image colors invert, showing low values as high colors

💡 Hint

Think about what caxis does to color scaling.

The caxis function sets the data value limits for color mapping. Values below 10 map to the lowest color, above 20 to the highest color, and the colorbar reflects this range.

Practice

1. What does the colormap function do in MATLAB?

easy

A. It changes the colors used in a plot.

B. It adds a title to the plot.

C. It changes the size of the plot.

D. It saves the plot as an image.

Solution

1. Step 1: Understand the purpose of colormap

   The colormap function sets the color scheme for plots that use colors to represent data values.

2. Step 2: Compare options with the function's role

   Only It changes the colors used in a plot. correctly describes changing colors used in a plot, while others describe unrelated actions.

3. Final Answer:

   It changes the colors used in a plot. -> Option A

4. Quick Check:

   Colormap changes plot colors = B [OK]

Hint: Colormap sets plot colors, colorbar shows their meaning [OK]

Common Mistakes:

• Confusing colormap with plot title or size
• Thinking colormap saves images
• Mixing colormap with colorbar function

2. Which of the following is the correct syntax to add a colorbar to a plot in MATLAB?

easy

A. colorbar<>;

B. colorbar[];

C. colorbar();

D. colorbar{};

Solution

1. Step 1: Recall MATLAB function syntax

   MATLAB functions use parentheses to call them, so colorbar() is correct syntax.

2. Step 2: Check each option's syntax

   Options A, B, and C use invalid delimiters (<>, [], {}) for function calls in MATLAB.

3. Final Answer:

   colorbar(); -> Option C

4. Quick Check:

   Function calls use parentheses = D [OK]

Hint: Use parentheses () to call MATLAB functions [OK]

Common Mistakes:

• Using square brackets [] instead of parentheses
• Using curly braces {} incorrectly
• Confusing syntax with other programming languages

3. What will be the effect of the following MATLAB code?

imagesc(peaks(10));
colormap(jet);
colorbar;

medium

A. Displays a grayscale plot without any colorbar.

B. Displays a colored plot with a jet colormap and a colorbar showing the color scale.

C. Shows an error because colorbar is missing parentheses.

D. Displays a plot but the colormap is ignored.

Solution

1. Step 1: Understand each function's role

   imagesc(peaks(10)) creates a colored image plot of data. colormap(jet) sets the colors to the 'jet' scheme. colorbar adds a color scale bar.

2. Step 2: Analyze the output

   The plot will show colors from the jet colormap and a colorbar indicating the color scale. No syntax errors occur.

3. Final Answer:

   Displays a colored plot with a jet colormap and a colorbar showing the color scale. -> Option B

4. Quick Check:

   imagesc + jet + colorbar = colored plot with scale [OK]

Hint: imagesc + colormap + colorbar shows colored data with scale [OK]

Common Mistakes:

• Thinking colorbar needs parentheses to work
• Assuming default grayscale without colormap
• Confusing colormap effect with plot type

4. Identify the error in this MATLAB code snippet:

surf(peaks);
colormap('hot');
colorbar;

medium

A. There is no error; the code runs correctly.

B. The colorbar function is missing parentheses.

C. The surf function cannot be used with colormap.

D. The colormap name should not be in quotes.

Solution

1. Step 1: Check colormap syntax

   Using colormap('hot') with quotes is valid in MATLAB to specify the colormap by name.

2. Step 2: Verify colorbar usage and surf compatibility

   colorbar without parentheses works in MATLAB as a command. surf works fine with colormap and colorbar.

3. Final Answer:

   There is no error; the code runs correctly. -> Option A

4. Quick Check:

   colormap('name') and colorbar command valid = A [OK]

Hint: Quotes allowed in colormap; colorbar works without () [OK]

Common Mistakes:

• Thinking colormap names must be unquoted
• Believing colorbar needs parentheses always
• Assuming surf can't use colormap

5. You want to create a heatmap of data with values from 0 to 100, using a colormap that smoothly changes from blue to red, and show a colorbar with ticks at 0, 50, and 100 labeled accordingly. Which MATLAB code snippet achieves this?

hard

A. imagesc(data);
colormap(hot);
colorbar('Ticks',[0 50 100],'TickLabels',{'Low','Medium','High'});

B. imagesc(data);
colormap([0 0 1; 1 0 0]);
colorbar('Ticks',[0 50 100],'TickLabels',{'Low','Medium','High'});

C. imagesc(data);
colormap([linspace(0,1,256)', zeros(256,1), linspace(1,0,256)']);
cb = colorbar;
cb.Ticks = [0 0.5 1];
cb.TickLabels = {'Low','Medium','High'};

D. imagesc(data);
colormap(jet);
cb = colorbar;
cb.Ticks = [0 50 100];
cb.TickLabels = {'Low','Medium','High'};

Solution

1. Step 1: Create a blue-to-red colormap

   imagesc(data); colormap([0 0 1; 1 0 0]); colorbar('Ticks',[0 50 100],'TickLabels',{'Low','Medium','High'}); attempts a custom colormap with two colors blue and red, but MATLAB requires more points or interpolation for smooth transition. imagesc(data); colormap(jet); cb = colorbar; cb.Ticks = [0 50 100]; cb.TickLabels = {'Low','Medium','High'}; uses jet, which is a smooth colormap but not strictly blue to red. imagesc(data); colormap([linspace(0,1,256)', zeros(256,1), linspace(1,0,256)']); cb = colorbar; cb.Ticks = [0 0.5 1]; cb.TickLabels = {'Low','Medium','High'}; uses a custom colormap with 256 points smoothly transitioning from blue to red, but the ticks are set incorrectly at [0 0.5 1] which do not match data range 0-100. imagesc(data); colormap(hot); colorbar('Ticks',[0 50 100],'TickLabels',{'Low','Medium','High'}); uses hot colormap which is not blue to red.

2. Step 2: Set colorbar ticks and labels correctly

   imagesc(data); colormap(jet); cb = colorbar; cb.Ticks = [0 50 100]; cb.TickLabels = {'Low','Medium','High'}; sets ticks at [0 50 100] matching data range and labels accordingly. imagesc(data); colormap([0 0 1; 1 0 0]); colorbar('Ticks',[0 50 100],'TickLabels',{'Low','Medium','High'}); sets ticks and labels but the colormap is only two colors, which may not smoothly interpolate. imagesc(data); colormap([linspace(0,1,256)', zeros(256,1), linspace(1,0,256)']); cb = colorbar; cb.Ticks = [0 0.5 1]; cb.TickLabels = {'Low','Medium','High'}; has incorrect ticks. imagesc(data); colormap(hot); colorbar('Ticks',[0 50 100],'TickLabels',{'Low','Medium','High'}); uses wrong colormap.

3. Step 3: Choose the best option

   imagesc(data); colormap(jet); cb = colorbar; cb.Ticks = [0 50 100]; cb.TickLabels = {'Low','Medium','High'}; provides a smooth colormap (jet) and correctly sets ticks and labels on the colorbar matching data range 0-100.

4. Final Answer:

   imagesc(data); colormap(jet); cb = colorbar; cb.Ticks = [0 50 100]; cb.TickLabels = {'Low','Medium','High'}; -> Option D

5. Quick Check:

   Jet colormap + correct ticks and labels = A [OK]

Hint: Use linspace to build smooth colormap; set colorbar ticks carefully [OK]

Common Mistakes:

• Using colormap with wrong colors
• Setting colorbar ticks without matching colormap scale
• Incorrect syntax for colormap or colorbar properties